TITLE: Basic Ultrasonography & Sonographic Artifacts
AUTHOR(S): George Henry, DVM, DACVR
ADDRESS (URL):
http://www.vin.com/Members/Proceedings/Proceedings.plx?CID=WVC2003&PID=3201&O=VIN
OBJECTIVES
Present basic physical principles required to understand diagnostic ultrasound imaging.
Discuss the basic controls of an ultrasound scanner and their function in producing a
diagnostic quality image.
Review basic descriptive terminology used for describing ultrasound images and
findings.
Review and discuss the cause and recognition of common ultrasound artifacts relevant
to the interpretation of ultrasound images.
GENERAL KEY POINTS
What is diagnostic ultrasound?
Diagnostic ultrasound is the use of sound waves to image the soft tissues and fluids of
the body.
Sound is transmitted through tissues by areas of compression and rarefaction at an
average speed of 1540 meters per second.
Image production relies on reflective surfaces within the tissues that reflect sound
waves back to the transducer!
Utilizes sound waves in the 2-15 MHz (Million Hertz (cycles per second)) range.
Reflective surfaces occur at the interface between tissues/substances with different
acoustic properties (acoustic impedance).
Acoustic impedance = velocity of sound in that tissue x tissue density
Air: 0.0004
Fat: 1.38
Water: 1.54
Brain: 1.58
Blood: 1.61
Kidney: 1.62
Muscle: 1.70
Bone: 7.8
Note that bone and air have very different acoustic impedance than the normal soft
tissues and fluid of the body.
The percent of sound reflection of a reflective surface is directly proportional to the
magnitude of the difference in acoustic impedance's of the tissues at an interface!
Interface: % Reflected
Blood-brain: 0.3
Kidney-liver: 0.6
Liver-muscle: 1.8
Blood-fat: 7.9
Liver-fat: 10.0
Muscle-fat: 10.0
Muscle-bone: 64.6
Soft-tissue-gas: 99.0
Interfaces between bone or air and soft tissues produce highly reflective interfaces that
prevent imaging of tissues deep to the interface. An air - soft tissue interface is
essentially like looking in a mirror. The ultrasound probe cannot see through that
interface because practically all of the sound is reflected back.
Image on the monitor is made of multiple scan lines of gray scale dots.
Depth, y axis, is determined by 1540 m/s times round-trip time of echo divided by 2.
Brightness of dot on display directly related to intensity of the echo.
Scan line determines x axis position.
Sound wave is produced as a pulse of sound.
Pulse = approx. 2-3 wavelengths.
Sound pulse produced approximately 1% of time.
Listening for returning echoes approximately 99% of time.
Resolution
Axial Resolution
Directly related to frequency of transducer.
Related to wavelength / spatial pulse length.
Increase frequency = Increase resolution.
Lateral Resolution
Determined by beam diameter or width.
Attenuation of the Sound beam produced by the following factors:
Reflection loss
Absorption loss
Scattering loss
Refraction loss
Increase frequency = Increased attenuation = Decreased penetration.
Transducer Selection
Select highest frequency transducer that will allow sufficient penetration to image
organ of interest!
Multiple frequencies used for many exams.
Ultrasound Image Display
A-mode (Amplitude Mode)
Line graph type display showing depth along x axis and y axis indicates intensity
(Amplitude).
No anatomical image.
First ultrasound was A-mode.
Used in some high resolution ocular exams.
B-mode (Brightness Mode)
Line of dots.
Depth along x-axis.
Brightness of dot indicates intensity of the echo.
M-mode (Motion Mode)
Initially was B-mode line imprinted on moving paper so the lines produced
represented movement of structures such as ventricular walls of the heart.
Present equipment sweeps the B-mode line across the monitor.
Still used for many cardiac measurements.
Usually will come as part of the cardiac package of a scanner.
Static B-mode
Articulated probe was used to sweep through anatomy and "paint" an anatomical
image of the soft tissues.
First ultrasound scanners to produce anatomical images.
Movement during the scanning seriously degraded images.
No one uses these scanners any more!
Real-time B-mode
Current scanners are of this type.
Image made from multiple B-mode scan lines placed together to form an
anatomical image of the tissues.
Image is updated frequently (frame rate) to give the appearance of real time
motion.
Ultrasound Transducers
A transducer is a device that changes one kind of energy into another.
Ultrasound transducers change electrical energy into mechanical energy (sound) and
then change the returning mechanical sound energy into electrical energy for
processing by the scanner.
Types of Ultrasound Transducers
Linear-Array Transducer
Produces rectangular image.
Advantage-wide image in near field (close to transducer).
Advantage-simple electronics (less expensive).
Disadvantage-not able to see larger image beyond a small superficial acoustic
window.
Mechanical Sector Transducer
Produces sector type image.
Advantage-relatively simple electronics (less expensive).
Advantage-small near field image that widens allows observation of structures
through a small acoustic window-e.g., cardiac ultrasound.
Disadvantage-only a small portion of structures in near field are displayed.
Disadvantage-usually has fixed focal zone.
Disadvantage-moving parts.
Phased-array, Curved-array Transducers
Produces sector type image.
Most common type used by newer higher quality equipment.
Advantage-allows dynamic changing of focal zones including multiple focal zones
to improve image quality.
Advantage-no moving parts.
Disadvantage-more expensive.
Annular Array Transducers
Similar to phased/curved array transducers except they use circular crystals to
produce a cone shaped sound beam.
Knowledge of cross-sectional anatomy is essential for recognition of structures and
abnormalities!
Consistent orientation of image planes important to decrease confusion and prevent
misinterpretation of relational abnormalities.
Example: For sagittal and parasagittal images of the abdomen orient cranial to the
left of the screen and caudal to the right of the screen.
Terminology is related to position, echoic intensity and echo texture.
Hyperechoic / echogenic / echo rich / high intensity (bright areas).
Anechoic / echo free (black areas).
Hypoechoic / echo poor (dark to medium gray scale areas).
Focal, multifocal, or diffuse describe physical extent of lesion or lesions in an organ.
Fine, medium, or coarse parenchymal texture refers to small or large "dots."
Uniform (homogeneous) or nonuniform (heterogeneous) can refer to texture and
echogenicity. Therefore, the terms should be combined to indicate what is being
described. "Heterogeneous appearance" is confusing as it does not indicate what is
heterogeneous-texture, echogenicity, or both.
General Scanner Controls
There are many controls on most ultrasound scanners that vary from machine to machine
and should be understood to produce the maximum image quality and information. A
number of control settings are stored in "presets" for various different ultrasound
examinations. These allow optimum beginning settings for different exams such as cardiac,
vascular, abdominal, small parts, extremities etc. The following list concerns the most
frequently used controls and their basic impact on the image produced.
Power Switch (on/off)-always best to turn equipment off before unplugging the
equipment.
Power / Intensity / Output Control-Some scanners have a knob labeled power,
intensity or output. This controls the power or intensity of the sound beam produced by
the transducers. It is basically like a volume control on a speaker. Most of the time this
will be set to the highest value depending on the equipment.
Gain (Amplification)-This is the overall gain control that controls the amount of gain
or amplification of the sound detected by the transducer. Increasing this control will
brighten the entire image. Setting the gain too high will brighten the image but will
also increase the "noise." Setting the gain too high will decrease observation of subtle
changes in echogenicity and echo texture.
Reject-Some scanners use the term "filter." This control allows "filtering out" selected
intensities from the image. This can be used to filter out some noise from the image.
Setting this control incorrectly can cause loss of useful information and should be used
with caution.
Time-Gain (Depth-Gain) Compensation
Individual controls for amount of gain/amplification of individual depth zones in the
image. This allows adjustment of the image so that there is a uniform
appearance/brightness of a structure.
A common example is a sagittal image of the liver will necessitate increased
gain/amplification of the deeper portions and decreased gain of the near field to
give a uniform medium echoic parenchyma.
Depth Control-This controls the depth of view indicated in mm or cm. Start with
deeper depth views to get orientation and then decrease depth as needed to view
superficial organs. Depth of view must be adjusted frequently to optimism the image
for interpretation.
Focal Zone Control
Available only on scanners that allow dynamic focus control. This control allows the
sonographer to change the depth of the focal zone for optimal imaging of target
structures. Most machines with this feature also allow multiple focal zones to further
enhance the image quality. However, using multiple focal zones will usually
significantly decrease the frame rate of the scanner. The "frame rate" is the rate at
which the image on the screen is updated. Frame rate is critical for faster moving
structures such as the heart. Dropping below 15 frames per second will produce a
"jerky" image if any movement is present.
Freeze Button
This button allows the current image to be "frozen" to allow printing, saving and/or
measurements of the image. When the freeze button is active, the transducer is not
active. The freeze button should be activated when no active imaging is being
performed for a long time. When the probe is active and not interfaced with tissue,
the transducer may heat up and shorten the life of the transducer. Some machines
will automatically go into freeze mode if there is no use of controls or buttons for a
set period of time.
Ultrasound Artifacts-What you see is not necessarily what or where you
think!
Acoustic Shadowing
Occurs at interfaces that reflect and/or absorb a significant portion of the ultrasound
beam.
Clean Shadows-Echogenic surface with dark acoustic shadow deep to the structure.
Commonly seen with mineral interfaces with tissues and fluid such as bone
surfaces, calculi, and foreign bodies.
Dirty Shadows-Usually a highly echogenic surface with shadow containing "noise"
that appears to indicate echoes are coming from deep to the shadowing interface.
These are usually produced by gas interfaces with tissues and fluid.
Size matters! Smaller mineral objects and small air bubbles may not show the typical
type of shadow! The sonographer must recognize that the "echoes" apparently
distal to these interfaces do NOT represent actual structures in the area of the
shadow!
Higher frequency transducers usually show better shadows!
Acoustic shadows are more distinct if the object is in the focal zone.
A small diameter calculus that is not as large as the ultrasound beam width may not
produce a shadow.
The "dirty" versus "clean" acoustic shadowing is not always definitive for mineral
versus gas. In some situations gas can produce a clean shadow.
Gas and fecal mater in the colon frequently produce a mixed type of acoustic
shadowing.
Reverberation Artifact
Due to highly reflective surface. Especially very smooth reflectors perpendicular to
the ultrasound beam. Produces dirty acoustic shadow with multiple parallel echoic
lines deep to the interface.
Gas such as in the lung, GI bubbles or free air within a body cavity are the most
frequent cause.
Metal such as metal sutures, bone plates, sewing needles, etc. will also produce
reverberation artifact.
Smaller width strong reverberation artifacts called "Comet tails" are commonly seen
with irregular gas interfaces.
"Ring down" artifact due to resonance in air bubbles appears similar to comet tails.
Mirror Image Artifact
Type of "multipath" artifact.
Associated with highly reflective curved surfaces
Produces duplicate of structure deep to the reflective surface
Liver appearing on the thoracic side of the diaphragm (intrapulmonary air and soft
tissue interface) is the most commonly recognized example.
Beware of a false diagnosis of diaphragmatic hernia due to this artifact!!
Through Transmission Enhancement
Increased echogenicity of tissue deep to an area or structure.
Most often seen with fluid filled structures.
Due to improved penetration of sound through fluid because of decreased attenuation
of the sound beam. Therefore the sound deep to the fluid filled structure will appear
brighter than adjacent tissue at the same depth. Sound adjacent to the fluid filled
structure passed through tissue that attenuated the sound beam more than
occurred to the sound beam passing through the fluid. Since echoes from a given
depth all the way across the image are amplified equally, the area deep to the fluid
filled structure will appear brighter.
Useful in identifying fluid within structures.
Slice thickness
Echoes from two structures within the width of the sound beam are combined.
Commonly causes pseudosludge in the gall bladder
Side-lobe or grating-lobe artifact
Weak echoes produced by the transducer outside of the main sound beam produced
weak reflections from echogenic interfaces out side of the main beam. These weak
echoes can be reflected back to the transducer and detected. The ultrasound
scanner assumes echoes occur along a straight line and places the echoes along the
main scan line. These low intensity echoes are usually not observed in relatively
echoic tissue as the main echoes are of higher intensity and "cover" up the weak
echoes. However, when anechoic or very hypoechoic areas are present in the
image, these weak echoes will show as echoes from within the anechoic structure.
Commonly seen as pseudoechoes within the urinary bladder from echoes produced
by echogenic gas interface within the colon.
Edge Shadowing
Most common with round structures.
Occurs at the edge parallel to the sound beam.
Causes dark shadow distal to the edge.
Due to refraction and reflection factors
SUMMARY
Ultrasound imaging requires a good knowledge of the physical principles of sound/tissue
interactions that produce the image in order to understand the normal and abnormal
tissue appearances.
The sonographer must understand that ultrasound is significantly different than other
diagnostic imaging modalities such as x-rays. Radiographs record transmission of xrays through tissues with different absorption properties (density) whereas ultrasound
is looking at reflected sound waves returning to the transducer that are not necessarily
directly related to "density" of the tissues.
The use and effect of multiple controls must be understood and utilized to optimize the
images obtained with the ultrasound equipment.
Interpretation of ultrasound images requires an excellent knowledge of cross-sectional
anatomy of the structures imaged.
Standard terminology in the description of sonographic images is necessary for accurate
recording and interpretation of sonographic findings.
The ability to recognize sonographic artifacts is necessary to prevent false diagnoses
that often lead to inappropriate treatment.
With the appropriate knowledge and skill, diagnostic ultrasound can significantly
enhance the diagnostic capability available to the practicing veterinarian.
References
1. Nyland TG, Mattoon JS. Small Animal Diagnostic Ultrasound 2nd Ed. Philadelphia; WB Saunders
Co, 2002
SEARCH RESULT #: 2
TITLE: Current Techniques in Ultrasonography
AUTHOR(S): John S. Mattoon
ADDRESS (URL):
http://www.vin.com/Members/Proceedings/Proceedings.plx?CID=WVC2003&PID=3200&O=VIN
OBJECTIVES
Acquaint practitioners with cutting-edge ultrasound techniques and equipment.
KEY POINTS
Special interventional ultrasound techniques
Power Doppler
Basics and application of harmonic ultrasound.
Ultrasound contrast agents.
Endoscopic ultrasound
Ultra-high frequency ultrasound
Extended field-of-view imaging
3-D ultrasound.
OVERVIEW
Percutaneous Antegrade Pyelography
1,2
Intravenous excretory urography is a useful, common technique for diagnosing obstructive
uropathy and for localizing the obstruction site. However, in instances of severe renal
dysfunction, the study may be inconclusive because of compromised glomerular filtration of
the iodinated contrast material, resulting in poor or absent opacification of the renal
collecting system and ureters. The intravenous contrast medium administered may also
cause further renal damage or elicit adverse reactions in a small percentage of patients.
Ultrasound guided percutaneous antegrade pyelography has been described as a technically
simple diagnostic procedure for confirmation of ureteral obstruction in the dog and cat.
Ultrasound guided percutaneous antegrade pyelography is performed by inserting a sterile
3.5-in, 22 gauge spinal needle into the renal pelvis through the greater curvature of the
renal parenchyma, opposite the renal hilus to avoid the renal artery and vein.
Nephropyelocentesis is performed to reduce the transverse diameter of the dilated renal
pelvis by half followed by infusion of iodinated contrast medium. Lateral and ventrodorsal
radiographs are then immediately obtained.
Percutaneous US-guided pyelocentesis can also be performed to collect urine samples,
especially in animals suspected of having pyelonephritis despite negative urine culture from
routine cystocentesis.
Suction Biopsy of Bladder and Urethral Masses3
Ultrasound can be used to direct urinary catheter placement to the site of bladder or urethral
lesions for aspiration/suction biopsy. When using a side-hole catheter, the location of the
side holes can be identified by the anechoic or hypoechoic defect within the catheter. The
side hole can then be positioned optimally against the mass to be aspirated.
Intraoperative Ultrasonography
4-11
Intraoperative ultrasonography (IOU) has found multiple uses in human medicine during the
past 15-20 years. Ultrasound guidance has been used to assist various surgical procedures,
determine lesion margins and resectability of lesions, detect pre-operatively unidentified
deep parenchymal lesions (e.g., insulinomas), detect and localize urinary and biliary calculi,
identify specific anatomical structures such as the common bile duct, determine flow patterns
within vessels, and assess tumoral vascular invasion. Similar applications exist in veterinary
medicine.28
Ideally, IOU is performed using small, dedicated, high resolution linear array transducers,
available in a variety of shapes to adapt better to the organ or procedure of interest. In the
absence of specialized equipment, intraoperative ultrasonography can be performed using an
appropriate transducer with standard ultrasound equipment. Sterilization of the transducer
and cable may be done using gas sterilization (ultrasound transducers cannot be
autoclaved). More commonly, the transducer and cable are thoroughly cleansed and covered
with a commercially available sterile plastic sleeve with sterile acoustic gel. Warm sterile
saline or sterile coupling gel is poured into the surgical field as an acoustic coupling agent.
IOU of the brain and spinal cord has been described in dogs. The normal brain and spinal
cord anatomy can be identified with appropriate equipment and with a surgical site large
enough to allow proper transducer contact. IOU can facilitate identification of deep seated
and nonpalpable lesions, limit tissue dissection, and reduce operating time. Ultrasoundguided biopsy of the cingulate gyrus and the head of the caudate nucleus on 10 clinically
normal dogs has been reported. Using 16-gauge Menghini needles, adequate tissue
specimens were obtained without complication. This procedure may be a useful technique for
definitive diagnosis of focal brain disease in veterinary patients. Ultrasonography may also be
used through post-operative bony defects to monitor post-operative changes, potential
complications and long-term response to treatment. Intraoperative ultrasound guidance may
be useful to assist therapeutic intralesional injection of anti-cancerous drugs. IOU has been
used as an aid to intraoperative radiation therapy planning. Precise ultrasound assessment of
lesion size, depth, volume and extension help determine the size and energy of the radiation
field.
IOU is used to locate foreign bodies and bony fragments. Ultrasonography is sensitive in
locating small foreign objects, especially if they are highly echogenic. Underlying anatomic
structures such as the joint capsule, vessels, nerves, and tendons can be recognized and
spared during surgical exploration. In addition to reducing tissue dissection and operating
time, IOU may supplement or replace intraoperative radiography in specific circumstances.
IOU can be used to atraumatically localize portosystemic shunts and assist with ligation of
shunt vessels. IOU is especially helpful in detection of intrahepatic portosystemic shunts that
may be otherwise difficult to identify during surgery if liver parenchyma completely
encompasses the shunt vessel.
A ligation technique using direct intraoperative ultrasound guidance has been reported in
the dog. Ultrasound is also helpful in assessing decreases in vessel diameter and portal blood
flow during the ligation procedure. Ultrasound has been used to visualize catheter placement
within portosystemic shunt vessels and to localize intravascular coil placement.
IOU requires teamwork between the surgeon and the experienced ultrasonographer to be
successful.Coordinating the surgery and diagnostic imaging is important to minimize time in
the operating suite. Logistical problems of scheduling, great dependence on an experienced
ultrasonographer, and the fact that ultrasound is a highly user-dependent modality remain
the principal limiting factors of IUO.
