CROSS SECTIONAL IMAGING/NUCLEAR SCINTIGRAPHY
Computed Tomography
CT is becoming a very common tool in the diagnosis of disease in our veterinary
patients. In the past, CT scanners were only available at veterinary schools and
now they can be found in numerous large small animal practices. Almost any
part of the body can be imaged with CT. The nice thing about CT as a cross
sectional modality is there is no superimposition of structures. The contrast
resolution is also more highly advanced when compared to radiography. In
humans, they are currently doing “whole body” CT as a screening tool to rule out
disease.
• Uses X-rays, X-ray tube, detectors, collimators – very similar to radiography in
how it works. Therefore, ionizing radiation is used. Normally the patient dose
received is higher with CT than with radiography.
• Patient placed in gantry – the CT table has a couch that slides into a round
tube called the gantry. Positioning for the exam varies depending on the part
of interest. The gantry essentially rotates around the patient to acquire the
information needed to form the image.
• Multiple samples are taken from around the patient and then reconstruction
can occur to make a slice – Must learn to visualize anatomy from the cross
sectional perspective rather than the 2D superimposed radiographs.
CT Generations – CT began to be used clinically in the 1970’s.
Multiple generations of scanners have been made.
Generation configuration
detectors
beam
min scan
time
First
translaterotate
1~2
pencil thin
2.5 min
Second
translaterotate
3~52
narrow fan
10 sec
Third
Rotate-rotate
256~1000
wide fan
0.5 sec
Fourth
Rotate-fixed
600~4800
wide fan
1 sec
Fifth
electron beam
1284
detectors
wide fan electron
beam
33 ms
How It Works
• Scout image is made first to pick the area to scan – multiple lines will come up
on the screen and they are set to extend, for example, from the tip of the nose
through the caudal aspect of the skull.
• Parameters set on the computer – determines the thickness of the slices, how
often the slices are taken etc.
• Scan begins – depending on the scanner the entire scan can be only a few
seconds
• Linear attenuation coefficient of tissues – the LAC is used very similar to tissue
densities seen radiographically – The LAC of the pixels is compared to water
and a Houndsfield number is obtained. Things that are more dense attenuate
more of the beam etc. Thus they will be whiter.
• Houndsfield units calculated
• Shade of grey assigned to a CT number
CT Principles
• The image is divided into small areas called pixels
• Each pixel has a location and attenuation value
• Using this information and very complex math formulas, the computer
constructs the image – we are fortunate the computer does all the calculations.
CT numbers
• High CT number = white because of increased attenuation
• Low CT number = black because of decreased attenuation
• Houndsfield scale
– Water is zero, air is –1,000 and bone is 1,000
• 256 shades of grey
Windowing
• Level
– Center portion of the Houndsfield scale that is being used
• Should be near the tissue of interest
• Width
– How much of the Houndsfield scale is used
• Values within the window will be various shades of grey - rest black or
white
Windowing - Use
• Narrow window – enhance contrast of the tissues
– Brain
• Wide window – area with high inherent contrast
– Lungs
• Soft tissue window – used to look for tumors etc
• Bone window – used to see if there is bony lysis present – this is particularly
important in the case of nasal tumors
• Reformatting – can not be better than original slice – decreased spatial
resolution – but the nice thing about being able to reformat is that the body
only has to be scanned once and then the other 2 planes can be derived from
that data and the scan time is therefore much shorter.
CT Terminology
• Density
– Hypodense – more black
– Isodense
– Hyperdense – more white
• IV Contrast can also be administered – then contrast enhancing, ring
enhancement etc can be used – the contrast will give an idea of vasculature
and margins can be more easily identified between adjacent structures
sometimes.
Magnetic Resonance Imaging (MRI)
•
•
•
Does not involve ionizing radiation
Uses magnetic field and radiofrequency pulses
Hydrogen protons of tissues (water) – The tissues of the body contain varying
amounts of Hydrogen protons – usually in the form of water.
• Water = like tiny magnets
• When placed into magnetic field H protons line up along field
MRI – its like the H protons stand up at attention in the direction of the magnetic
field.