Power Doppler
12-14
This form of color Doppler is extremely sensitive to detection of very low blood flow. Power
Doppler allows visualization of small vessels and tissue perfusion that are undetectable with
conventional Doppler interrogation. For example, renal interlobular vessels only a few dozen
microns in diameter can be imaged in fine detail. Large vessels seen with conventional color
Doppler can also be morphologically examined with power Doppler. Power Doppler can now
discriminate blood flow direction. Although it is often used alone, power Doppler is commonly
utilized with other advanced ultrasound technologies such as harmonic contrast imaging and
3-D image reconstruction.
Harmonic Imaging 15-22
Conventional B-mode ultrasounds relies on pulsed production of ultrasound waves at a given
fundamental frequency (e.g., 6 Mhz). The ultrasound waves propagate through tissues and
encounter reflectors that produce backscatter waves that return to the transducer. When
scanning in convention mode, transmitted and received ultrasound pulses are of the same
frequency. This is often referred to as imaging in the fundamental mode.
When an ultrasound wave propagates through tissue, distortion of the typical sinusoidal
waveform occurs which can result in harmonic wave formation. Harmonic frequencies are
integer multiples of the fundamental frequency, with the second harmonic occurring at twice
the fundamental frequency (e.g., 12 MHz second harmonic frequency from a 6 MHz
fundamental frequency). Reflected harmonic waves can produce a detectable signal if the
transducer is able to receive at the harmonic frequencies. As the harmonic frequency
increases (2nd, 3rd, 4th etc.) the amplitude of the wave diminishes and usually only the
second harmonic will have enough amplitude to produce a usable signal. Since the energy of
the harmonic waves is much less than the original transmitted fundamental wave, a
harmonic mode image can only be produced if the receiving transducer eliminates the
fundamental signal as a component of image formation. This method of image formation is
sometimes referred to as tissue harmonic imaging or native harmonic imaging.
In some large, difficult to image patients, harmonic imaging may reduce some of the
artifacts that result in poor image quality in fundamental mode. Image degradation in
fundamental mode is often the result of low amplitude pulses produced by reverberation,
multiple scattering, side lobe and grating lobe artifacts. Because these artifacts arise from
the fundamental transmission frequency and are of low amplitude, they contribute negligible
signal to an image derived only from the second harmonic receive frequency.
Tissue harmonic imaging technology was developed to improve image quality in
particularly large, difficult to image human patients. Because the small animal patient
population is significantly smaller and higher frequency transducers are generally used for
routine imaging, the value of tissue harmonic imaging may be less in veterinary medicine.
However, the concept of harmonics is still important in understanding other advanced or
newer ultrasound techniques such as contrast enhanced ultrasound.
Intravenous Ultrasound Contrast Media
23-31
Ultrasound contrast agents are a relatively new addition to ultrasound imaging. Almost all
ultrasound contrast agents are formulated as stable microbubble suspensions that serve as
microreflectors following intravenous injection. The microbubble particles are typically 1-7
microns in diameter; large enough to be retained in the vascular system but small enough to
pass through capillary beds without obstruction them. In most instances the circulation halflife is a few minutes after which the bubbles dissolve, rupture or are phagocytized by
reticuloendothelial cells.
Initially, contrast microbubbles were "homemade" preparations formulated by mixing
saline with a small quantity of the patient's blood then agitating and reinjecting the mixture.
Albumin in the blood served as the mechanism for stabilizing microbubbles that formed in
the agitated saline. Commercial microbubble suspensions have since been formulated to
increase the uniformity of bubble diameter, increase the stability of the bubble surface layer
or modify the core gas to prolong the circulating half-life, and to impart characteristics that
optimize imaging efficacy.
Microbubble shell formulations include albumin, galactose, phospholipids and synthetic
polymeric materials. Newer agents may combine two or more components to form a single
complex layer or they may have a double capsule layer with the two layers composed of
different materials to impart specific biological and imaging characteristics. The microbubble
core generally contains air, nitrogen or a perfluoro gas such as perfluoropentane (EchoGen,
Abbott Labs). Some bubbles are provided as a preformed suspension a while others require
the addition of a diluent to initiate bubble formation in the vial. A few are injected as a liquid
and only convert to the gas phase when warmed to body temperature.
Only a few formulations are currently approved for human use and it is likely that there
will be many modifications to the existing formulations during the next few years. Since
these agents are all being developed for human use, many include human albumin as a part
of the exposed microbubble shell. Although there are reports of uncomplicated use of these
agents in dogs and other small animals, repeated use in a veterinary patient could and most
likely would result in an adverse antigenic response.
Contrast Harmonic Imaging
After intravenous injection, contrast microbubbles distribute within the vascular space. As
the ultrasound beam impinges on the microbubbles, they produce significant backscatter
toward the transducer because of the high impedance between the bubble surface layer and
the gas core. Even though they are highly reflective, because the bubbles are small and
sparsely distributed, they alone do not account marked image enhancement.
When ultrasound waves emitted at a given fundamental frequency interact with circulating
microbubbles, they cause the microbubbles to resonate generating their own returning
ultrasound waves at both fundamental and harmonic frequencies. The returning harmonic
frequencies may account for a majority of the signal returning to the transducer. Among
other variables, the composition of the microbubble and the wavelength of the fundamental
transmit frequency can determine the strength of the returning signal which, in turn,
determines the efficacy of the formulation as a contrast agent. Generally, the microbubble
shell characteristics and the fundamental transmission frequency must be matched to
optimize the contrast response. Ultrasound microbubble contrast imaging can be utilized
using wideband transducers, transducers tuned to specific harmonic frequencies, with color
and power Doppler and with three-dimensional reconstructions.
Power Doppler Harmonic Imaging
When ultrasound waves of high energy interact with contrast microbubbles, the pressure
propagated through the bubbles can cause them to rupture. The mechanical index (MI) of a
transmitted ultrasound beam is defined as the peak pressure in the image field divided by
the center frequency of the ultrasound beam and is an indirect measure of the force acting
on the microbubbles. Power Doppler ultrasound using high MI insonation causes bubble
destruction that, in turn, generates a substantial signal. Although the signal generated may
be quite high and the contrast enhancement properties excellent, bubble destruction results
in significantly reduced contrast circulating time. In addition, this method of contrast imaging
requires a continuous replenishment of bubbles into the imaging field for enhancement to be
sustained.
As the name implies power Doppler harmonic imaging integrates both the high mechanical
index of Power Doppler and the selective reception of harmonic frequency ultrasound waves
to generate an image with high contrast signal and low background tissue noise.
Gastrointestinal Ultrasound Contrast Agents
32-35
Ultrasound evaluation of the abdomen commonly is compromised by the presence of
gastrointestinal gas. In veterinary medicine, fasting and water administration (per os or via
stomach tube) have been described and appeared to improve the evaluation of the cranial
abdomen. In humans, the use of a cellulose-based suspension as an oral gastrointestinal
contrast agent has been studied. This work led to the development of an orally administered
contrast agent (SonoRx®, Bracco Diagnostic Inc., Princeton, NJ, USA). This agent is made
off simethicone (antifoaming agent) coated cellulose which forms a suspension that exhibits
homogeneity, reflectivity and cohesiveness in the GI tract. By dispersing and displacing gas
bubbles, an optimal sonic window is created for evaluating the upper abdomen. Clinical
studies in humans showed improvement in evaluating the stomach, duodenum, pancreas,
splenic vein, and abdominal aorta. At similar doses, SonoRx® was considered more effective
than water. Recent clinical studies showed that SonoRx® is a safe and well tolerated
contrast agent with only few minor side effects such as mild diarrhea and nausea. The use of
an oral gastrointestinal contrast agent is currently being studied in normal dogs.
Endosonography
36-41
Endoscopic ultrasonography refers to the use of small high-frequency transducers
incorporated into the tip of an endoscope providing excellent near field resolution.The
ultrasonographic endoscope is introduced into a body cavity perorally, transrectally or
transvaginally. Orientation of the probe and topographic recognition are initially difficult but
can be facilitated by the use of standard endoscope positions. Endoscopic ultrasonography
has been in human medicine used to evaluate the esophagus, stomach, colon, prostate,
vagina and uterus, and the heart and great vessels. Structures adjacent to the digestive
lumen, such as the hilus of the liver, common bile duct, pancreas, spleen, kidneys, urethra,
lymph nodes, and abdominal vessels can also be optimally imaged.With some instruments,
direct endoscopic visualization and biopsy can be combined. Miniature endoscopic Doppler
probes have been designed to study the location and flow characteristics of gastrointestinal
vessels in order to identify suspected bleeding sites.Endosonography is not yet considered a
routine diagnostic method in human medicine, but provides complementary information to
that obtained with conventional endoscopy and transcutaneous ultrasonography. Although
potential clinical applications exist in veterinary medicine, high equipment cost and
anesthesia requirements may limit its use.
Laparoscopic Ultrasonography
42-47
Laparoscopic ultrasound refers to the insertion of a fixed or flexible ultrasound transducer
through a laparoscopic port to image directly intra-abdominal or intra-thoracic structures.
This technique completes the visual dimension of conventional laparoscopy by allowing the
surgeon to see into abdominal or thoracic organs and structures. While laparoscopic
ultrasound is technically challenging, a potential reduction in morbidity/mortality related to
unnecessary open surgery is the principle advantages of this procedure. Doppler
interrogation of abdominal organs and has been reported. Laparoscopic ultrasonography may
provide the surgeon information important in planning the surgical procedure or optimize the
medical management of the case. Detection of small masses not identified prior to
laparoscopy may assist with staging of oncological cases.In addition, guidance for the
aspiration or biopsy of these masses, using either fine or large gauge needles, is considered
to be a very effective and a safe procedure.
Because conventional laparoscopy in veterinary medicine is an underutilized modality,
common use of laparoscopic ultrasonography will likely not be realized in the near future.
Endovascular Ultrasonography
48
Ultrasound transducers can now be made small enough to be incorporated into a tiny
catheter and introduced intravenously to specific sites.Cardiac evaluation using endovascular
ultrasound catheters can provide exceptional images of the heart. One of the most promising
areas of clinical application is its use for quantitating the degree of arterial stenosis and for
monitoring the effects of angioplasty in peripheral and coronary arteries. . Organs adjacent
to catheterized vessels can also be seen in great detail. Three-dimensional (3-D) intraluminal
ultrasound imaging can provide important information on spatial relationships. Interventional
procedures such as stent deployment or localization of myocardial ablation catheters are
possible.
Ultrasound Biomicroscopy
49-52
Ultrasound biomicroscopy (UBM) is the use of ultra-high frequency transducers to achieve
extraordinary resolution. Currently, UBM is used primarily to study the eye.First reported in
1990, resolution in the 35-50 µm range is possible, rendering images that have nearly a
histological appearance. By comparison, modern 10MHz transducers have a resolution of
approximately 150 µm. Because tissue penetration is only 4-5 mm, biomicroscopy is limited
to the anterior chamber, where extensive work has been done evaluating the anterior
chamber angle, iris and ciliary process, and in the localization of occult foreign bodies. Ultrahigh frequency transducers will continue to develop and their use will be expanded to other
areas such as evaluation of nerves, ligaments, tendons and peripheral vasculature.
Microimaging (i.e., mice) is a new research frontier.
Extended Field of View Imaging
Now available in certain high-end ultrasound lines, extended field of view imaging provides
expansive, panoramic images. A sequence of image frames are acquired as the transducer is
moved along the long axis of the anatomy. The image is reconstructed using mathematical
algorithms into a single, static extended field of view image.
The ability to obtain a broad anatomic overview of the imaged structures allows
visualization of more information in a single still image than in conventional real-time
imaging. Clinically, this greatly enhances the evaluation of widespread disease, allows
comparison of normal and abnormal structures or tissues in the same field, improves
reproducibility in serial examinations, and facilitates measurements of large structures which
might otherwise need several images pieced together to determine size. The downside to
current extended field of view imaging technology is that only static images can be viewed.
However, technological advances should make real-time viewing available in the near future.
Three-Dimensional (3-D) Ultrasound
53-57
Two-dimensional ultrasound relies on the acquisition of images in multiple scan planes from
which to build a mental three-dimensional (3-D) image. There are circumstances however in
which it is difficult for even an experienced sonographer to develop an accurate mental 3-D
image of spatial relationships of a particular structure or pathology. Advancements in data
processing and memory capabilities, and the development of two-dimensional transducers
capable of expanding the field-of-view into the third dimension are now available. The 3-D
image is constructed using advanced computer software from volume data acquired in a
series of B-mode scan planes that are either parallel to each other or separated by regular
angular increments. The 3-D image can be manipulated in a number of ways (rotation,
zooming) to allow unprecedented examination of ultrasound images.
There are two basic types of 3-D ultrasound reconstruction. Feature based (surface
rendering) images provide a profile of the surface of structures. The main advantage of
feature based 3-D ultrasound is fast reconstruction. However, some data is permanently lost
in the reconstruction process and further data manipulation is not possible. The second type
of 3-D ultrasound is volume rendering (voxel based reconstruction ). Each pixel acquired in
the 2-D ultrasound images is placed into the proper 3-D location. All image information is
retained allowing different reconstruction at a later time or site. The disadvantages are large
data sets and complex reconstruction algorithms.
Four-dimensional ultrasound (4-D) provides functional data in three dimensions. It is the
newest form of 3-D ultrasound and is being investigated in echocardiography and
neurosonology applications.
SUMMARY
Advanced interventional techniques and the use of specialized ultrasound equipment will
undoubtedly become more common place in veterinary medicine in the future.
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SEARCH RESULT #: 3
TITLE: Gastrointestinal Ultrasonography
AUTHOR(S): John S. Mattoon
ADDRESS (URL):
http://www.vin.com/Members/Proceedings/Proceedings.plx?CID=WVC2003&PID=3199&O=VIN
OBJECTIVES
To familiarize practitioners with the appearance of the normal gastrointestinal tract.
Discuss various disease processes of the gastrointestinal tract.
Correlate radiographs with ultrasound findings when appropriate.
KEY POINTS
The principle limitation of gastrointestinal tract ultrasound is the presence of luminal
gas.
Radiographs are an especially important precursor to ultrasound of the gastrointestinal
tract.
Mural and mass lesions, obstructions, intussusceptions and foreign bodies, and
gastrointestinal motility may be reliably assessed during the ultrasound examination.
OVERVIEW
Ultrasound examination of the gastrointestinal tract is a safe, noninvasive imaging procedure
that can provide important diagnostic information despite limitations imposed by the
potential presence of intraluminal gas. Ultrasound may be performed as a first-line diagnostic
procedure when gastrointestinal disease is the presenting illness. However, in most cases
survey radiographs should precede the ultrasound examination. The most important reason
for this is to assess the amount, location and pattern of intestinal gas, which must be taken
into consideration prior to and during an ultrasound examination. Ultrasound examination of
the gastrointestinal tract may be indicated to further investigate findings made from survey
abdominal radiographs such as gastric distention or bowel dilatation. Used together,
ultrasound and radiography often provide complimentary as well as confirmatory
information.
Ultrasound can provide information on bowel wall thickness, which cannot be reliably made
from survey abdominal radiographs. Some degree of functional information can be obtained
by observation of gastric contractions and small intestinal peristalsis.
Suspicious findings on an ultrasound examination may be reevaluated during follow-up
examination or the clinician may decide to perform additional diagnostic procedures such as
a contrast gastrointestinal study, endoscopy, or an exploratory laparotomy. Ultrasound
should be completed prior to a barium upper gastrointestinal examination, as barium
effectively inhibits transmission of the ultrasound beam. Iodinated contrast material used for
contrast gastrointestinal radiography (e.g., iohexol) is not a barrier to the ultrasound beam,
however.
It must be acknowledged that despite abundant literature regarding the ultrasound
appearance of gastrointestinal tract disease, ultrasound is an unreliable method for a
definitive, histologic or cytologic diagnosis. In fact, it is often not possible to distinguish
between neoplastic and non-neoplastic conditions, let alone cell type. Biopsies or aspirates
are often needed to establish a definitive diagnosis. Ultrasound is an excellent method by
which to safely obtain samples of gastrointestinal tract pathology.
Examination Technique
Ideally, the patient should be fasted overnight. While often not practical in a busy clinical
setting, withholding food helps ensure an empty stomach and reduces the likelihood of
luminal gas, the single biggest limitation to ultrasound examination of the gastrointestinal
tract. Sedation is not usually necessary, but if needed, xylazine should be avoided because it
causes gastric stasis leading to massive gaseous distention. The ventral abdominal hair is
clipped (a number 40 blade is routinely used) and the patient typically placed in dorsal
recumbency. Five MHz, 7.5 MHz or higher frequency transducers are used, with higher
frequency transducers offering the best resolution of bowel wall layers. In small dogs or cats,
a stand off pad can be useful to image bowel loops near the abdominal wall. The need for
stand off pads has been greatly reduced with the popularity of electronic transducers that
greatly increase near field resolution compared with older mechanical sector scanners.
Transverse and longitudinal sections of the stomach, proximal duodenum, remaining small
bowel and colon should be systematically imaged. Knowledge of the anatomic relationship of
the various segments of bowel to adjacent abdominal organs is helpful. Various anatomy
textbooks provide a useful resource.
Imaging the patient in lateral recumbency from the non-recumbent surface is preferred for
routine examinations by some sonographers. The patient must then be rolled over to image
from the opposite side. Sometimes it is necessary to examine the patient while standing, or
in lateral recumbency due to patient restlessness, discomfort, or when medical concerns
preclude scanning in dorsal recumbency (e.g., vomiting and potential aspiration).
Examination with the patient standing is useful because it allows fluid within the stomach
and intestinal tract to gravitate ventrally and serve as an acoustic window. The recumbent
side of the patient may be imaged by using an examination table with a hole or notch in it,
allowing access from below (this table is invaluable for cardiac ultrasound examinations).
This technique uses gravity dependent fluid within the bowel lumen as an acoustic window.
For example, the pylorus may best be imaged from the right side, with the patient in right
lateral recumbency.1
Complete visualization of the gastric outflow tract is quite dependent on the amount of
luminal gas present. In addition, deep-chested dogs tend to be more difficult to examine
because the stomach is partially enclosed within the rib cage. Scanning the patient while
standing may be beneficial in these instances. Brachycephalic dogs are usually more
aerophagic and thus the stomach is often more difficult to evaluate.
Water can be placed into the stomach via a stomach tube to help visualize the wall and
pyloric region although this is infrequently done.2 The use of oral ultrasound contrast agents
is being investigated in human and veterinary medicine.3-9 They create an ideal environment
for gastrointestinal imaging by acting as antifoaming agents, dispersing and displacing gas
bubbles. This creates a homogeneous, relatively sonolucent environment allowing an
acoustic window to deeper portions of the gastric lumen and wall.