• Radiofrequency pulse passed through patient
• Protons flip and spin
• Pulse turned off and H protons return to normal state = relaxation – so the
once attentive H protons now can relax – when they relax the machine
watches how much energy is released and notes time to relax etc. From this
data T1 and T 2 images are created in which soft tissue differences are
superb.
• T1 – CSF is black, fat is white – best for anatomical evaluation
• T2 – CSF is white, fat is black
• Tissues that have little H protons have little signal and are black
– Air, bone, moving blood
• Good for soft tissue imaging though
•
•
•
•
Paramagnetic contrast agent – Gadolinium – can be administered IV and
usually a T1 scan is used – the contrast will be taken up by abnormal tissue
and even greater change in relaxation times can be noted.
No reformat – must scan all planes
– Thus much longer scan than CT
Transverse, sagittal, dorsal – All of these individual scans must be performed.
There is no reformatting in MRI. Thus, the scan times are MUCH longer.
For example a MRI of the head might be something like this:
- T1 transverse
- T1 sagittal
- T1 dorsal
- T2 transverse
- T2 sagittal
- FLAIR- special one that’s usually done
- T1 transverse with Gad
Each of these scans will take about 9 minutes each – so 7 times 9 = 63
minutes just to scan (not including the set up time for the computer) – if
anything else is required based on what is seen on original images some
special sequences can be performed. But, spatial resolution is great on every
sequence.
MRI Machines
• Can vary from .3 Tesla to 7 Tesla for routine working machines – Currently
almost ever veterinary school in North American either has their own MRI
scanner or has access to one. In the recent past, people were also using
mobile MRI scanners to stop by their practice at various time intervals.
• Many are superconducting – use helium – these are the best
• Magnet is always on and must be contained in a Faraday cage (blocks stray
radiofrequency signals)
• Open and closed magnets
MRI Terminology
• Intensity
– Hyperintense
– Isointense
– Hypointense
• Contrast enhancing with Gadolinium
MRI Safety
• Augment T waves on EKG
• Light flashes – Mild skin tingling – people report their skin sometimes tingles
• Involuntary muscle twitching
•
Increased body temperature – heat is a definite response in the region that is
being scanned
• Projectile effects – if the cleaning person goes into the scan room with the little
floor polisher – well… duck
• Effects on surgical implants – ferrous – it can cause movement of the implants
and also localized heating around them
• Magnetic foreign bodies
• Life support devices
Tattoos – if a tattoo is relatively new (<6 months) the tattoo can spread – the inks
they use are metallic and until fibrous tissue has surrounded the tattoo – a
portrait of Madonna could look like Madonna in her 80’s – everything spreads
out.
MRI Contraindications
• Pacemaker – when the scan is on – we can barely even read the EKG – no
telling what the scanner would do to a pacemaker
• Intra-cranial implants, clips – especially with aneurysm clips in the brain – ya
don’t want those to move
• Metallic foreign bodies – bullets, shrapnel – especially welders who have
previously had shards in their eye
• Implanted electrical pumps, mechanical devices
Nuclear Scintigraphy (Nuc Med)
The Basics
• Radionuclides (radioisotopes) are used – They are injected IV, administered
per rectum, per os etc.
• They undergo decay over time
• Linked to a radiopharmaceutical
• Determines the area of distribution
• Gamma rays come from the patient
– Radioactive – ionizing radiation is involved
• Gamma camera detects the gamma radiation
• Good for physiologic function stuff
• Does not provide good anatomical info
The Ideal Radionuclide
•
Technetium 99m
• Short half life = 6 hours
• Binds to radiopharmaceuticals
• Cheap to purchase
The Gamma Camera
• The gamma rays produce scintillations
• They are converted to electrical signals and multiplied by photomultiplier tubes
• The computer records the strength and location of the scintillation events
Types of Scanning
• Static
– Images are acquired of structures at a single point in time
• Dynamic
– Images are acquired of a structure over a period of time
• Provides functional activity – portal scintigraphy, angio studies, GFR, etc
• Time activity curves
– Activity in a region is followed over time and a graph made
Nuclear medicine exams should have some prominent diagnostic reason for
performing the scan (should not just be used as a screening tool or used in place
of a good physical exam or lameness work up).