Stomach and Duodenum
With the transducer placed just caudal to the xiphoid process, in a sagittal body plane, the
stomach is seen caudal to the liver. In a sagittal body plane, the canine stomach will be
viewed in a transverse (cross-sectional) axis, since it is positioned essentially perpendicular
to the spine. The feline stomach is oriented with its long axis more parallel to the spine and
is imaged in a more longitudinal axis when the ultrasound beam is directed in a sagittal body
plane. It should be noted that the normal canine stomach extends well to the right of
midline, while the feline pylorus is often positioned on or near midline.10
From midline, the transducer is moved to the left, more or less along the costal arch to
include as much of the stomach within the field of view as possible. The fundus is the most
laterally located part of the stomach. From the fundus, the transducer is slowly directed back
toward midline observing the gastric body. The ventral stomach wall and the greater and
lesser curvatures are well seen in this plane. Particular attention is paid to the greater
curvature, as the left lobe of the pancreas is adjacent (caudal and dorsal) to it.
As the transducer is moved to the right of midline, the antrum and pylorus of the stomach
are imaged. Maintaining a transverse image of the antrum and pylorus requires transducer
rotation clockwise while moving cranially. The pyloric sphincter can be recognized because of
its thicker muscular wall and small, narrow lumen.
The transition from pylorus to duodenum is seen as a transition from the muscular portion
of the pylorus to the typical layered appearance of the duodenum. The transducer must be
manipulated to maintain a transverse image of the pyloric outflow tract and duodenum, now
requiring counterclockwise transducer rotation to image the descending duodenum in crosssection. The duodenum may then be followed distally along the right body wall. The right
kidney is nearby, usually medial to the descending duodenum. The right lobe of the pancreas
is located between the descending duodenum and the right kidney within the
mesoduodenum. It is sometimes possible to image the duodenum, pancreas, right kidney,
and sometimes the ascending colon (medial to the right kidney) simultaneously in a
transverse image. Once studied in cross-section, the transducer can be rotated 90° for
evaluation in a longitudinal axis of the duodenum and then moved medially to assess the
right lobe of the pancreas in long axis.
The body and fundus of the stomach should be imaged in its long axis following transverse
scans. The transducer is repositioned at the level of the xiphoid process with the ultrasound
beam now directed in a transverse body plane. It is moved cranial and caudal as well as from
right to left to complete the longitudinal gastric study in the dog. In cats, the transducer
must be oriented obliquely to image the stomach in a true longitudinal axis.
Small Intestines and Colon
The small intestines are studied by systematic transverse and sagittal imaging of the
abdominal cavity. Depth of field should be set to visualize the deepest portion of the
intestinal tract and then be decreased to best visualize the more superficially positioned
intestinal segments. The transducer is positioned in a sagittal plane and is moved back and
forth from left to right and right to left, while moving the transducer from cranial to caudal to
image the entire small intestinal tract. The transducer is then positioned in a transverse body
plane and the left, central and right portions of the abdomen are imaged in a cranial to
caudal manner. Sections of small intestine will be viewed sagittally, transversely and in
various oblique images, dependent on transducer and intestinal tract position. The intestinal
tract should be assessed for uniformity in diameter, wall thickness (3-5 mm), discrete wall
layers, luminal contents and peristalsis.
Although the colon has a layered appearance similar to the small intestine, it can routinely
be identified because of its fairly consistent location and the presence of acoustic shadow and
reverberation artifacts created by fecal material and gas. The colonic wall is usually not as
thick as small intestine (primarily due to a thinner mucosa) and is commonly more
echogenic. Because of its relatively fixed position, the colon may often be scanned in its
entirety, including the junction of small intestine with colon (cat) or cecum (dog). The
ascending colon is located medial and ventral to the right kidney and right lobe of the
pancreas. The transverse colon is caudal to the greater curvature of the stomach and left
lobe of the pancreas. Thus the colon serves as a useful landmark for pancreatic
identification. The descending colon usually be imaged along the left side of the abdomen
and followed distally as it courses dorsal to the urinary bladder and enters the pelvic inlet.
Normal colonic peristalsis is not usually observed, unlike the small intestine.
Normal Anatomy
The gastrointestinal tract as seen sonographically as alternating hyperechoic and hypoechoic
layers, whether viewed in long-axis or in cross-section. Optimally, five discrete layers of the
GI tract can be seen, corresponding to the luminal/ mucosal interface, mucosa, submucosa,
muscular layer, and the serosa.11-13 When empty, the intestinal lumen is defined by a thin
layer of hyperechoic mucus separating the near and far field wall. The mucosal layer is
hypoechoic, nearly anechoic in many instances and is the thickest layer of intestinal wall. The
mucosal layer is separated from the thin hypo- to anechoic muscle layer by a thin
hyperechoic submucosal layer. The outer surface of the intestine is defined by a thin
hyperechoic serosal layer. These layers can be easily and routinely seen with high quality
equipment. With older, lower frequency transducers and less resolving equipment, however,
discrete bowel wall layers are not always seen. In this case, the bowel wall appears as three
layers, an echogenic mucosal layer/luminal interface, a hypoechoic mucosal/muscle layer
and the outer hyperechoic serosa.
The appearance of the stomach is quite variable depending on degree of distension and
luminal contents. In the near field, the stomach wall is seen as a thin, layered, curvilinear
structure caudal and directly adjacent to the liver when moderately to fully distended. When
compared to the small intestine, the echogenic submucosal layer is often noted to be thicker
and more prominent, especially in cats. The stomach wall measures 3-5 mm in thickness in
the dog (perhaps thicker in large dogs), and is similar in the cat when distended.14 Gastric
wall invaginations into the lumen represent rugal folds, easily seen when the stomach is not
fully distended. These disappear when the stomach is fully distended. When the stomach is
empty or nearly so, the rugal folds and interrugal spaces of the fundus and body create a
radial or "spoke wheel" pattern when the stomach is viewed in cross section, especially in
cats. When stomach gas is present, only the near-field portion of the stomach can be
imaged. This is because gas rises to the ventral portion of the stomach (near field) creating
strong acoustic shadows and reverberation artifacts that obscure the more dorsal aspect of
the stomach. This is a serious limitation during ultrasound evaluation of the stomach and
positional maneuvers of the patient and transducer are necessary to study the entire
stomach. Conversely, when the stomach is fluid-filled or contains ingesta that allows
transmission of the ultrasound beam, the majority, if not all of the stomach can be
evaluated. The stomach wall should be evaluated for thickness and continuity of layers,
assessed for evidence of peristalsis, and gastric contents should be noted.
An empty stomach can have a remarkably different appearance from one that is distended.
The wall will be thicker and more echogenic than expected, and the appearance may be
mistaken for diffuse pathology. The stomach should be reassessed in a more distended state
to confirm normalcy or pathology.
The small intestinal wall is only several millimeters thick, measured between the outer
echogenic serosal surface and the mucosal-luminal interface. In dogs, the intestinal wall is
reported to be 2-3 mm thick,13 while in cats an average of 2.1 mm thick has been
reported.14 Our observation is that the duodenum may be thicker than the jejunum,
sometimes up to 5 mm thick in a normal dog, while the jejunum is often a millimeter or two
thinner. Careful observation of the common bile duct entering the duodenum may allow
visualization of the duodenal papilla. The feline duodenal papilla has been reported to
average 2.9-5.5 mm thick.15 Flat indentations sometimes seen in the duodenal mucosa along
the antimesenteric margin likely represent Peyer's patches. Duodenal ulcers may have a
similar appearance, although with ulceration the intestinal wall is expected to be thickened.
In some cases, the ileum can be identified by its location in the right mid-to cranial abdomen
and its relationship with the ascending colon and cecum. The ileum may also be identified by
a thicker echogenic and irregular submucosal layer.2 The canine colon is usually 2-3 mm; the
cat colon has been reported to be an average of 1.7 mm thick.14
In most cases, the appearance of the small intestine is dictated by the type and amount of
luminal content. The small intestine may contain anechoic fluid or have a quite echogenic
appearance due to an admixture of gas and fluid. When empty, a "mucous pattern" is
present, seen as an echogenic line in the center of the intestinal segment (lumen) separating
the hypoechoic muscular layers. Gas is hyperechoic, and large amounts will hinder the
examination of the far (distal) bowel wall and the abdominal organs. Gas-filled small
intestinal loops will create reverberation artifacts and sometimes acoustic shadowing.
Normal values for the diameter of small intestinal segments have not been published in
the veterinary ultrasound literature. Radiographic assessment of the small intestine is
probably more beneficial than ultrasound in cases of suspected bowel distention, since
established parameters do exist. 16 Observation of both distended and empty loops of small
intestine during the ultrasound examination should prompt the sonographer to consider
obstructive bowel disease, or other segmental intestinal tract pathology such as
inflammation or neoplasia.
Contractility of the stomach and bowel should be assessed. There are typically 4-5
peristaltic contractions per minute of the stomach and duodenum.2,13 The jejunum contracts
1-3 times per minute.2,13 The colon is rarely seen to contract during an ultrasound
examination. Therefore, peristalsis (or lack of peristalsis) can be used to help differentiate
small bowel from colon.
The colon is usually gas filled and imaged as a hyperechoic curvilinear structure in
transverse section and an echogenic linear structure longitudinally. The presence of gas
creates an acoustic shadow, as the ultrasound beam is reflected and therefore structures
deep to the colon are not imaged. Familiarity with the appearance of the descending colon is
important. Due to its close proximity to the urinary bladder, the colon can mimic cystic
calculi or lesions within the urinary bladder wall.17,18
Diseases of the Stomach
Dilatation
A fluid and/or gas dilated stomach may secondary to atony or reduced peristalsis, a
mechanical outflow obstruction, or be a normal finding in some cases. Relevance of gastric
dilation must be correlated with clinical, radiographic and other pertinent information.
Positional studies may be indicated to allow observation of the entire stomach.
Fluid gastric distention is recognized by a large fluid-filled stomach. The dilated stomach
may dominate the field of view and may limit visualization of the liver. The stomach wall may
appear to be thinner than normal and rugal folds will be absent. The gastric fluid may be
anechoic but is often heterogeneous and echogenic. It may be sonolucent, allowing
visualization of the far field gastric wall as well potential foreign material within the lumen.
A gas dilated stomach is immediately recognized by a large highly echogenic interface in
the near field, with acoustic shadowing and/or reverberation artifact. Only the near field wall
can be evaluated in this instance.
The presence of gastric dilatation should prompt the sonographer to search for an etiology.
Gastric dysfunction, either primary or secondary, may be suspected if gastric contractions
are reduced in number or intensity. Gastric motility may be reduced or absent in cases of
chronic outflow obstructions as the stomach tires, however. Therefore, observation of
reduced or absent gastric peristalsis does not necessarily exclude chronic foreign body
ingestion or pyloric stenosis as the primary cause of gastric dilatation. The gastric fluid
should be carefully evaluated for the presence of a discrete foreign body that can be
detected in some cases. The pyloric outflow tract should be evaluated for evidence of wall
thickening, and peristalsis with propagation of gastric contents through the pyloric canal into
the duodenum should be assessed. Focal as well as diffuse gastric diseases are often
associated with dilation of the stomach.
Gastric Foreign Bodies
Many common foreign bodies are identified by intense acoustic shadowing, such as rubber
balls, plastic toys and rocks.2,19,20 Rubber balls create a very hyperechoic curvilinear line with
acoustic shadowing. The balls may be intact or only pieces of the ball may be seen.
Fortunately, these balls (racquet balls, tennis balls, handballs) are often radiopaque and can
be identified on survey radiography. Sometime the balls will be punctured during ingestion
and contain echogenic fluid. Other foreign bodies may be diagnosed by a characteristic
appearance, such a cob of corn.
Trichobezoars (hair balls) are a fairly common gastric foreign body in cats, dogs and
rabbits. They do not have a distinct appearance but are of mixed echogenicity and often
create some degree of acoustic shadowing, They may be slender and cylindrical in shape
(especially in cats), but may occupy the entire gastric lumen (more common in dogs and
rabbits). In the latter instance, they may even be mistaken for a huge gastric mass. As
mentioned, a key finding in many cases of gastric foreign material is a markedly distended
stomach. Contractions may be increased, but if long-standing, the stomach may have
reduced motility.
There are obviously a plethora of potential gastric foreign bodies, and of course material
imaged in the stomach may be normal ingesta. An important question to ask is how long has
it been since the last meal? Normal dogs and cats will completely empty their stomachs in 810 hours. Suspicious findings may be reevaluated later in the day or following day, or
followed up with other diagnostic procedures.
Pyloric Stenosis
Pyloric stenosis causing a gastric outflow obstruction in most instances causes gastric
dilatation, as described above. Vigorous contractions may be observed without propagation
of gastric contents through the pylorus and into the duodenum, although as mentioned,
gastric atony can occur over time as the stomach tires.
Chronic hypertrophic pyloric gastropathy has been described as concentric hypoechoic
thickening of the pylorus.19,21,22 Congenital pyloric stenosis may look similar. 2 In one case of
congenital pyloric stenosis the wall was thickened and relatively hyperechoic with poor
visualization of wall layers. Focal mass lesions of the pyloric outflow tract or irregular wall
thickening may represent neoplasia or inflammatory lesions. Mass lesions, either malignant
or benign (e.g., benign polyps) may be seen projecting into the pyloric lumen.
The ultrasound findings discussed above may be confirmed with a barium or iodinated
contrast material upper gastrointestinal examination. In many instances, the diagnosis is
clearly evident when information from both imaging modalities is considered. Endoscopy is
an effective diagnostic tool to further evaluate the gastric outflow tract.
Diffuse Gastric Wall Thickening
Diffuse thickening of the stomach is often indicative of nonmalignant disease such as
parvovirus infection, lymphocytic/plasmacytic or eosinophilic infiltrate, uremic induced
gastritis or gastritis for other causes such as dietary indiscretion. Lymphosarcoma and mast
cell disease are neoplastic diseases to consider when the stomach wall is diffusely thickened.
Regional lymphadenopathy can be indicative of the severity of gastric disease. More severe
lymphadenopathy is usually associated with neoplasia.
The stomach wall will be diffusely thickened, sometimes minimally (e.g., mild-moderate
inflammation, lymphocytic/plasmacytic or eosinophilic infiltrate) or may be quite severe (1-2
cm or more) with parvovirus infection or uremic gastritis. Minimal thickening usually
preserves the normal layered appearance of the stomach wall. Severe thickening usually
obliterates these layers. The rugal folds also become thickened. Mineralization of the gastric
mucosa that may occur with chronic renal disease has been described sonographically as a
hyperechoic line at the mucosal-luminal interface, usually without acoustic shadowing.23
Lymphosarcoma is the most common diffuse neoplastic disease of the stomach. The gastric
wall becomes uniformly hypoechoic and thickened with loss of normal layers. 24,25 As noted
below, gastriclymphosarcoma can also be a focal disease.
Differentiation between these various diseases requires integration of the signalment,
clinical signs, lab data, radiology interpretation, etc. Ultimately, a fine needle aspirate or
biopsy may be necessary to make a final diagnosis.
Focal or Mass Lesions of the Gastric Wall
In most instances, focal gastric lesions represent a neoplastic process. Mass lesions of the
stomach are identified as focal areas of gastric wall thickening. These lesions usually
obliterate the normal layered anatomy of the stomach wall and can become quite large. Their
appearance may be homogeneous or very complex. Gastric dilation is often present.
Gastric carcinomas, lymphosarcoma, leiomyoma and leiomyosarcoma are examples of
focal gastric neoplasia and have been described in the veterinary ultrasound literature.24-32
Gastric leiomyomas are characterized by small, sessile, homogeneous focal masses in older
animals.2,32 They may be a source of intermittent vomiting or be an incidental finding.
Gastric leiomyosarcomas usually are seen as large, complex masses.2,32 Internally, these
masses may have areas of necrosis and hemorrhage, accounting for their complex
ultrasound appearance. Hematemesis is often a presenting sign.
Lymphosarcoma usually presents as an area of uniform hypoechoic gastric wall thickening.
Wall thickness has been reported to be 8-25 mm thick in feline lymphosarcoma. Loss of
gastric wall motility may be observed. Regional lymph node enlargement is also a common
finding in both dogs and cats.
Gastric carcinoma is less common than the gastric neoplasms previously discussed.2,31 One
ultrasound feature of this type of tumor is what has been termed "pseudolayering",
described as a moderately echogenic zone surrounded by outer and inner poorly echogenic
lines. This appearance was felt correlated with the unevenly layered tumor distribution seen
histologically. Lymph node enlargement is commonly seen, indicative of metastasis.
These lesions are usually readily biopsied with ultrasonic guidance. As mentioned
previously, misdiagnosis of a gastric mass lesion or diffuse thickening can occur when the
stomach is assessed when empty.33
Ulcers
Ultrasound has been used to diagnose gastric ulcers.2, 34 A hyperechoic area representing gas
accumulation within a focal area of gastric wall is seen. There is usually a noticeable
depression in the mucosal surface. Gastric fluid distension is often present and regional
reduction in gastric wall peristalsis may be observed. The area surrounding the gastric ulcer
should be studied for evidence of focal perigastric fluid accumulation and hyperechoic
mesentery, evidence of ulcer perforation and focal peritonitis. Ultrasound may be used to
assess gastric ulcer healing, with reduction of wall thickness and eventual reestablishment of
normal wall layering. Care must be taken to differentiate trapped air bubbles within gastric
rugae from true gastric ulcers.
Gastropexy Adhesions
Ultrasound has been used to evaluate surgical gastropexy.35,36 The site of gastropexy is
easily identified by observing the stomach wall move in unison with the abdominal wall
during respiration, indicative of an adhesion. The stomach wall is thickened at the site of
gastropexy and wall layers are lost.
Diseases of the Small Intestine
Dilatation
Dilation of the small intestine may be caused by diffuse infiltrative disease (infectious or
dietary enteritis) or obstruction from mass lesions, luminal foreign bodies or adhesions.
Small intestinal dilatation is seen as distension of the small intestine with fluid, gas, or a
combination of both. As mentioned, specific ultrasound parameters for bowel diameter are
not established, and in questionable cases, radiographs may be helpful to verify dilation of
bowel. Bowel wall thickness may be normal, appear thinner than expected, or be thickened,
depending on etiology.
Mechanical obstruction results from the presence of foreign material, mass lesions,
strictures or intussusceptions. Differentiating obstructive from non-obstructive small bowel
dilatation is possible when dilated and normal intestinal segments are both seen or when the
site or cause of obstruction is identified. Depending on the location of an obstructive lesion,
the dilation may be segmental (proximal obstruction), or involve the entire small bowel if the
obstruction is at the level of the terminal small intestine or ileocecocolic region. In proximal
small intestinal obstructions, both dilated (proximal to obstruction) and normal (distal to
obstruction) segments of intestine will be present. With mechanical obstruction, the degree
of bowel distention is dependent on whether or not the obstruction is partial or complete and
on the duration of the obstruction. When segmental small intestinal distension is recognized,
an attempt should be made to define the site of obstruction and differentiate a foreign body
from a mass lesion or intussusception (see below).
When the entire small intestine is dilated, it is important to differentiate diffuse intestinal
disease (e.g., parvovirus infection) from a distal small bowel obstruction. Localization of the
colon and following it to the level of the cecum is helpful, as an obstruction in the colon (e.g.,
colocolic intussusception) or terminal small bowel (e.g., ileocolic intussusception or ileal
neoplasia) can mimic diffuse bowel dilation from non-obstructive causes. A small portion of
normal, non-distended small bowel (distal to the obstruction) is a clue that a distal
obstruction is present. Of course, an UGI may be necessary to absolutely rule-out a distal
bowel obstruction.
Peristaltic activity varies when small bowel dilation is present, from complete absence to
hypermotility. Hypermotility is probably more common with acute mechanical obstructions
from foreign material and infectious or dietary induced enteritis than with chronic partial
obstructions.
Diffuse or Segmental Bowel Thickening
Diffuse thickening of the small intestine is seen in cases of lymphocytic/plasmacytic enteritis,
corona or parvovirus, dietary indiscretion, and intestinal lymphosarcoma. 24,25,37,38 We have
documented one case of mild bowel wall thickening and partial loss of bowel layers in acute
hemorrhagic gastroenteritis (HGE), which resolved in several days. Thickening is mild to
moderate (e.g., 1 or 2 mm thicker than normal) in most cases. Careful observation of
various intestinal segments may reveal that some areas are slightly thicker than others. It
must be noted that mild lymphocytic/plasmacytic enteritis may not be detectable
sonographically, yet be confirmed on histologically. The normal layered appearance of the
bowel wall will usually be preserved and motility will be normal. Assessment of regional
lymph nodes is helpful, as lack of enlargement or mild lymphadenopathy is more of an
indicator of inflammatory bowel disease, whereas marked lymphadenopathy is more
indicative of neoplasia.
Intestinal lymphosarcoma is usually presents with more severe, advanced bowel pathology
than that described above. The bowel wall is usually quite thick (5-25 mm), hypoechoic, and
layers are lost.24,25 Transmural circumferential thickening (symmetric or asymmetric) is the
most common form of intestinal lymphosarcoma in dogs and cats. Other forms of intestinal
lymphosarcoma include transmural-bulky, in which larger, complex lesions are seen; a
transmural-nodular form, characterized by nodular lesions within the wall and metastasis to
regional lymph nodes; the transmural-segmental form, in which only a segment of bowel is
affected; and a mucosal infiltrative pattern, with subtle thickening and mottled echogenicity
of the mucosa and preservation of intestinal layers. The mucosa infiltrative pattern of
lymphosarcoma is difficult to identify in some instances, and if seen, may mimic nonneoplastic enteric disease. As mentioned, the absence or presence and severity of
lymphadenopathy can be used as an indicator of benign or malignant disease.
Segmental small bowel thickening occurs most commonly with intestinal lymphosarcoma,
adenocarcinoma, or undifferentiated sarcomas. Duodenitis as a sequel to pancreatitis,
hypergastrinemia, or portosystemic shunts (duodenal ulceration) is a common form of
segmental enteritis. The bowel may have a corrugated appearance. In some regions
infectious mycoses can produce segmental or diffuse bowel wall thickening. Segmental
intestinal thickening may also be present proximal to and at the site of an intestinal
obstruction (inflammation and/or muscular hyperplasia).
Ultrasound guided aspirates or core tissue biopsies of bowel wall thickening are routinely
performed. Mesenteric lymph nodes may also be sampled.
Mass Lesions
Primary intestinal tumors (other than lymphoma) are often imaged as mass lesions by the
time the patient is presented for clinical signs of bowel neoplasia. Common intestinal masses
include leiomyosarcoma and carcinoma.2,32,39 One notable exception is feline adenocarcinoma
of the ileocecocolic region, which may cause clinical signs while quite small, and palpate as a
small mass lesion that can be detected sonographically (Fig. 35). Palpable mass lesions may
be imaged while the sonographer or an assistant holds the lesion for positive identification.
Intestinal mass lesions can usually be readily identified if clinical signs are present. The
mass may be quite variable in appearance. Focal, concentric thickening of the bowel may be
present, or the thickening may be eccentric in location, the latter a common finding with
leiomyosarcoma.32 Larger lesions are usually complex, with mixed echogenicity. While it is
not difficult to identify large mass lesions, it may be more of a challenge to associate the
mass with the bowel. Key points are the presence of gas within the mass and dilatation of
the obstructed bowel proximal to the lesion. An attempt to image the dilated proximal bowel
as it enters the mass and/or normal bowel exiting the mass is critical. Metastasis to regional
lymph nodes and occasionally to the liver or other organs can occur40
Intussusception
An intussusceptions is a telescoping of bowel within itself. It can occur along any portion of
the gastrointestinal tract, named according to the section of bowel involved.
Intussusceptions occur within the jejunum, ileocolic or ileocecal junctions, or within the colon
(colocolic). Rarely do they involve the stomach or duodenum. They often occur in puppies
and kittens secondary to primary intestinal disease such as enteritis from intestinal
parasites, bacterial or viral infections. As discussed above, intussusceptions are one cause of
mechanical bowel obstruction.
Intussusceptions have a characteristic ultrasound appearance that in most cases allows a
definitive diagnosis to be made with confidence. 2,19,41 Cross-sectional views of an
intussusception show a multilayered, concentric, target-like lesion due to the multiple walls
and wall layers that comprise the mass. On a sagittal image, multiple layers of bowel wall
are seen "stacked" on one another. There may be varying amounts of luminal fluid present
within the intussusceptum (the inner or invaginated segment) or intussuscipiens (the outer
or receiving portion of bowel). Hyperechoic mesentery is often incorporated into the
intussuscipiens as it accompanies the intussusceptum. In some instances the concentric or
layered appearance is distorted and not as easily recognized because of inflammation and
edema. Distended bowel proximal to the obstruction will be present. Indeed, distended small
intestine maybe the first ultrasound abnormality identified in cases of intussusception.
Linear Foreign Bodies
Linear foreign bodies such as string, cellophane, pieces of cloth, pantyhose, etc., may be
diagnosed with ultrasonography by recognizing the characteristic plicated appearance of the
small bowel.2,19 The affected bowel may be fluid and gas dilated or just appear thickened and
bunched. The foreign body may be identified as an echogenic luminal structure. Foreign
material may be noted in the stomach (e.g., cellophane wrap from ham or roast, with the
thick twine descending into the small bowel). Peritonitis from bowel wall leakage in long
standing cases is suggested if free peritoneal fluid is detected, the mesentery is hyperechoic
with poor sonographic detail, and lymphadenopathy is present. Ultrasound-guided
abdominocentesis would provide a diagnosis.
Disease of the Cecum and Colon
Perhaps the most common abnormality found during examination of the colon is fluid
distention, indicative of diarrhea. The fluid is usually quite echogenic and swirling motion can
be observed, indicative of low viscosity. Diseases of the cecum and colon are usually best
diagnosed by direct visualization during endoscopy. However, ultrasound examination of the
colon and to a lesser extent the cecum can be rewarding in cases of colitis, focal wall
thickening, mass lesions or segmental disease. Colitis usually manifests itself by wall
thickening. Wall layers may be preserved in mild cases or be absent in more severe disease.
Alteration of wall layer echogenicity is often present. Focal colonic wall lesions may not be
immediately obvious during the ultrasound examination; the colon must be specifically and
carefully imaged.
SUMMARY
Ultrasound is a reliable imaging modality for assessment of the gastrointestinal tract.
Some gastrointestinal diseases allow a definitive ultrasound diagnosis to be made, such
as foreign bodies and intussusceptions.
While it is uncommon that the sonographic appearance of focal or diffuse
gastrointestinal disease is specific enough for a definitive diagnosis to be made,
ultrasound can be used to safely guide needle aspirates or obtain tissue core biopsies.
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SEARCH RESULT #: 4
TITLE: Hepatic Ultrasonography
AUTHOR(S): George Henry, DVM, DACVR
ADDRESS (URL):
http://www.vin.com/Members/Proceedings/Proceedings.plx?CID=WVC2003&PID=3204&O=VIN
OBJECTIVES
Review the normal sonographic structure of the liver in the dog and cat.
Relate abnormal sonographic findings to specific hepatic diseases.
Discuss what diagnostic information can and cannot be obtained from ultrasound.
GENERAL KEY POINTS
Ultrasonography provides more specific information than radiography of the liver.
Normal ultrasound findings do not rule out liver disease.
Abnormal ultrasound findings may not be pathognomonic.
Ultrasound guided biopsy or fine needle aspirates are usually required for specific
disease diagnosis.
Doppler ultrasonography is becoming more useful in examination of the liver.
KEY CLINICAL DIAGNOSTIC POINTS
Examine the entire liver in a systematic approach
Sub-xiphoid, midline, sagittal -> sweep beam from midline to right and left lateral
aspects.
Move transducer along costal arch in sagittal/parasagittal plane.
Sub-xiphoid, midline, transverse -> sweep beam ventral to dorsal.
Right and Left 10th to 12th intercostal spaces.
Abnormal findings should be viewed in at least two orthogonal planes!!!
Helps characterize extent and morphology of lesion.
Prevents erroneous identification of artifact as a lesion!
Liver bounded cranially by the diaphragm.
Do not mistake mirror image artifact for diaphragmatic hernia!
Liver bounded ventrally by falciform fat.
Do not mistake fat for liver especially in fat cats.
In some cases the falciform fat may be the same echogenicity but always more
course echo texture than liver.
Liver bounded caudally by right kidney on the right, stomach centrally, and spleen on
the left.
The gall bladder and portal vein divide the left liver and right liver.
Identification of individual left and right liver lobes difficult unless peritoneal effusion
is present.
Branches of the hepatic artery and bile ducts are not usually seen in the liver
parenchyma.
Location of hepatic arteries can be identified with Doppler ultrasonography.
Hepatic veins appear as branching anechoic tubular structures with no observable wall.
Portal veins appear to have echogenic walls due to surrounding fibrofatty tissue.
Identifying a greatly enlarged or very small liver is relatively easy; however, definitive
determination of liver size is not practical.
Distance between diaphragm and stomach can help gage liver size.
Increased extension of liver along the ventral abdomen indicates enlargement.
Rounding of liver lobes indicates enlargement.
Poor visualization of liver without stomach gas indicates a small liver.
Abdominal radiographs are helpful in evaluation of liver size.
Liver has uniform medium echotexture interrupted only by hepatic and portal veins.
Liver parenchyma should be equal to or slightly more echogenic than the cortex of the
right kidney.
Liver parenchyma should be less echogenic than the spleen.
The gall bladder is usually a round or oval shaped structure of variable size containing
anechoic bile.
A small amount of echogenic debris may be seen within the gall bladder in normal dogs
and cats.
The gall bladder wall is usually not observed or seen as a thin echoic line in normal dogs
and cats.
KEY ETIOLOGIC AND PATHOPHYSIOLOGIC POINTS
Focal parenchymal disease
Cysts
Usually not clinical significant unless numerous cysts are present (polycystic
disease).
More commonly seen in the cat.
Usually have thin, well-defined walls with anechoic contents causing distal acoustic
enhancement.
Cyst with thicker, irregular walls, and internal echoes is more likely to be clinical
significant.
Percutaneous aspiration of the cyst with ultrasound guidance with cytology and
bacteriological culture may be indicated.
Serial examinations also help identify probable benign lesions that do not change
over time.
Hematoma
Internal appearance changes with age of lesion.
Acute parenchymal hemorrhage is echogenic.
Later, the hematoma can appear anechoic or hypoechoic.
The margins are usually irregular and poorly defined.
A variable pattern is seen as the hematoma ages over weeks or months.
Variable appearance of hematomas can be similar to necrosis, abscess or
neoplasia.
Abscess
Not common in dogs and cats.
Most common organism in the dog is E. coli.
Often associated with bacterial disease of other organs or organ systems.
Unfortunately, abscesses can appear anechoic, hypoechoic, hyperechoic and
mixed depending on the age of the abscess.
Some abscesses will have observable walls.
Ultrasound guided aspiration with cytology and culture provide diagnosis in most
cases.
Fine needle aspirates of abscess have not been shown to increase the severity or
spread the infection.
Serial ultrasound exams useful for evaluation of treatment.
Hepatic necrosis
Hypoechoic or mixed focal or multifocal lesions secondary to chemical, toxic,
infectious, or immune-mediated insults.
Biopsy is usually required to differentiate from hepatitis, multifocal abscesses or
neoplasia.
Nodular hyperplasia
Usually hypoechoic nodules in liver but can be isoechoic, mildly hyperechoic or
mixed.
Sonographic appearance has not been reported in the cat.
Biopsy required to rule out neoplasia.
May occur in up to 70% of older dogs.
Neoplasia
Ultrasound serves primarily as a means of identifying lesions in the liver and
assisting in obtaining tissue samples and monitoring the progress of the disease.
Metastatic neoplasia is more common than primary hepatic neoplasia in the dog
and cat.
Focal and diffuse lesions are possible with neoplasia in the liver.
Experience has shown that tumor type cannot be determined by ultrasonographic
appearance alone.
Tumors of the same cell type can have a variety of appearances even within the
same animal.
Aspirates and biopsy are necessary for definitive diagnosis of these liver lesions.
With the above statements in mind, some general guidelines may be helpful in
predicting tumor type.
Hypoechoic focal or multifocal parenchymal lesion in a young animal with
peripheral and/or abdominal lymphadenopathy suggests lymphosarcoma.
Solitary hyperechoic lesions are often hepatocellular carcinoma.
Focal or multifocal hyperechoic or mixed masses are usually carcinoma.
Hypoechoic focal or multifocal lesions do not correlate with any particular
neoplasia.
Focal or multifocal thin-walled cystic lesions in older cats are usually benign
biliary cystadenomas.
Diffuse Liver Disease
Ultrasonographic recognition of diffuse liver diseases is more challenging than
observing focal or multifocal disease.
Echogenicity of the liver must be compared to that of the kidneys and spleen at a
similar depth and gain settings.
Subjective evaluation of liver echogenicity is prone to error because of variability of
ultrasound equipment and experience of the sonographer with significant variability
between observers.
Decreased echogenicity
Reported with lymphoma, leukemia, and amyloidosis.
Hepatitis may show as normal or decreased echogenicity of the liver.
Passive congestion may result in decreased echogenicity.
Increased echogenicity
Reported with fatty infiltration, steroid hepatopathy, chronic hepatitis, cirrhosis,
and less commonly lymphosarcoma.
Mixed echogenicity
Diffuse neoplasia may present as an ill-defined mixed pattern.
Canine superficial necrolytic dermatitis (hepatocutaneous syndrome) causes a
unique honeycomb or Swiss-cheese-like ultrasound pattern within the liver with
characteristic skin lesions in older dogs.
Cirrhosis may look similar but is commonly characterized by a normal to small
irregular liver and does not have characteristic skin lesions.
Biliary Disease
Biliary sediment and calculi
Variable amounts of echoic sediment may be seen in normal non-fasting dogs and
cats.
Significance of sediment is questionable and does not correlate well with
hepatobiliary disease.
Calculi are rare and not necessarily associated with clinical signs.
Calculi should fall to the dependent portion of the gall bladder with different
positional views.
Gallbladder wall thickening
Wall thickening can be seen with acute or chronic hepatitis, cholecystitis or
cholangiohepatitis.
Right-sided heart failure, hypoalbuminemia, and neoplasia can also cause wall
thickening.
Acute cholecystitis
Acute inflammatory disease usually causes thickening due to edema causing a
hypoechoic layer between two echogenic layers.
Pain may be induced during ultrasound of the gallbladder region (Murphy sign).
Emphysematous cholecystitis is an acute form of the disease that results in wall
thickening, increased echogenicity of the wall or lumen with "dirty" shadowing
caused by gas produced by bacteria.
Chronic cholecystitis
Usually presents with a less acute history and physical findings.
Thickened gallbladder wall usually seen due to inflammation and fibrosis.
Polyps may occur with chronic inflammation.
Gallbladder mucoceles
Characterized by gallbladder distention, thickening of the wall, and biliary sludge
or intraluminal masses that do not move.
There may be intraluminal echogenic membranes or striations often described as a
stellate pattern.
It is not always possible to differentiate thick biliary sludge from the wall of the
gallbladder making the wall appear thicker in some cases.
Fluid or edema may be seen as a hypoechoic ring around the gallbladder that may
indicate possible early rupture of the gallbladder.
Biliary obstruction may be present.
Exploratory surgery is usually necessary to treat these once the mucocele forms.
Neoplasia of the gallbladder
Tumors of the gallbladder are rare and may not be easily differentiated from
surrounding hepatic neoplasia.
KEY THERAPEUTIC POINTS
Ultrasound serves primarily as a means of identifying lesions in the liver and assisting in
obtaining tissue samples and monitoring the progress of the disease.
SUMMARY
Ultrasonography of the liver in dogs and cats is a useful non-invasive method of further
identification of suspected hepatic disease. Knowledge of the various sonographic
appearances of diseases can assist in determining the likely prognosis and need for further
diagnostic tests. Ultrasound serves primarily as a means of identifying lesions in the liver and
assisting in obtaining tissue samples and monitoring the progress of the disease.
References
1. Nyland TG, Mattoon JS. Small Animal Diagnostic Ultrasound 2nd Ed. Philadelphia; WB Saunders
Co, 2002
SEARCH RESULT #: 5
TITLE: Lower Urinary Ultrasonography
AUTHOR(S): John S. Mattoon
ADDRESS (URL):
http://www.vin.com/Members/Proceedings/Proceedings.plx?CID=WVC2003&PID=3198&O=VIN
OBJECTIVES
Understand and recognize ultrasound artifacts that may mimic disease when imaging
the urinary bladder.
Discuss the ultrasound appearance of various diseases of the ureters, urinary bladder,
and urethra.
Correlate the radiographic appearance to the ultrasound appearance of disease when
applicable.
KEY POINTS
Ultrasound is very useful in assessment of the lower urinary system, often a first-line
imaging modality.
Common diseases of the lower urinary system such as calculi, cystitis, and neoplasia are
usually readily diagnosed.
Knowledge of ultrasound artifacts that may occur when imaging the urinary bladder is
important to avoid potential mis-diagnoses.
OVERVIEW
Ultrasound is a common diagnostic procedure for evaluation of the lower urinary tract.1-4 It is
often used as a first-line imaging modality due to its noninvasive nature and relative ease of
the procedure. Calculi, blood clots, bladder wall thickness, mass lesions, diverticula,
ureteroceles and even ectopic ureters can be evaluated. However, as with most areas of the
body, ultrasound assessment of the ureters, bladder, and urethra may be complemented
with radiographic procedures.