Bone Scans
• One of the most common scans we do – both limbs etc. are obtained for
symmetry.
– Equine – use Tech99m and bind to a phosphonate radiopharmaceutical –
usually MDP is used.
• 3 phases:
• Vascular phase (not often performed) – the RP is injected IV. The patient is
positioned for scanning of the area of interest before injection. A dynamic
study is usually performed to watch blood flow to the area = nuclear
angiogram.
• Soft tissue phase – 5-10 minutes after injection the ST phase can begin.
Abnormalities in the soft tissues can be appreciated – such as with muscle,
tendons, ligaments – Ultrasound of the abnormal area can be performed after
the patient is no longer radioactive.
• Bone phase – usually begins 2-4 hours post injection – in young horses the
scans can begin earlier as bone uptake is quicker. The MDP will incorporate
into the bone matrix making it very visible. If there are areas of increased
bone turnover or highly active areas such as growth plates – these will
produce greater RP uptake. Increased uptake is colloquially called “hot”.
Areas with decreased bone turn over will be photopenic (decreased uptake).
This is seen with OCD. Images of the abnormal nuc med area can be made
with radiographs after the patient is no longer radioactive.
Items to Consider
• Age of the animal
– Young animals – growth plates
– Older animal – longer time to distribution of radiopharmaceutical
• Must scan both limbs etc even if only one is suspected of being abnormal
•
Symmetry is your friend
• Animals are radioactive for a time after the scan
Thyroid Scintigraphy
• Technetium99m Pertechnetate
• Uptake in thyroid glands is compared to uptake in salivary glands – should be
equal
• Hyperthyroid – Benign adenoma
– Thyroid glands exceed salivary glands
• Functional thyroid tumors
– Patchy irregular inconsistent pattern
Portosystemic Shunts
• Technetium 99m is placed in the rectum and dynamic images every 4 seconds
are acquired over 2-3 minutes
• Non-invasive, quick, accurate, quantitative but will not characterize the shunt
location as intra or extra hepatic. Sometimes portoazygous shunts can be
detected as such.
• Liver then heart = normal
• Heart then liver = abnormal (shunt)
• Time Activity Curves – important – once the dynamic scan is complete, all the
images are “summed” together to make one image. From this image, regions
of interest can be drawn around the liver and heart individually and the
radioactivity in those regions can be calculated. Also a shunt fraction can be
obtained. The TAC can be used to see who came in loud and strong first – the
heart or liver.
Other Scan Types
• Renal Scans
– To determine GFR and ERPF – anatomic or function activity can be
determined. Used to evaluate for renal failure, before transplants primarily.
• Cardiac Scans – not used much in animals due to replacement with
echocardiography
• Hepatobiliary Scans
– Hepatocyte function, function of the reticuloendothelial system, biliary
function – US has essentially replaced the need for nuc med to evaluate the
liver and biliary tract
• Gastrointestinal scans – Used to evaluate esophageal motility, gastric
emptying and GI bleeding
• Lung Scans – ventilation and perfusion – uses Macro aggregated albumin
bound to Tech 99m. Used to evaluate for EIPH, COPD, PTE primarily.
• Infection and tumor imaging – Used for abscesses or implant infection like with
a total hip prosthesis which is failing
Nuc Med Safety
• Higher energy radiation
– Especially before injection
• Urine from horses – wear plastic booties when entering a horses stall
• Bedding
• Isolation
• Lead for workers – not work – if you wear lead – the radiation essentially gets
slowed down enough by the lead to stick in your body
– Wear plastic gloves to keep off hands
• Wear monitoring badges, rings
Release Protocol
• Isolation of the animals is necessary
• Limited contact with the animal
– Very sick animals may not be best to inject
• Bedding must be monitored before it can be removed
• Animal must be released after scanning with Geiger counter
Note: At this time the Chalk River reactor in Canada has been shut down and is
expected to remain down for at least a year. This reactor produces about half of
the world’s medical isotopes. The reactor in Amsterdam supplies about 30% to
the world and will be closed for maintenance soon. The other large reactor is in
South Africa and will not be able to keep up with the demand. These shut downs
impact veterinary medicine directly since Tech99m is the most common
radiopharmaceutical we use.