Examination Technique and Normal Anatomy
High frequency transducers are best suited for examining the urinary bladder and urethra,
even in large dogs. A linear array transducer is optimal for visualization of near field
structures and is therefore quite useful for imaging of the ventral bladder wall, trigone, and
urethra. A standoff pad may be helpful in obtaining a good image in smaller patients or when
the bladder is poorly distended if a linear array transducer is not available. Examination of
the urinary bladder is optimized when moderately distended with urine. If not distended,
artifactual bladder wall thickening will be present which can mimic disease. If the bladder is
not distended at the time of the examination, a diuretic such as furosemide may be
administered intravenously at very low doses to achieve rapid filling. Occasionally, the
bladder may be infused with sterile saline, or the examination performed later in the day.
Attention to output, overall gain and time-gain compensation controls is quite important
during urinary bladder evaluation. Because the bladder is a fluid-filled structure, the
ultrasound beam is only minimally attenuated as it passes through the bladder. This results
in acoustic enhancement of the far field bladder wall (the dorsal bladder wall in conventional
dorsal recumbency). Unless the output and gain controls are reduced substantially from
levels used to scan other areas of the abdomen, the bladder wall will be poorly resolved and
much too echogenic. Another consideration is the ventral bladder wall. Because the ventral
bladder wall is very superficial to the surface of the skin, near-field gain must be drastically
reduced to image it at all. It may be necessary to use a standoff pad to properly visualize the
ventral bladder wall in small patients. Near-field resolution is more of a problem with
mechanical sector transducers due to inherent near-field "ring-down artifact and the narrow
"wedge-shaped" image at the top of the monitor. Newer electronic transducers have much
better near-field resolution, with linear array configurations offering the very best in
superficial image detail. Finally, scanning with too much output and/or gain will create false
echoes within the urine pool, causing normally anechoic urine to become echogenic. Failure
to properly adjust the ultrasound machine for optimal visualization of the urinary bladder will
undoubtedly result in non-visualization of ventral and dorsal bladder wall lesions, small cystic
calculi, and misinterpretation of the appearance of the urine.
The bladder should be scanned in sagittal and transverse planes, typically from the ventral
body wall with the patient in dorsal recumbency. Lateral approaches or scanning the
standing patient are also helpful. The bladder wall is normally very thin (1-2 mm) and
imaged as a bright, echogenic curvilinear structure.5 Under optimum conditions, the layers of
the bladder wall can be resolved, with the hypoechoic muscular layers sandwiched between
echogenic mucosal and serosal surfaces. Normal dog and cat urine is usually anechoic. Tiny
suspended echoes can be seen in cat urine and may be normal lipid droplets.
The trigone region of the urinary bladder should be routinely imaged, as it is a common
site for neoplasia and is the location of the vesicoureteral junction. Distention of the bladder
is helpful. Also, tipping the examination table such that the pelvis is lowered may help
distend the trigone and proximal urethra. Two small soft tissue projections from the dorsal
surface of the trigone may occasionally be seen, representing the intramural portion of the
ureters as they enter the bladder, the vesicoureteral junction.6,7 Periodic "steaming" of bright
echoes of urine from the ureters into the bladder lumen may be observed in some patients
("jet effect"), and can also be observed with color Doppler examination.8,9 This observation is
quite useful when searching for ectopic ureters.
The proximal urethra should be routinely evaluated when possible. It is seen as a small,
thin walled tubular structure that typically fades from view as it extends into the pelvic inlet.
The proximal portion can be imaged in most dogs and cats. In male dogs, the prostatic
urethra may also be seen within the gland. Distal to this the urethra becomes difficult to
visualize as it extends deep into the pelvic inlet. In male dogs, the membranous (perineal)
and penile portion of the urethra can be studied using a high frequency transducer suited to
very near-field imaging or a transducer coupled with a standoff pad.
There are several ultrasound artifacts that are commonly encountered when the urinary
bladder is imaged. These artifacts are important to be aware of because their appearance
can mimic pathology. These include edge-shadows (a form of refraction artifact),
reverberation, slice thickness, and side lobe artifacts. An edge-shadow artifact is created
when the incident ultrasound beam does not strike the bladder wall perpendicularly, due to
the curvilinear nature of the cranial portion of the bladder wall. This may result in a focal
anechoic area of the bladder wall near the apex of the bladder. This artifact may be mistaken
for a focal area of loss of bladder wall integrity or it may hide the presence of a diverticulum
or other pathology. Reorientation of the ultrasound beam so that it is perpendicular to the
wall is necessary if this artifact is encountered.
Reverberation, slice thickness, and side lobe artifacts are apparent as spurious echoes
within the lumen of the urinary bladder. Recognition is important so that pathologic
significance is not placed on them. Slice thickness artifact shows itself as bright echoes
within the bladder lumen that may mimic echogenic sediment. It is created when the
ultrasound beam width is positioned partially within and partially outside the bladder. The
portion of the ultrasound beam outside the bladder encounters tissue that is echogenic and
erroneously places these echoes within the image of the bladder lumen.
Reverberation artifacts are recognized by repeating, horizontally positioned linear echoes
displayed within the bladder lumen. Lowering the transducer's output and gain and use of a
standoff pad best eliminates reverberation artifacts. Slice thickness and reverberation
artifacts can be decreased or eliminated by changing the transducer's position and imaging
in multiple planes.
Side-lobe artifacts are also common when imaging the urinary bladder. Side-lobe artifacts
are created by spurious laterally oriented ultrasound waves that contribute erroneous
information to the appearance of bladder. These are usually apparent as artifactual echoes
within the bladder lumen, and may look identical to the slice thickness artifact. While they
may be difficult to eliminate (side-lobe echoes are an inherent consequence of ultrasound
beam formation), reorientation of the ultrasound beam perpendicular to the structure for
maximum resolution should allow the sonographer to determine an artifact from a true
anatomic or pathologic structure.
Ureters
Ureters are not normally visualized during an ultrasound examination. This is due to their
small size. However, there are a number of pathologies that affect the ureter that can be
diagnosed with ultrasound, mainly because the ureters become partially or completely
obstructed and therefore enlarge. Ureteral pathology is often associated with renal or urinary
bladder disease.
Dilatation
In most cases of dilated ureters (also termed hydroureter), there is concurrent dilation of the
renal pelvis (hydronephrosis), easily recognized as an anechoic separation of the central
hyperechoic renal sinus.10 A dilated ureter may be followed medially and caudally away from
the kidney as a continuum of the dilated renal pelvis. The entire ureter, both ureters, or only
a segment of ureter may be dilated, depending on etiology. Ureteral calculi, obstruction at
the level of the trigone of the bladder by mass lesions or cystic calculi, inflammation or
infection, ectopia, may cause dilatation and rarely ureteral pathology caused by an
ovariohysterectomy procedure or neoplasia.11-17 Diseases of both the retroperitoneal (e.g.,,
hemorrhage or neoplasia) and peritoneal spaces can affect the ureters, since they occupy
both locations.18
Hydroureter is recognized as a dilated tubular structure in long axis (sagittal plane) or as a
round structure in cross-section. It can generally be differentiated from two large tubular
structures in the same vicinity, the caudal vena cava or abdominal aorta. Doppler
examination readily distinguishes between vascular structures with characteristic blood flow
patterns and a dilated ureter in which urine flow cannot be detected. Urine within the
ureteral lumen is anechoic in uncomplicated cases of obstruction, but may become echogenic
with infection or inflammation. The ureteral wall is normally a thin echogenic structure, but
may become thick and irregular when inflamed or infected. Once a dilated ureter is
diagnosed, the sonographer must search for an etiology.
It should be mentioned that excretory urography is still the most sensitive imaging
modality in detection of mild, subtle dilatation or distortion of the renal collecting system
(mild hydronephrosis or pyelonephritis) and the ureters.
Calculi
Ureteral calculi are often the cause of hydroureter.19,20 This is more commonly seen in cats
than in dogs. Ureteral calculi are identified as focal areas of intense echogenicity, nearly
always associated with acoustic shadowing.21 The ureter is usually dilated, but this is not
always the case. If the kidney and ureter are not dilated, identification of a ureteral calculus
may be a daunting task. Usually when ureteral calculi are present, calculi will also be
identified in the kidneys and/or urinary bladder.
It should be noted that all calculi image as intense hyperechoic foci, whether or not they
are radiopaque or radiolucent on radiographs. Acoustic shadowing is usually identified, but in
some cases this is not present due to small size and/or low frequency transducers. Using the
highest frequency transducer possible, setting the focal point at the level of the calculus, and
using a minimum of gain and output can maximize acoustic shadowing. Even so, the tiniest
calculi may not shadow.
Small mineralizations seen in the retroperitoneal space on abdominal radiographs,
suspected to be ureteral calculi, can be verified on an ultrasound examination. Of course,
radiolucent calculi cannot be identified on survey radiographs, requiring an excretory
urogram to make the diagnosis by focal filling defects within the contrast pool of the ureter.
Ectopic Ureters
In some instances, ectopic ureters can be diagnosed during an ultrasound examination.22
However, ultrasound should not be considered a definitive diagnostic technique. Excretory
urography and vaginourethrography, especially used in conjunction with fluoroscopic
examination are additional imaging techniques to be considered. Computed tomography (CT)
is used in human medicine and its utility in veterinary medicine is being investigated.
Cystourethroscopy is also a valid technique.
The key points when evaluating a patient for ectopic ureters are identification of the right
and left vesicoureteral junctions and "ureteral jets."6-8 As mentioned, it is sometimes
possible to see echogenic, swirling urine streaming from ureteral orifices. Ideally, the bladder
is imaged in cross-section such that both the right and left ureters can be visualized in one
scan plane. If imaged in a sagittal plane, it may be difficult to conclusively identify both the
right and left ureteral jets. Color Doppler evaluation has made detection of ureteral jets
easier.9 Visualization of both ureteral jets rules out ectopia in most instances. However,
some forms of ectopic ureters enter and empty into the bladder in a normal location and
then continue distally along the urethra where they may have one or more additional
openings, accounting for the clinical signs. Also, ureteral jets are not identified in all patients.
It is sometimes possible to identify an abnormal location of ureteral termination, such as in
the bladder neck or within the prostate gland. Dilatation makes it easier to follow the course
and locate the abnormal termination of the ureter.
Ureterocele
Ureteroceles are congenital dilations of the distal-most portion of the ureter.23 As their name
implies, they are thin-walled, cyst-like structures located within the bladder wall or
projecting into the bladder lumen. Concurrent ectopia and hydroureter may be present.
Ureteral Rupture
Definitive diagnosis of rupture of the ureter is difficult with ultrasound and best left for
intravenous urography. A ruptured ureter should be considered when retroperitoneal or
peritoneal fluid is seen with a history of abdominal trauma.
Ureteral Evaluation Using Antegrade Pyelography
Ultrasound guided placement of a needle into the renal pelvis and injection of iodinated
contrast material has been described as a method to evaluate the ureter when conventional
intravenous urography is non-diagnostic.17,24 Following injection of contrast material,
radiographs are made.
Urinary Bladder
Cystitis
Bladder wall thickening associated with cystitis is classically most severe cranioventrally,
although thickening may certainly be generalized. Wall thickness varies upon the severity of
pathology and the degree of bladder distention. Care must be taken when interpreting wall
thickness in nondistended bladders, as the wall will be thicker than in a distended state. In
polypoid cystitis, focal areas of wall thickening will be imaged in addition to generalized
thickening. Polyps may be seen, some with a thin stalk, others more broad-based and
malignant appearing. Of course, an inciting cause for the cystitis should be searched for such
as calculi or a urachal remnant.
Emphysematous cystitis can be diagnosed sonographically by visualization of highly
echogenic gas within the bladder wall or in the bladder lumen.25-28 Gas causes the bladder
wall to be very echogenic and irregular. Acoustic shadowing will be present to some degree,
dependent on the amount of gas present. Intraluminal gas will localize to the non-dependent
portion of the bladder (e.g.,, along the near field ventral wall when the patient is scanned in
dorsal recumbency). Intraluminal gas will redistribute dorsally if the patient is scanned from
the ventral abdomen while standing. Gas in the urinary bladder is most commonly associated
with cystitis caused by the presence of gas-forming bacteria (E. coli), secondary to diabetes
mellitus.25 Glucose in the urine is an excellent growth media for E. coli. Rarely, gas-forming
Clostridial infection will be encountered.28 If there is any doubt as to the presence of gas
within the bladder, it can be confirmed with abdominal radiography.
Urachal Diverticulum
Remnant of the urachus can act as a nidus for chronic bladder inflammation and infection.
The urachal remnant will appear as a focal small out pouching in the apex of the bladder
wall. The adjacent bladder wall will be thickened. A patent urachus is uncommon in dogs and
cats, although is regularly encountered in foals and calves.
Cystic Calculi
Radiopaque or radiolucent calculi are imaged as highly echogenic foci within the dependent
portion of the bladder, usually exhibiting strong acoustic shadowing.1,2,4,21 Size, shape and
number varies from large solitary or multiple stones to one or two tiny calculi. Feline calculi
can to be flatter, more discoid in shape. Shadowing sediment ("sand") may also be seen,
which although gravity dependent, is easily suspended when the bladder is "bounced" with
the transducer or the patient is repositioned. If in doubt, the patient may be scanned in a
standing position, with calculi or sediment now positioned along the dependent ventral
bladder wall. An offset may be necessary to resolve smaller calculi/sediment from near-field
reverberation artifact in this position. Occasionally with severe chronic cystitis, mural
mineralization can occur, or small calculi may adhere to the bladder wall. This too can
usually be diagnosed by repositioning the patient. Calculi should be measured, particularly
small ones. This may be important when small calculi are diagnosed are not found at
surgery. Small, 1-2 mm calculi can pass on their own, especially in face of aggressive fluid
therapy.
Proper machine settings are quite important when imaging the urinary bladder. This is
especially true when evaluating the dependent bladder wall for the presence of small calculi.
If the gain settings are not reduced to minimal levels to allow proper visualization of the
dorsal bladder wall, small calculi and weak acoustic shadows will be hidden in the overly
echogenic bladder wall.
Because the colon is directly adjacent to the urinary bladder dorsally (and/or lateral),
colonic gas may mimic shadowing calculi within the bladder or "hide" smaller stones. The
colon may indent the bladder wall, and its crescent-shaped hyperechoic appearance with
acoustic shadowing can truly mimic a cystic calculus. It is therefore important to positively
identify the colon. This is best done by visualizing the colon in cross-section as well as in a
sagittal plane. Usually the colon is seen as a crescent-shaped highly echogenic interface with
acoustic shadowing on cross-section and as a long, linear shadowing structure on long axis.
Echogenic Urine
In addition to echogenic urinary sediment that is gravity dependent and "settles out" during
the examination, it is not uncommon to encounter echogenic "particles" suspended within
the urine. As normal canine and feline urine is anechoic, it is easy to identify anything that is
echogenic within the bladder lumen. In some cats, tiny suspended particles may be lipid
droplets that can be a normal finding. Rabbits and horses normally have extremely
echogenic urine. The echogenic particles are evenly distributed in these cases and are quite
numerous and dense. However, particulate matter evenly suspended within the urine pool
can be indicative of infection, hemorrhage, crystals, or casts and other debris arising from
the kidneys. Often in the case of chronic infection or hemorrhage, the echogenic material
within the urine is not uniform in distribution or shape. It may acquire a more organized
appearance when followed over time and irregular linear strands or bizarre-shaped
structures may form. Fortunately, urinalysis nearly always provides a definitive diagnosis.
Blood Clots
Blood clots may result from trauma, bleeding disorders, neoplasia or cystitis. Immature clots
may be heterogeneous in shape and echogenicity. As clots organize, they appear as more
discrete, hyperechoic, non-shadowing structures of various size and shape. They may be
adherent to the bladder wall and difficult to distinguish from a polyp or mass lesion in some
instances. Followed over time, they may persist for weeks and by themselves act as a nidus
of irritation.
Bladder Rupture
Ultrasound can be useful in cases of bladder rupture. Free abdominal urine will be seen as
anechoic areas between intra-abdominal organs. The urinary bladder may be difficult to
identify in severe cases of trauma. It will be seen as linear echoic bladder wall remnants
"floating" in the free abdominal fluid. In other cases, the bladder may be partially distended
and the disruption of the wall may not be apparent. As noted previously, the "drop-out"
created by edge shadowing must not be mistaken for a site bladder wall tear. Positive
contrast cystography remains the gold standard for assessment of bladder integrity.
Assessment of Bladder Size
Obviously, the size of the urinary bladder varies greatly, dependent on many factors. Time
intervals since urination and fluid therapy are common considerations. Pathologically, a
massively distended urinary bladder may be indirect evidence of an obstruction of the
bladder neck or urethra, or indicate neurological disease. Patients with hyperadrenocorticism
may have markedly distended bladders, a result of excessive urine production and reduced
muscle tone. Chronic massive distention can lead to functional loss of bladder wall integrity,
with urine literally oozing from the bladder. This is not an uncommon finding in cats that
have urinary obstruction.
Conversely, a persistently small urinary bladder may indicate chronic infection, scarring,
and lack of the ability to distend. This is a common consequence of feline urologic syndrome
(FUS).
While bladder volume can be determined using electronic calipers and measurement
software of the ultrasound machine, there is currently no practical application for bladder
size determination in various disease states.
Bladder Masses
Transitional cell carcinomas are the most common type of bladder tumors, although other
types exist. Transitional cell carcinomas of the bladder have been described. 29,30 Focal and
irregular bladder wall thickening is usually present. These masses may be quite large, but
this is one form of tumor that may be detected when it is fairly small, due to clinical signs
that may occur early in the course of the disease. The trigone and proximal urethra are
common sites, and extension from a prostatic neoplasm may occur. Certainly, many tumors
have been diagnosed in the body or apex of the bladder as well, so location alone is not
enough to establish a diagnosis. Some tumors may be seen as a more diffuse thickening of
the bladder wall and be difficult to distinguish from cystitis.
Since benign tumors cannot be reliably differentiated from malignancies based on
ultrasound appearance alone, cytological or histological samples must be obtained. If
urinalysis is not diagnostic, an ultrasound guided suction aspiration of the mass by urinary
catheterization can be performed.31 The catheter is advanced into the bladder from the
urethra and the tip is placed next to the mass and suction is applied, visualized in real-time.
There is some risk in directly sampling urinary masses via a traditional percutaneous
ultrasound guided aspiration or biopsy, as seeding of the needle tract with neoplastic cells
can occur.
The ureters and kidneys should also be evaluated for evidence of dilation from obstruction
by a trigonal mass. Examination of regional sublumbar lymph nodes for evidence of
enlargement is helpful in cases of metastasis. The surface of the caudal lumbar vertebrae
and pelvic bones may be assessed for evidence of metastasis. This is suspected when the
surfaces are roughened, as normally they are smooth. It is difficult to differentiate metastatic
bony reaction from spondylosis sonographically. Abdominal, spinal, and thoracic radiographs
should be made in cases of suspected urinary tract neoplasms.
Urethra
As discussed, the proximal urethra can usually be visualized with high quality ultrasound
equipment. Familiarity with the normal appearance of the canine and feline urethra will aid
the veterinarian recognize various pathologies. The most common diseases affecting the
urethra are the accumulation of calculi and neoplasms.
Urethral Calculi
As for renal and cystic calculi, urethral calculi are identified as focal accumulations of highly
echogenic structures within the urethral lumen, with distal acoustic shadowing. The urethra
may be distended proximal to the calculi. Often, the urethral wall will be noticeably
thickened.
Calculi can lodge anywhere along the urethra, but accumulation within the prostatic
urethra in male dogs is common. In obstructed cats, very small linear accumulations of
calculi or sediment can be seen in the urethra. The urethra may be thickened from
inflammation and edema. The membranous and penile canine urethra can be scanned
transcutaneously using high frequency transducers designed for small parts or near-field
imaging, or a stand off pad can be used to allow adequate evaluation when such transducers
are not available. This is routinely performed in dogs with known urinary calculi. Survey and
contrast radiography are still the mainstay imaging procedures of choice for urethral
evaluation.
Urethral Neoplasia
Neoplasms of the urethra are encountered in male and female dogs.31 They may be primary
or extend from bladder neoplasia. The ultrasound appearance of transitional cell carcinoma
of the canine urethra has been reported.32 In males, urethral neoplasia is often associated
with the prostate. In the female dog, marked thickening of the urethra usually identifies
urethral neoplasms. Primary urethral neoplasms are less common in cats.
SUMMARY
Ultrasound has become a routine imaging modality for evaluation the lower urinary tract.
While many diseases are readily diagnosed with ultrasound, the practitioner must be aware
of artifacts that can mimic disease. Further, radiography must be considered complementary
to ultrasound when imaging the lower urinary system.
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1978;172:931-933.
32. Hanson JA Tidwell AS: Ultrasonographic appearance of urethral transitional cell carcinoma in
ten dogs. Vet Radiol Ultrasound 1996;37:293-299.
SEARCH RESULT #: 6
TITLE: Overview of the Diagnostic Power of Bone Scanning
AUTHOR(S): Gregory B Daniel
ADDRESS (URL):
http://www.vin.com/Members/Proceedings/Proceedings.plx?CID=WVC2003&PID=3208&O=VIN
OBJECTIVES
Introduce the basic principles of bone scintigraphy.
Review the principles of interpretation.
Show examples of how bone scintigraphy could add in the diagnosis of skeletal disease.
OVERVIEW
Bone scintigraphy is one of the most commonly performed nuclear medicine scans performed
in man, horses and dogs. Bone scintigraphy is indicated in a variety of skeletal disorders
because of its high sensitivity and the ease at which the entire skeleton can be images.
Nuclear scintigraphy represents images of physiology in contrast to radiology represents
morphology. Nuclear scintigraphy can detect diseases that alter physiology before there are
morphologic changes in structure. Since there is often a lag period of 14 days before there
are radiographic changes in bone density, bone scintigraphy can detect bony abnormalities
earlier thus providing better case management. Although nuclear scintigraphy is more
sensitive than other imaging modalities, it is often less specific. But specificity of bone
scintigraphy has advanced with improvements in image quality and increasing experience in
clinical interpretation. However; it is still recommended that scintigram be combined with
conventional radiographs to determine the cause of the bony lesion.
Indications For Bone Scintigraphy
Diagnosis of occult lameness or localization of bone pain when is difficult, following
negative radiographs or in cases of multiple bone lesions.
Detection of osseous metastasis.
Preoperative evaluation of primary bone tumors.
Evaluation of healing response.
Bone survey for generalized bone disease {ex. panosteitis}.
Evaluation of blood flow to bone in cases of sequestrum or peripheral vascular injury.
Radioisotope And Dosage
Today bone imaging is almost exclusively performed using 99m Tc-labeled diphosphonate
which show high sensitivity of skeletal abnormalities. These organic analogs have virtually
replaced the older inorganic phosphates such as pyrophosphate (PYP). The diphosphonates
are preferred because there is less protein and red cell binding compared to the phosphates
therefore the soft tissue clearance is superior providing better bone to soft tissue ratios. The
Hydroxyethylene diphosphonates have faster renal clearance than MDP and imaging 1 hour
post injection is possible.
99m
Tc-Methylene Diphosphonate ( 99m Tc-MDP)
99m
Tc-Hydroxyethylene diphosphonate ( 99m Tc-HEDP)
99m
Tc-Hydroxymethylene diphosphonate ( 99m Tc-HMDP)
Typical dosages
3-5 mCi Cat
5-20 mCi Dog
120-200 mCi Horse
Bone Imaging Protocol
Vascular Flow Phase
Use a bolus administration apparatus.
A dynamic acquisition is acquired immediately following injection.
Acquisition parameters 64 x 64 x 8 matrix. Frame rate of 1 frame per second for 60-90
seconds.
Used to evaluate blood flow to an area.
With inflammation there will be increased blood flow.
If the bone is not viable there will be a decrease or a void of blood flow.
Soft Tissue Phase
5 minutes following injection. Images are acquired 5-10 minutes following injection.
Bone localization can occur within 15-20 minutes especially in young horses.
Static images are obtained, matrix 256 x 256 x 16. Lateral images of the area of
interest are acquired for 60-90 seconds. Similar images of the contralateral limb should
be also obtained. Because of the time limitation of the soft tissue scan only one or two
anatomic sites can be evaluated.
The soft tissue phase images determine the presence of the radiopharmaceutical in the
extracellular space along with that in the vascular pool. The uptake is passive and is
caused by increased blood blow to an area and expansion of the extracellular fluid
space from edema.
This technique has also been used to differentiate between cellulitis and acute
osteomyelitis. The scintigraphic appearance of cellulitis is that of a diffuse increase
activity during the vascular phase and soft tissue phase with either a slight uptake or
normal uptake during the corresponding bone phase images. Acute osteomyelitis will
cause an intense increase uptake in the affected bone during the vascular and soft
tissue phases that persist during the bone phase images.
If there is rapid bone uptake it may be difficult to differentiate between true soft tissue
uptake and early bone localization. In these cases, 99m Tc-DTPA can be used as a soft
tissue imaging agent.
Bone Phase
Image acquisition is delayed to allow the radiopharmaceutical to be cleared from the
soft tissues. Bone uptake stops after 4 hours in laminar bone but can continue in
woven bone.
Delayed bone images (24 hours) have been used to detect osteomyelitis in which the
intensity of uptake will continue to increase after 4 hours.
Mechanism Of Radioisotope Elimination
99m
Tc-MDP is cleared primarily by renal excretion.
A small quantity will be secreted from the sweat glands (horse).
Bone Physiology
Bone has a remarkable combination of physical properties-high tensile and compressive
strength while at the same time having elasticity. Despite its strength and hardness, bone is
dynamic tissue being constantly renewed and reconstructed throughout the lifetime of the
animal. Bone scintigraphy documents these dynamic changes in bone physiology. There are
three types of bone cells:
Osteoblast-produces bone matrix-organic.
Osteocyte-synthesize bone matrix-inorganic.
Osteoclast active in bone reabsorption.
Normally an equilibrium state exists between osteoclastic and osteoblastic activity. In
many disease states, the bone will respond by changing the balance between bone
production and bone reabsorption. Bone scintigraphy measures the degree of osteoblastic
activity. Radiographs show the net effect of the osteoblastic and osteoclastic activity. Even
lesions that radiographically appear osteolytic will often have increased uptake on bone scan
because both the rate of reabsorption and bone production are increased. These lesions
appear lytic on radiographs because the rate of reabsorption is occurring at a faster rate than
production. Since most bony lesions result in increased osteoblastic activity, bone
scintigraphy is very sensitive to detecting bone lesions. Exceptions to this rule are multiple
myeloma and acute osteomyelitis. Increased muscular activity results in an increase in
strength of both muscle and bone. Bone responds to stress loading by adding new bone
along the lines of stress (Wolf's Law). Under normal circumstances, the muscle tones up
faster than the bones do, resulting in a mechanical imbalance. The result is stress may be
applied to the bone before its has increased in skeletal strength. If the stresses continue or
are excessive, the deformation of the bone by muscular activity may extend beyond the
bones elastic range resulting in microfractures. As the number of microfractures increase,
small cortical cracks occur. Eventually, as the fracture point is exceeded, structural failure
occurs. Radiographs are usually insensitive in detecting the early changes of stress induced
remodeling of bone. Pain often precedes the radiographic changes and the animal will be
clinically lame before there are radiographic abnormalities. Bone scintigraphy often becomes
positive concurrent with the onset of pain and is better suited to diagnose and monitor these
changes.
Bone has a limited response to injury or disease. There are many more diseases than
there are avenues of bone response. Scintigraphy is known for its sensitivity but is criticized
for the lack of specificity. As the state of the art improves, pattern recognition is becoming
more important and characteristic pattern are known for many diseases.
Mechanism Of Localization
The mechanisms of skeletal localization of pyrophosphate and diphosphonates are not
completely understood but is felt to be as result of absorption via chemical bonding on the
surface of the hydroxyapatite crystals (inorganic component). Since the diphosphonates are
absorbed onto the exposed surface of the hydroxyapatite crystal, regions with large surface
areas (such as the metaphysis of long bones) allow enhanced absorption. Enzyme and
enzyme receptor sites appear to have a small contribution to binding.
Factors Responsible For Uptake
Increased osteoblastic activity-bone turn over the most important factor in uptake.
Increase blood flow-increased blood flow enhances uptake but not in a linear fashion
because the process is diffusion limited. A 3 to 4 time increase in blood flow will result
in approximately 35% increase in bone uptake.
Other factors.
Increased extraction efficiency.
Increased surface area for absorption.
Increased capillary membrane permeability.
Local pH changes.
Electrical potentials.
Extra-Skeletal Sites Of Normal
99m
Tc-MDP Include
The radiopharmaceutical is primarily excreted in the urine resulting in kidney and
bladder activity.
Nasopharynx and oropharynx.
Glandular tissue of the breast and lacrimal apparatus.
Blood pool activity will also be seen as not all the radiopharmaceutical localizes into
these tissues.
Lesion That Can Result In Decreased Or Normal Uptake
Cold spots can be caused by pathological conditions that prevent delivery of the tracer
to the bone. Such conditions include:
Ischemia or avascular necrosis.
Aggressive metastatic tumors.
Fulminating osteomyelitis (seen in infants, d.t. thrombosis).
Multiple myeloma.
Histiocytosis X [eosinophilic granuloma complex in man].
Bone infarcts (acute phase).
idiopathic.
Many of these conditions will result in an increase uptake in later stages of the disease
process.
Normal Appearance
Soft tissue Phase
There should be uniform distribution of the radioisotope within the soft tissues. Thicker areas
are expected to have greater uptake than thinner area. Vascular structures are seen as
linear area of high concentration of radioactivity. Always take the contralateral area for
comparison. Note that prominence of vascular structures decreases rapidly and could result
in normal asymmetry.
Principles of Interpretation
Compare uptake to contralateral limb
Correlate area of uptake to specific anatomic location
Grade intensity of Uptake
Bone Phase Images
Differentials for Focal High Intensity Uptake
Fracture (Recent)
Tumor
Infection
Enthesiopathy
Fibrous Dysplasia
Degenerative Joint Disease
Differentials for Generalized High Intensity Uptake
Hyperparathyroidism
Differentials for Localized "Cold" Uptake
Overlying attenuation
Previous radiation therapy in the area
Local vascular compromise
Multiple Myeloma
Signalment of the patient can also affect the distribution of radiopharmaceutical. In young,
growing animals, there will normally be increased physeal and metaphyseal activity. There
can also be instances where the soft tissue shows uptake of the radiopharmaceutical during
the bone phase. Etiologies include
1. Dystrophic or metastatic mineralization
2. Tumor
3. Poor soft tissue clearance
4. Myositis (horses)
5. Local anesthetic injection (horses)
6. Pulmonary uptake due to
a. Cushing's Disease
b. Metastasis
c. Heterotopic bone formation
d. Previous lung scan
7. Hepatic Uptake due to
a. Mineralized granuloma
b. Metastasis
c. Hematoma
d. Radiopharmaceutical Impurities
e. Previous Sulfur Colloid Scan
SUMMARY
Bone Scintigraphy is a sensitive diagnostic test that can identify sites of bone disease.
Scintigraphic uptake is basic on physiologic responses of the bone to disease. Physiologic
changes occur before morphologic changes and therefore the bone scan can detect disease
before it is seen radiographically.
References
1. Kransnow AZ, Hellman RS, Thiminis ME et al. Diagnostic Bone Scanning In Oncology Seminar
Nuc Med 27:107, 1997
2. Ogilvie GK, Allhands RV, Reynolds HA. Use Of Radionuclide Imaging To Identify Malignant
Mannary Tumor Bone Metastases In Dogs, J AM Vet Med Assoc 195:220, 1989
3. Forrest LJ, Thrall DE. Bone Scintigraphy for metastasis detection in canine osteosarcoma. Vet
Radiol & Ultrasound 35:124, 1994.
4. Cooley DM, Waters DJ. Skeletal Metastasis As The Initial Clinical Manifestation Of Metastatic
Carcinoma In 19 Dogs. J Vet Intern Med 12:288, 1998.
5. Lamb CR. The Principles And Practice Of Bone Scintigraphy In Small Animals, Semin. Vet Med
Surg. 6: 140, 1997.
SEARCH RESULT #: 7
TITLE: Reproductive Ultrasonography
AUTHOR(S): George Henry, DVM, DACVR
ADDRESS (URL):
http://www.vin.com/Members/Proceedings/Proceedings.plx?CID=WVC2003&PID=3203&O=VIN
OBJECTIVES
Review ultrasonographic examination of the male and female reproductive tracts of the
dog and cat.
Review pregnancy diagnosis, fetal evaluation and gestational aging of the queen and
bitch.
Present a summary of sonographic findings related to common diseases of the
reproductive tract.
KEY CLINICAL DIAGNOSTIC POINTS
High frequency transducers (=> 7.5 MHz) are best for evaluation of normal ovaries,
uterus, prostate, testes, and are required for subtle reproductive abnormalities in the
dog and cat.
5.0 MHz transducers may not reveal more subtle parenchymal changes but are usually
able to observe mid to late-term pregnancies, larger pyometras, ovarian masses,
enlarged prostates, and testicular tumors.
A standoff pad may be necessary in smaller patients or with the use of lower frequency
transducers.
Linear array transducers work well for testicular imaging because of excellent near field
imaging.
The application of alcohol and then ultrasound gel may allow pregnancy diagnosis and
evaluation without the necessity of clipping the hair. However, this can compromise
the image and subtle lesions can be missed.
Higher quality transducers allow observation of the normal ovaries immediately caudal
to the caudal pole of the kidneys in the dog and cat. However, lack of identification of
normal ovaries in the dog and cat may occur due to their small size or being shadowed
by gas in the bowel.
A full urinary bladder is beneficial for scanning of the uterus, prostate and pelvic canal.
A full bladder serves as a good acoustic window to the uterus and acts as an
anatomical marker for location of the uterus and prostate.
As is always the case, imaging of the particular reproductive structures in multiple
planes is required for accurate evaluation and diagnosis.
Transrectal imaging of the canine prostate improves evaluation of the gland but requires
expensive endorectal probes.
The vagina and vulva are better evaluated with direct visualization.
KEY ETIOLOGIC AND PATHOPHYSIOLOGIC POINTS
Ovary-Normal
Normal canine ovaries are reported to be roughly 1.5 cm in length, 0.7 cm in width,
and 0.5 cm in thickness in a 25-pound dog with only a small variation in size due to
weight. Feline ovaries are smaller as expected.
The ovaries are comprised of cortex and medulla. However, these are not
differentiated on ultrasound in the dog and cat.
Ovary during the estrus cycle.
The ultrasound appearance of the ovaries varies during the estrus cycle.
During anestrus and early proestrus the ovaries appear oval or bean shaped with
a homogeneous echogenicity similar to the renal cortex.
Anechoic follicular cysts may be identified initially at day 2 to day 7 of proestrus.
Multiple small cysts that cannot be resolved as individual cysts may actually
increase the echogenicity of the ovary.
The surface of the ovary may become irregular as the cysts enlarge to
approximately 1 cm in diameter as ovulation approaches.
In some cases, a decrease in number and size of follicles indicates ovulation has
occurred.
The ovary may develop a mixed hypoechoic and hyperechoic pattern post
ovulation.
By day 6, post ovulation, the ovaries are hypoechoic due to the presence of
corpora lutea and appear more rounded and lobular. Anechoic cystic structures
may still be recognized.
Multifocal anechoic, hypoechoic and hyperechoic areas are present at the
beginning of diestrus, representing corpora lutea and corpora hemorrhagica.
Detection of the precise time of canine ovulation by ultrasound is not thought to be
reliable because canine follicles do not collapse after ovulation.
Cystic Ovaries
Canine follicular and luteinizing cysts have been described in the dog.
The ultrasound appearance is characterized by anechoic contents with thin walls and
distal acoustic enhancement.
Size of the cysts varies from small to large.
Ultrasound cannot reliably differentiate the various types of ovarian cysts. However,
luteinizing cysts often have thicker walls.
Associated pathology may include pyometra, cystic endometrial hyperplasia, and
hydrometra.
Ovarian Neoplasia
Three categories of ovarian tumors are recognized:
Epithelium tumors-adenoma and adenocarcinoma.
Gonadal stromal tumors-granulose cell, thecoma, and luteoma.
Germ cell tumors-dysgerminoma.
Unilateral tumors are more common but bilateral adenocarcinomas have been
reported.
Ovarian tumors are identified by location and by exclusion of renal, lymph node and
splenic tumors.
Ovarian tumors may be solid or mixed solid and cystic or primarily cystic with smooth
or irregular margins.
Uterus-Normal
The normal nongravid uterus can sometimes be imaged dorsal to the urinary bladder
and appears as a solid homogeneous, hypoechoic structure. The lumen is usually
not seen and the endometrium and myometrium cannot be differentiated.
The nongravid uterine horns usually are not observed as they are even smaller than
the body of the uterus.
The uterus can be differentiated from bowel by the lack of peristalsis, lack of
intraluminal gas, and absence of the layered appearance of small bowel.
Pyometra
In older bitches, cystic endometrial hyperplasia usually precedes pyometra.
Sterile luminal mucin and fluid accumulation may occur causing hydrometra or
mucometra.
Ultrasound is the modality of choice to diagnose pyometra.
Mild to severe luminal enlargement with anechoic to echoic contents is characteristic.
Luminal contents are usually homogeneous and a slow. swirling pattern may be
observed.
Usually both uterine horns are involved, but segmental or unilateral pyometras have
been reported.
The uterine walls may be smooth and thin or irregular and thick. The wall generally
thins as distention increases.
Centesis of a suspected pyometra is not recommended.
Serial ultrasound can be used to evaluate response to medical treatment if
ovariohysterectomy is not performed.
Uterine Stump Pyometra
Diagnosis of a stump granuloma or stump pyometra can be challenging.
High frequency transducers are best to evaluate a questionable lesion.
The appearance of stump pyometra can vary.
A large complex mass lesion just cranial to the pubis, between the colon and urinary
bladder, is the typical finding.
Pregnancy Diagnosis
Ultrasound imaging has been used to detect pregnancy as early as 10 days after
breeding in the dog and 11 days after breeding in the cat.
Ultrasound can be used to determine fetal numbers, but the accuracy decreases as
fetal number increase beyond 6. Early fetal death and resorption can influence the
number of viable fetuses at parturition.
Gestational date based on breeding dates in the dog can over estimate gestational
age since conception can take place as late as 7 days after the last breeding.
Ultrasound examination for pregnancy is usually performed between 21 to 30 days
post last breeding. Examination prior to this time may show pregnancy, however,
bowel gas might prevent observation of the smaller gestational sacs resulting in a
false negative.
Gestational age is defined for this discussion as days following the LH (luteinizing
hormone) peak and is considered to be 65 days plus or minus 1 day.
The first sign confirming pregnancy is detection of an anechoic gestational sac.
Observation of cardiac activity and, later fetal movement indicates fetal viability.
Gestational Age Prediction
The following formulas are from Nyland and Mattoon, Small Animal Diagnostic
Ultrasound, 2nd Ed.
Gestation age in the dog-± 3 days
Less than 40 days
GA = (6 X GSD) + 20
GA = (3 X CRL) + 27
More than 40 days
GA = (15 X HD) + 20
GA = (7 X BD) + 29
Days before parturition in the dog
DBP = 65-GA
Gestational age in the cat-± 2 days
Greater than 40 days
GA = (25 X HD) + 3
GA = (11 X BD) + 21
GSD = Gestational Sac Diameter
GA = Gestational Age
CRL = Crown-Rump Length
HD = Head Diameter
BD = Body Diameter
Postpartum Uterus
Normal involution of the canine uterus is complete in 3 to 4 weeks.
Normal involution of the feline uterus is complete in 24 days.
Fetal Monitoring
Ultrasound may be used to detect fetal resorption, abortion, fetal abnormalities, fetal
death before or during parturition and fetal stress.
Comparison of one fetus with another can help identify an abnormal fetus.
If embryonic death occurs before 25 days, complete resorption of the embryo results.
If death of the fetus occurs after 35 days, abortion is usually the result.
Fetal death is recognized first as loss of cardiac activity.
Recognition of fetal structures diminishes rapidly after death with only mineralized
structures identified after 24 hours.
Fetal heart beat and movement should be recognized easily in near term fetuses.
Fetal stress results in a decreased fetal heart rate.
The normal fetal heart rate is roughly twice the maternal heart rate.
M-mode of the fetal heart produces an accurate fetal heart rate determination.
Mammary Tissue
Examination of mammary tissue can be helpful in determining the extent of neoplasia
and regional lymphadenopathy.
Examination of mastitis may determine if an abscess is present.
Prostate-Normal
The normal prostate surrounds the pelvic urethra, beginning at the trigone of the
urinary bladder.
The urethra may be centrally or eccentrically located in the prostate and can be
recognized as a hypoechoic linear structure.
In the sagittal plane the prostate is round to oval in shape.
In the transverse plane, the prostate is semi-oval with a flatten dorsal border and a
bilobed appearance.
The normal ultrasonographic appearance of the normal prostate varies with age and
intact or neutered status.
The appearance of the prostate can also be influenced by the type of ultrasound
probe and settings used.
The prostate of the young to middle-aged dog has a homogeneous parenchyma with
a medium to fine texture. Echogenicity is variable.
The prostate should be smoothly marginated.
A thin hyperechoic rim representing the capsule may be seen but a definitive capsule
is not always seen.
The size of the normal canine prostate gland varies with body size, age and breed.
Scottish terriers have been reported to have larger prostates than other breeds.
Measurements of the prostate in dogs have been reported. However, expected
normal size of the prostate in any individual dog is not exact.
Enlargement of the prostate gland due to benign prostatic hypertrophy may be
considered a normal occurrence in aging intact dogs.
Measurements of the prostate are of benefit in evaluation of treatment of prostatic
disease.
Benign Prostatic Hyperplasia (BPH)
Occurs commonly in intact dogs over 4 years of age.
Enlargement due to BPH may be an incidental finding.
Enlargement due to BPH may cause problems with defecation and urination when
significant enlargement occurs.
Enlargement may be symmetrical or asymmetrical.
Echogenicity varies from hypoechoic to hyperechoic.
Usually shows some evidence of inhomogeneity.
Inhomogeneity of echotexture is usually less severe than seen with prostatitis.
There should not be disruption of the capsule of the prostate or enlargement of the
sublumbar lymph nodes.
Prostatitis
Multiple bacterial organisms have been identified in acute and chronic prostatitis.
Ascending urinary tract infection or septicemia are the most common causes.
May be diffuse, focal or multifocal.
Usually produces heterogeneous mixed pattern of hypoechoic and hyperechoic areas.
May detect mild sublumbar lymphadenopathy.
Pockets of echogenic fluid may represent abscess formation.
In some cases may appear similar to benign prostatic hyperplasia.
Inflammatory prostatitis may appear relatively normal and requires aspirates to
make the diagnosis.
Cannot differentiate neoplasia from prostatitis and may have secondary prostatitis
with neoplasia.
Fine needle aspirates of several areas in the prostate usually are needed for definitive
diagnosis.
Prostatomegaly that does not decrease following castration and treatment for
infection should be suspect for underlying neoplasia.
Prostatic Neoplasia
Prostatic neoplasia typically occur in older, intact, medium to large breed dogs.
Prostatic neoplasia can also occur in neutered males.
Adenocarcinomas and undifferentiated carcinomas are the most common neoplasia of
the prostate.
The appearance of prostatic neoplasia commonly causes a heterogeneous appearance
and may contain cyst-like areas.
Extension of the pathological changes to the urethra or neck of the bladder, regional
lymph node enlargement, and disruption of the capsule are important signs of
prostatic neoplasia.
An enlarged prostate in a neutered male is highly suspicious of neoplasia and
biopsies should be obtained.
Prostatic Cysts
Cysts may be seen with hyperplasia, prostatitis, and neoplasia as already indicated.
Prostatic cysts are commonly observed as an incidental finding.
Aspirates of cysts should be obtained as some cysts may show evidence of infection.
Larger cysts may be drained to relieve symptoms caused by very large cysts followed
by castration.
Paraprostatic Cysts
Paraprostatic cysts are not uncommon and are often seen radiographically as a
"double bladder" sign.
Originate from remnants of the mullerian ducts or as extensions from a prostate lobe.
May become very large and dominate the abdomen.
Appear as anechoic cystic structures with smooth thin walls or irregular thicker walls.
Anechoic contents may show echogenic sediment or focal echogenic material.
Occasionally internal septa or membranes are present.
Surgical removal is the treatment of choice.
Testes-Normal
The testes are easy to ultrasound and should be routinely examined with an
abdominal examination.
The canine testes are echogenic with a medium, homogeneous echo texture.
The mediastinum testis is seen as an echogenic central linear structure on the
midsagittal view and as an echogenic dot on the transverse view.
The head and body of the epididymus are isoechoic to the testis.
The tail of the epididymus is hypoechoic and more coarse in appearance than the
testis.
Ultrasound examination of the testes is performed commonly to assess changes
found on palpation, and to differentiate testicular, epididymal and scrotal diseases.
Ultrasound examination for retained testicles is not uncommon. The retained testicle
is usually smaller but retains the normal appearance of the testes unless neoplasia
is present.
Testes-Neoplasia
Three common neoplasia are seen in the testicle.
Interstitial Cell Tumors.
Sertoli Cell Tumors.
Seminomas.
The sonographic appearance of testicular tumors varies, however, differentiation
from normal testicular parenchyma is usually easy.
Sertoli cell tumors are more common in retained testicles.
Orchitis
Orchitis is often found along with epididymitis.
Abscess formation is reported to be common.
Orchitis may appear similar to neoplasia; however, extratesticular fluid and
epididymal enlargement is more common with infection.
KEY PROGNOSTIC POINTS
Ultrasound serves primarily to identify suspect abnormal tissue changes and help guide
biopsy or aspiration of specific lesions for improved definitive diagnosis and treatment.
Generally, the more complex the parenchymal appearance, the higher neoplasia should
be placed on the differential list.
Serial ultrasound examinations are useful and recommended for evaluation of response
to treatment. Unexpected changes or poor tissue response to treatment may warrant
additional diagnostic tests.
Heart rate and movement of the fetus are the key factors in assessing fetal viability.
Prostatomegaly that does not decrease following castration and treatment for infection
should be suspect for underlying neoplasia.
An enlarged prostate in a neutered male is highly suspicious of neoplasia and biopsies
should be obtained.
SUMMARY
Ultrasonography of the reproductive organs of the dog and cat is a common and very useful
non-invasive method of identification of suspected disease. The sonographic findings can
significantly assist in determining the likely prognosis and selection of further diagnostic
tests. Ultrasound serves as a means of identifying lesions in the reproductive organs and
assisting in obtaining tissue samples and aspirates and monitoring the progress of the
disease. The information gained from ultrasound examination enhances the diagnosis and
treatment of many reproductive processes and diseases of the reproductive organs.
References
1. Nyland TG, Mattoon JS. Small Animal Diagnostic Ultrasound 2nd Ed. Philadelphia; WB Saunders
Co, 2002
SEARCH RESULT #: 8
TITLE: Splenic Ultrasonography
AUTHOR(S): George Henry, DVM, DACVR
ADDRESS (URL):
http://www.vin.com/Members/Proceedings/Proceedings.plx?CID=WVC2003&PID=3202&O=VIN
OBJECTIVES
Provide an overview of sonographic examination of the spleen of the dog and cat.
Present and discuss sonographic findings observed with diseases of the spleen.
Discuss the decision making process for further diagnostics based on sonographic
findings.
GENERAL KEY POINTS
Common indications for imaging the spleen are splenomegaly, abdominal or splenic
mass, trauma, and peritoneal effusion (especially hemoperitoneum).
The primary value of ultrasonography is its ability to recognize focal versus nonfocal
parenchymal disease, solid versus fluid or cavitary lesion, evaluate blood flow to and
from the spleen, and provide guidance for aspirates or biopsy of the lesion.
KEY CLINICAL DIAGNOSTIC POINTS
The spleen is located caudal to the stomach with the dorsal extremity (head) under the
rib cage and the ventral extremity (body and tail) extending along the left lateral
abdomen or across the ventral abdomen.
The head of the spleen can be scanned through the left 11th or 12th intercostals spaces
or longitudinally from the left ventral abdomen. It is important to remember to
examine the head of the spleen and not just the more easily viewable ventral
extremity!
A higher frequency (7.5-10 MHz) probe should be used to improve resolution of the
splenic parenchyma.
An offset pad may be required to prevent near field artifact from interfering with
evaluation of the superficial portions of the spleen.
The size of the spleen is variable and must be assessed subjectively.
Clinically insignificant changes in the size of the spleen are commonly seen in the
dog.
Enlargement of the feline spleen other than with tranquilizers or anesthesia is more
likely clinically significant.
The splenic capsule is smooth, regular and will cause an echogenic line when struck
perpendicular by the ultrasound beam.
The splenic parenchyma is homogeneous with finely textured, medium to high-level
echogenic pattern.
The spleen is normally more echogenic than the cortex of the kidney with the liver being
usually less echogenic than the spleen.
Splenic arteries are difficult to see without the aid of Doppler.
Splenic veins appear as branching anechoic structures that exit the spleen along the
hilum of the spleen. The main splenic vein empties into the portal vein.
Diffuse splenic enlargement can be caused by a number of diseases as well as
secondary to certain medications.
KEY ETIOLOGIC AND PATHOPHYSIOLOGIC POINTS
Diffuse Splenic Congestion
Anesthetic agents and tranquilizers are common causes of splenomegaly with normal
to slightly hypoechoic appearance.
Venous congestion from right heart failure usually does not cause marked
splenomegaly.
Obstruction of the portal vein may result in splenomegaly.
Marked splenomegaly with a course "lacey" hypoechoic pattern can be observed with
splenic torsion, splenic vein thrombosis, inflammation leading to infarction,
parenchymal necrosis, or abscess formation.
Doppler is usually necessary to differentiate passive congestion or inflammation from
vascular thrombosis seen with splenic torsion or thrombosis.
Chronic splenic congestion may appear more echogenic than normal.
Multiple parallel echogenic lines within the splenic parenchyma may be seen due to
severely dilated vessels.
Differentiation of splenic vein thrombosis due to torsion versus without torsion is
difficult even with Doppler examination. However, treatment, splenectomy, is
usually the same for both cases.
Systemic infectious diseases of bacterial and fungal etiology may cause secondary
splenic enlargement with normal or reduced echogenicity.
Infiltrative diseases usually cause splenomegaly with normal or reduced echogenicity.
Extramedullary hematopoiesis and amyloidosis can cause diffuse splenomegaly.
Diffuse lymphoma, malignant histiocytosis, mastocytosis, myeloma, and leukemia
may reduce splenic echogenicity and produce a coarser appearance.
Focal or Multifocal Splenic Disease
Focal lesions of the spleen are more easily observed than diffuse changes.
Although focal lesions are easier to detect, the similar appearance of multiple
diseases prevents definitive diagnosis without aspirates or biopsy.
The sonographic findings must be correlated with history, physical exam findings and
laboratory data to arrive at a tentative diagnosis.
Hematomas
Similar to hematomas in the liver, the sonographic appearance can be extremely
variable.
Initially, hematomas can appear hyperechoic unless there is a larger collection of
unclotted blood that will appear anechoic or hypoechoic.
Cyst-like lesions may be old hematomas due to trauma.
Splenic hematomas cannot be differentiated from hemangiosarcoma based on
sonographic appearance. Fine needle aspirates may provide evidence of
neoplasia, however, hemodilution of the sample often prevents identification of
neoplastic cells. A negative aspirate unfortunately does not rule out neoplasia.
Hematomas will usually decrease in size over time providing evidence for a
diagnosis of hematoma with serial exams.
Neoplasia
Focal or multifocal neoplastic lesions are commonly poorly defined, anechoic,
hypoechoic , or complex lesions.
The most common neoplasias are sarcomas such as hemangiosarcoma and
lymphosarcoma.
Less common neoplasia observed in the spleen include leiomyosarcoma,
fibrosarcoma, osteosarcoma, chondrosarcoma, liposarcoma, myxosarcoma,
rhabdomyosarcoma and fibrous histiocytoma.
The type of neoplasia cannot be determined by sonographic appearance.
Echogenic focal lesions
Focal fat deposits are occasionally observed in the spleen especially in cats. Many
of these deposits appear adjacent to the hepatic veins in the hilum of the spleen.
Hyperechoic foci with acoustic shadowing can be seen with fibrosis and
calcification secondary to previous hematomas, old infarcts, or granulomatous
lesions such as seen with histoplasmosis.
As already indicated, hematomas, and primary or metastatic neoplasia may
produce a hyperechoic lesion.
KEY THERAPEUTIC POINTS
The primary value of ultrasonography is its ability to recognize focal versus nonfocal
parenchymal disease, solid versus fluid or cavitary lesion, evaluate blood flow to and from
the spleen, and provide guidance for aspirates or biopsy of a lesion.
SUMMARY
Ultrasonography of the spleen in dogs and cats is a useful non-invasive method for
examination of the spleen. Although sonographic differentiation of many diseases of the
spleen is not possible, knowledge of the various sonographic appearances of diseases can
assist in determining the need for further diagnostic tests. Ultrasound serves primarily as a
means of identifying lesions in the spleen and assisting in obtaining tissue samples and
monitoring the progress of the disease.
References
1. Nyland TG, Mattoon JS. Small Animal Diagnostic Ultrasound 2nd Ed. Philadelphia; WB Saunders
Co, 2002
SEARCH RESULT #: 9
TITLE: Upper Urinary Ultrasonography.
AUTHOR(S): John S. Mattoon
ADDRESS (URL):
http://www.vin.com/Members/Proceedings/Proceedings.plx?CID=WVC2003&PID=3197&O=VIN
OBJECTIVES
Recognition of normal sonographic anatomy of the kidneys in various scan planes.
Discussion of various disease processes of the kidneys.
Comparison of radiographic findings with ultrasound findings when applicable.
KEY POINTS
Normal kidneys can have various sonographic appearances depending on image plane
and location, an important consideration when differentiating normal from pathologic
kidneys.
The ultrasound appearance of renal diseases are usually not pathognomonic. Findings
must be interpreted in conjunction with physical examination, blood work, urinalysis,
and in many cases cytology or histology.
OVERVIEW
Ultrasound examination of the urinary tract has become a routine procedure in veterinary
medicine. Ultrasound is often one of the initial diagnostic studies used to evaluate the urinary
system because it safely provides essential anatomical information such as size, shape, and
organ internal architecture. Advantages over conventional radiography include the ability to
study the kidneys in the presence of peritoneal or retroperitoneal fluid and in emaciated
patients when lack of intra-abdominal fat hinders radiographic renal visualization.
Subcapsular fluid accumulation, small renal masses, and evaluation of the ureters are
additional examples of advantages over conventional radiographic imaging. Doppler studies
allow assessment of renal blood flow. Ultrasound is commonly used by skilled veterinarians
to safely obtain renal biopsies or aspirates. Ultrasound is also safe and noninvasive.
Although ultrasound is superior to conventional diagnostic radiography in many instances,
there are also limitations. In some large or obese patients, the kidneys may be difficult to
fully examine. Bowel gas can interfere with the examination. Excretory urography is better
for qualitative assessment of renal function, visualization of nondilated ureters, study of
subtle pelvic or diverticular pathology, and diagnosis of the loss of urinary tract integrity
such as leakage from renal, ureteral, or urinary bladder trauma. Therefore, diagnostic
information potentially gained from survey radiographs, and excretory urograms should not
be overlooked, as it is often complimentary to the ultrasound findings.
Ultrasound Technique and Normal Kidneys
A 5.0 MHz transducer may be used for medium to large dogs while a 7.5 MHz or higher
frequency probe is best suited for smaller dogs and cats. Use of higher frequency
transducers allows the best resolution and image detail, the trade-off being depth
discrimination. Linear array transducers provide excellent anatomic detail of superficially
located kidneys in smaller patients (e.g., small breed dogs, cats, rabbits, ferrets, guinea
pigs, etc.) The hair coat is clipped and acoustic gel is applied to the skin to properly
acoustically couple the transducer to the patient. Most sonographers find it easiest to
examine the patient in dorsal recumbency, examining the urinary system from the ventral
abdomen. Others image with the patient in left and right lateral recumbency.
Three distinct anatomic regions of the kidney can be normally recognized; the renal cortex
(intermediate echogenicity), the medulla (hypoechoic to nearly anechoic) and the renal
pelvis/sinus (highly echogenic). Renal cortical and medullary tissues are routinely evaluated
for relative echogenicity and sonographic distinction between them. The cortex is
hyperechoic relative to the medulla and a distinct demarcation between them should be
present. Often an echogenic rim separates cortical from medullary tissue. This is seen in
normal and abnormal kidneys. The medulla is very hypoechoic, sometimes to the degree that
inexperienced sonographers may mistake its echogenicity as anechoic and make an incorrect
diagnosis of hydronephrosis. Normal kidneys should have a smooth contour bordered by a
thin echogenic capsule. It must be remembered that the ultrasound appearance of the
kidneys is dependent on the image plane and the particular location of the slice of the
ultrasound beam.
The normal appearance of the central portion of the kidney when viewed in a sagittal plane
is an echogenic peripheral cortex surrounded by a very hypoechoic medullary tissue. If the
sagittal image is positioned medially, close to the renal hilus, an echogenic "disc" of tissue
will often be present centrally, representing hilar fat. At this location, cross-sectional views of
the renal artery and vein may be seen, while the ureter is generally too small to resolve. The
artery is dorsal to the vein and Doppler evaluation can distinguish between them. Detail of
the renal hilus is highly dependent on the quality of the ultrasound equipment used,
transducer frequency and patient factors such as obesity. Scanning further medially, the
kidney becomes "bilobed" due to the indentation of the renal hilus and renal vessels. If the
kidney is imaged far laterally, medullary tissue will not be present and the resulting image
will be one of a uniformly echogenic and smaller kidney. The appearance of the kidney in this
lateral sagittal image must not be mistaken for a pathologically small and echoic kidney with
loss of corticomedullary definition.
In comparison to other organs, the canine renal cortex is usually less echogenic than the
splenic parenchyma, and may be hypoechoic to isoechoic relative to the liver. Transducer
frequency plays a role in this observation, as the use of high frequency transducers (7.5-10
MHz or greater) may alter this relationship. In particular, higher frequency transducers tend
to make the renal cortex more echogenic, especially in cats. Familiarity with normal
relationships on your equipment is important. Also worth noting is that individual
sonographers prefer different amounts of gain and time-gain compensation settings when
performing ultrasound examinations. Thus, direct comparisons between images from one
sonographer to the next and between different machines is sometimes difficult.
Urine cannot normally be seen in the renal pelvis or diverticula, nor can the ureters be
imaged. Occasionally, aggressive diuresis will cause minimal pelvic dilatation (several
millimeters). Iodinated contrast agents will cause a slight increase in renal size (due to
increase in medullary size) but do not appear to have an effect on renal echogenicity.
The left kidney is located caudal to the stomach, dorsal and medial to the spleen along the
left side of the abdomen. In dogs, the left kidney is usually imaged using the spleen as an
acoustic window. With the transducer positioned in a sagittal body plane to the left of midline
the spleen will be seen in the very near field as an echogenic, finely textured organ. The left
kidney is deep (dorsal) to this and is generally easily found. Once visualized, the depth of
field should be adjusted so that the entire kidney is located within the image display. The
entire kidney is evaluated by gently rocking the transducer medial to lateral, keeping the
cranial and caudal poles within view. When scanning the left kidney in dogs, splenic tissue is
often noted laterally, and deep to the kidney. This occurs because the head of the spleen
may curve toward midline dorsally.
The right kidney may be more difficult to image than the left. This is because of its more
cranial location and the presence of the pyloric outflow tract and duodenum ventrally (within
the near field) which may contain gas. The right kidney is generally located more dorsal and
cranial than the left kidney and therefore an increase in depth of field, gain and output may
be necessary relative to that required for the left. The right kidney is bounded cranially by
the liver, within the renal fossa of the caudate liver lobe. The transducer is positioned along
the right side of the cranial abdomen just caudal to the costal arch in a sagittal body plane.
By slowly sweeping the ultrasound beam laterally and medially, the right kidney is generally
found. It may be necessary in some cases to direct the ultrasound beam cranially,
underneath the costal arch. In this case, the right kidney will be positioned obliquely or
vertically on the screen with the cranial pole in the deeper portion of the field and the caudal
pole in the near field. This appearance is simply due to angulation of the insonating
ultrasound beam and is not indicative of the true orientation of the right kidney.
Once the kidney is evaluated along its sagittal axis, the transducer is rotated 90° to obtain
transverse (cross-sectional) views. Transverse images of the kidney are made from the
cranial to caudal poles by sliding the transducer along the length of the kidney. The hilus of
the kidney should be imaged to assess the renal artery, vein and ureter.
The kidneys may also be imaged in a dorsal plane. This is done by placing the transducer
on the right or left lateral abdominal wall. It is quite simple to obtain this view from the
standard sagittal plane by simply sliding the transducer from the ventral abdomen to the
lateral surface of patient. The lateral renal cortex will be positioned in the near field, the
medial portion of the kidney will be in the far field. The kidney has a slightly different
sonographic appearance when sectioned in this plane. The medullary tissue is larger and the
kidney is larger in a lateral to medial dimension when compared to a sagittal view. Also, the
renal arteries and veins (rarely ureter) are viewed in long-axis in this plane. The dorsal plane
is very useful to image the adjacent adrenal glands, caudal vena cava, aorta and portal vein.
Ultrasound determination of kidney size and correlation with body weight or surface area
has been attempted with limited success in the dog. This is primarily due to large variation in
kidney length and volume among normal dogs and difficulty in accurate mensuration. Feline
measurements are a bit more promising. The normal cat kidney has a length of
approximately 4 cm. Larger male cats (especially intact males) have the largest kidneys,
small spayed females the smallest. While there is not strict agreement on the size of normal
kidneys in cats, most sonographers use a range of 3-5 cm, keeping in mind the size, age,
and gender of the patient. Radiographic assessment of renal size has been established for
years in dogs and cats and may provide additional information to the ultrasound
examination.
Absence of a Kidney
This may be due to agenesis or occasionally nephrectomy. Agenesis of the right kidney is
more common than the left. The remaining kidney, if normal, may be hypertrophied to
compensate. It will be larger than normal but retain normal internal architecture. The right
kidney may be difficult to visualize in certain patients (usually large dogs with a lot of
intestinal gas). Various windows should be utilized in an attempt to visualize the kidney
before a diagnosis of an absent kidney is made. This is a situation where abdominal
radiographs may provide conclusive evidence of the absence of a kidney.
Diffuse Renal Disease
As for other organs, diffuse renal disease is much more difficult to diagnose with ultrasound
than focal or multifocal disease. This is because not all diseases cause a change in the
sonographic appearance of an organ. There are many instances in which renal failure is
present, yet the ultrasound appearance of the kidneys is considered normal. This is currently
one of the limitations of ultrasound. Thus, it is important to remember that diffuse renal
disease may be present without observed changes on the ultrasound examination. Although
sometimes frustrating, it is a fact that ultrasound is much better at identifying focal or
multifocal disease than diffuse pathology. It must further be emphasized that when
ultrasound abnormalities are identified, the appearance may not be specific for a particular
disease process. Nonetheless, many forms of renal pathology do show themselves on an
ultrasound examination. Further, the absence of observed ultrasound pathology in the face
of renal failure can help the veterinarian with a list of reasonable differential diagnoses, by
excluding certain diseases which do characteristically show ultrasound abnormalities.
Diffuse renal disease may cause increased cortical echogenicity with enhanced
corticomedullary distinction or result in decreased definition between the cortex and medulla
as a result of disease affecting both of these regions. Diseases diffusely affecting the kidney
include acute and chronic glomerulonephritis and interstitial nephritis, bacterial infections
(e.g., Leptospirosis), acute tubular necrosis from toxins (e.g., ethylene glycol toxicosis),
amyloidosis, endstage kidneys and nephrocalcinosis. In the cat, lymphosarcoma, feline
infectious peritonitis (FIP) and metastatic squamous cell carcinoma have been reported to
cause hyperechoic cortices, with maintained corticomedullary definition. Reduced cortical
echogenicity, or multifocal hypoechoic nodules or masses have also been described with
lymphosarcoma. The size of the kidneys may be normal, enlarged, or small with diffuse renal
disease. Acute nephritis, FIP, lymphosarcoma generally cause renal enlargement. As
previously mentioned, determination of normal renal size for a particular patient is
problematic. Therefore serial examinations may be necessary to detect renal size changes in
response to therapy.
Ethylene glycol toxicity (antifreeze) often will produce extremely hyperechoic cortices and
medullary tissue. Severe cases may produce complete acoustic shadowing. A rim of
hypoechogenicity has been described at the corticomedullary junction in some cases of
ethylene glycol toxicity. Concurrent peritoneal, retroperitoneal and subcapsular fluid can be
observed in some cases. Subtle to moderate renal enlargement is usually present.
A general increase in renal echogenicity (cortical and medullary), with loss of the
corticomedullary junction is noted in cases of acute and chronic inflammatory disease,
amyloidosis, some types of toxicity and endstage kidneys in dogs and cats. Endstage kidneys
are small, distorted, irregular, and may not resemble a kidney at all. Endstage renal disease
may be seen in older patients, or in young or even juvenile patients, a result of congenital
renal dysplasia. The appearance of the kidneys may be asymmetrical. This is seen regularly
in cats with renal failure. The opposite kidney may be appear normal and in fact hypertrophy
in an effort to compensate for diminished renal function.
The renal medullary rim sign has been described in a number of disease processes,
including hypercalcemic nephropathy (lymphosarcoma), ethylene glycol ingestion,
pyogranulomatous vasculitis (feline infectious peritonitis), acute tubular necrosis of
undetermined etiologies and chronic interstitial nephritis. It is often seen in dogs with
portosystemic shunts. It is recognized as a very echogenic rim parallel to the
corticomedullary junction and usually results from mineral deposits within the outer
medullary tubular lumens or tubular basement membranes. In the case of FIP, mineralization
is not seen histologically. It should also be noted that the medullary rim sign has been
described in normal cats caused by a band of mineral within the lumens of the renal tubules.
The medullary rim sign thus provides an ultrasonographic finding indicating primary renal
disease in some, but not all patients. This rim sign is also frequently seen in dogs and cats
without clinical or biochemical signs of renal disease. Thus, interpretation of this sonographic
finding must be correlated with other pertinent data.
It has also been shown that the degree of cortical echogenicity is positively correlated to
the amount of fat vacuoles in the cortical tubular epithelium of cat kidneys. Kidneys with a
plentiful amount of fat vacuoles demonstrated a great difference between the hyperechoic
cortical tissue and the hypoechoic medulla. The cortical echogenicity becomes similar to the
highly echogenic renal sinus. Cats without a large number of cortical fat vacuoles had less
echogenic cortices. Thus, the definition between the cortex and the medulla is less apparent,
as the two regions of the kidney are more similar in echogenicity.
Aspiration or biopsy of the kidneys using direct ultrasound guidance is very useful in cases
of diffuse renal disease. Certain diseases such a lymphosarcoma are very amenable to a
definitive cytological diagnosis via fine needle aspirate (FNA), whereas other diseases may
require a small tissue sample for histological diagnosis. While ultrasound guided FNA or
biopsies are relatively safe procedures, they are not without risk. Care must be taken to
avoid the renal hilus so that laceration of the renal artery and vein does not occur. Sampling
the cortical tissue and directing the needle away from the mid-portion of the kidney will
reduce risk. Many veterinarians sample the lateral cortical tissue, directing the needle
parallel to the long axis of the kidney and abdominal wall. It should be noted that small,
chronically diseased kidneys are at higher risk for hemorrhage. The risk vs. benefit of renal
biopsies or FNA should always be considered.
Renal Cysts
Renal cysts may be inherited or acquired in dogs and cats. Inherited polycystic disease
occurs in Persian cats and Cairn Terriers. Renal cysts may be single or multiple, large or
small and unilateral or bilateral. In many instances, renal cortical cysts are an incidental
finding. Cysts are usually round, anechoic, have smooth thin walls and show distal acoustic
enhancement. They may be so extensive that deformation of the kidney results. Distortion of
the collecting system may occur. Multiple, tiny cysts may not be individually resolved and
appear sonographically as hyperechoic renal tissue. Concurrent hepatic or biliary cysts may
be seen.
Differential diagnoses include uniform blood clots, unclotted blood, hematomas, abscesses,
lymphoma and necrosis. These conditions can mimic the sonographic appearance of cysts,
although they usually lack one or more of the criteria for cysts listed above. Lymphoma
nodules may appear anechoic (usually they are hypoechoic), but will not show far wall echo
enhancement, and show less, if any acoustic enhancement. One "trick" is to increase the
gain setting, which will cause a solid, parenchymal lesion to increase in echo intensity. Cysts
will not show internal echoes when this is done. Low-level echoes can be seen in the
periphery of some cysts, due to slice thickness artifact.
Cysts occurring with cystadenocarcinomas may be irregular in shape and very complex.
Cystadenocarcinomas are associated with female German Shepherds and accompany nodular
dermatofibrosis. Aspiration or biopsy for may be necessary to reach a definitive diagnosis
although in routine practice aspiration of renal cortical cysts is not necessary.
Solid Masses
Solid renal masses usually are malignant. They may be hypoechoic, hyperechoic, mixed
echogenicity or even isoechoic. Although these patterns are not specific for tumor type,
uniformly hypoechoic masses or nodules often indicate lymphoma. Hyperechoic masses are
less common, but have been reported with chondrosarcoma, hemangiosarcoma and
metastatic thyroid adenocarcinoma. Granulomas are an example of non-neoplastic solid renal
masses. Viscous debris within an abscess or hematoma may incorrectly be diagnosed as a
solid mass.
Focal Hyperechoic Areas in the Renal Cortex
Non-neoplastic hyperechoic focal lesions in the kidney include calcification, fibrosis, gas and
infarcts. Parenchymal calcification may simulate pelvic calculi. Chronic infarcts are wedgeshaped, and the broad-based periphery of the lesion may be depressed. Acute renal infarcts
may also be hyperechoic and then become hypoechoic days later. Radiography may help
differentiate gas from calculi. Indeed, an excretory urogram can provide complimentary
information to the renal sonogram.
Complex Renal Lesions
Many renal masses are complex, containing a mixture of anechoic, hypoechoic and
hyperechoic regions. Large masses that destroy renal architecture may be difficult to
recognize as renal in origin. Etiologies include hematomas, primary or metastatic neoplasia,
granulomas, abscesses, and occasionally acute infarcts. The most common primary
malignant renal neoplasm in dogs is adenocarcinoma; hemangioma is the most common
benign renal tumor. Primary lymphosarcoma is the most common neoplasm of the cat
kidney.
Pelvic Dilation, Hydronephrosis and Hydroureter
Dilation of the renal pelvis is recognized as an anechoic separation of the central hyperechoic
renal sinus. Mild pelvic dilation has been observed with diuresis and may also be seen in one
kidney from increased urine production when the opposite kidney is diseased or absent.
Pelvic dilation and increased size and distortion of the renal diverticula can occur in cases of
pyelonephritis. The mucosa of the renal pelvis may become hyperechoic and irregular. In
addition, hyperechoic focal or multifocal areas may be seen within the renal medullary tissue,
and hypo or hyperechoic areas may be recognized in the cortex. When a dilated renal pelvis
is found, the sonographer should attempt to follow the ureter to ascertain if it is dilated and
to investigate for potential etiology of the dilation. Mild dilatation of the renal diverticula may
be difficult to detect and differentiate from renal vessels. In this instance, Doppler evaluation
is informative, especially color Doppler. Excretory urography is still the most sensitive
imaging modality in detection of mild, subtle dilatation or distortion of the renal collecting
system (mild hydronephrosis or pyelonephritis).
Moderate or massive pelvic dilatation is readily apparent sonographically as a central
anechoic region within the kidney. Long standing ureteral occlusion results in varying
degrees of hydronephrosis, the most severe case being a fluid filled "sac", with only a thin
rim of cortical tissue remaining. The differential list for causes of moderate or severe pelvic
dilation includes congenital disease, pyelonephritis, and obstruction to urine flow. Renal,
ureteral or bladder calculi are common causes, as are bladder masses. Rarely, extrinsic
ureteral masses will be diagnosed. Determining if the condition is unilateral or bilateral is
important in establishing an etiology. Asymmetric pelvic dilatation and distortion of the
collecting system may occur from an intraparenchymal renal mass or cyst.
Renal and Ureteral Calculi
Renal calculi are imaged as intensely hyperechoic foci with distal acoustic shadowing.
Radiopaque and radiolucent calculi will both demonstrate shadowing. To obtain maximal
shadowing, the highest frequency transducer should be used, the calculi should be within the
focal zone and the ultrasound beam should be directed perpendicular to it. Small calculi may
not show acoustic shadowing, as they are smaller than the width of the ultrasound beam.
Anechoic pelvic dilatation makes imaging the calculi easier, confirming their location within
the renal pelvis or collecting system. Differentiating pelvic calculi from nephrocalcinosis is
frequently asked of the sonographer. Location of the calcification is paramount importance.
Differentiating between the two conditions may alter the approach to therapy. Distribution of
the calcification on survey radiography and an IVP is useful as well.
Perinephric Pseudocysts in Cats
Large subcapsular or perinephric accumulations of fluid have been diagnosed in cats and
occasionally dogs. The cause is unknown in most instances. These large "kidneys" are
typically palpated during a physical examination, thus prompting the ultrasound study. The
kidney is seen floating in encapsulated anechoic fluid. Kidney size, shape, internal
architecture and function may be normal, or there may underlying renal disease (e.g.,
lymphoma, FIP). Due to the presence of fluid, the kidney will appear more echogenic overall
than normal. Aspirate of the fluid usually yields a transudate or modified transudate. Therapy
is usually by surgical removal of the capsule, although these pseudocysts have been drained
via ultrasonic guidance. Surgery does not cure the disorder in all cases, as some patients
develop ascites. If the pseudocysts are drained, they will invariably recur in several weeks to
a month or more. Attempts to chemically ablate the pseudocysts by instillation of
tetracycline has been tried by the author, but is no longer practiced due to recurrence
(although the recurrence is prolonged versus drainage alone) and severe adverse reaction in
some patients (107°F).
Other Causes of Renal Subcapsular Fluid
Urine, blood, exudates or transudates may occur around the kidneys. Coagulopathies or
trauma may lead to retroperitoneal hemorrhage. Underlying renal pathology and pertinent
history or laboratory data may give the sonographer a clue as to etiology. Renal FIP and
lymphosarcoma often show small accumulations of subcapsular fluid. Ethylene glycol toxicity
can produce subcapsular, retroperitoneal and free peritoneal fluid.
SUMMARY
Familiarity of the various appearances of normal kidneys is important so as to not
misdiagnosis disease. Although ultrasound has become an important noninvasive imaging
modality, it must be remembered that not all renal diseases manifest as sonographic
abnormalities. When seen, ultrasound pathology is often nonspecific, necessitating fine
needle aspirates or biopsy for a final diagnosis.
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