INTRODUCTION TO DIAGNOSTIC IMAGING
How it all began:
On November 8 1895, Wilhelm Conrad Roentgen discovered X-rays. He
discovered the X ray while working in his laboratory by accident. A cathode ray
tube was in the room and he noticed a glow of light coming from a
phosphorescent screen. He began placing objects between the CRT and the
screen. When he placed his hand between the 2, he could see the bones of his
fingers. He noted at that time that platinum and lead would obstruct the rays.
One of the first images he obtained using film was of his wife’s hand in which the
exposure was 15 minutes long. He knew he had stumbled onto something
fascinating and sequestered himself in his lab for the next couple of months.
He later published his findings and shared with the intelligent his new finding. In
1901, the discovery of the X ray received a Nobel Prize for Physics.
The Radiological Society of North America (RSNA) today has thousands of
members that grew from the early 1900’s American Roentgen Ray Society
(ARRS).
How did it make its way into medicine?
In the early years, there were no “real radiologists” only individuals who would
look at the plates of images and compare them to autopsy and surgical findings
in light of the patient’s clinical complaint. Over time, they could tell what the
disease was from looking at the plates – thus the radiology specialty was born.
In veterinary medicine, veterinarians who are specifically trained in diagnostic
imaging belong to the American College of Veterinary Radiology (ACVR).
Currently, radiologists are one of the most sought after specialist not only in
academia but also in large private practices. In all of Canada, there are only
approximately 10-15 individuals currently working as radiologist.
What is Radiology?
In the past, radiology was used to denote a branch of medicine that used x-rays
for the diagnosis and treatment of disease. More recently, with the advent of
MRI, nuclear medicine, ultrasound etc. – which do not use x-rays to aid in a
diagnosis – Diagnostic Imaging is a more appropriate term.
So then why is diagnostic imaging so important?
A large number of patients that visit general and referral practices undergo some
form of diagnostic imaging in the work up of their clinical signs. It is imperative to
realize that imaging is an ancillary diagnostic modality which is performed in light
of clinical signs, history, physical exam findings, and numerous other diagnostic
tests. However, often times, the diagnosis is made on the imaging study and
even a normal exam can be an important diagnostic aid.
During this course, we are going to learn how images from different modalities
can help us, help our patients.
Forms of Diagnostic Imaging:
These modalities are often complementary in finalizing a diagnosis for the
patient.
Diagnostic Radiology / Radiography:
- uses x rays to produce the image, transmitted through patient
- size, shape, density and location of structures can be evaluated
- static images can be made with film/cassettes or digitally
- dynamic studies can be performed with fluoroscopy - where things are
viewed in real time
- black = lots of the x rays get through the part to reach the film
- white = most of the x rays are attenuated before reaching the film
- (digital) – image is made based on the attenuation of the various
structures
- Contrast Agents
o Allows visualization of structures not commonly seen on routine
studies
o Barium
o Iodine
o Can be administered IV, orally, per rectum, intrathecally etc
 GI series – Barium is given in syringe by mouth or by tube
into stomach – allows visualization of GI tract internally
 EU – Iodine is given IV so that the kidneys, ureters, and
bladder can be seen.
 Myelogram – non-iodinated contrast is given intrathecally to
visualize the spinal cord.
o Can be dynamic and static studies
Ultrasonography
- uses sound waves to produce an image, transmitted from transducer
through patient
- sending out and listening for echoes
- good for evaluating soft tissue structures in the abdomen and also the
heart
- size, shape, echogenicity (brighter dots and darker dots) and location of
structures (will be displayed further from the top of the screen if a structure
is deeper because it takes longer time for echo to return to transducer
- the internal architecture can be evaluated
- images seen are dynamic, US can not penetrate air or bone
- operator dependent and knowledge of anatomy is important
Computed Tomography (CT)
- use x rays to produce the images, transmitted through patient
- black and whites are like radiographs
- image the body is cross sections (slices), multiple detectors are used
- size, shape, density, location and no superimposition of structures
- available at referral centers, scan times very quick
- great for heads, nasal – think for example, where a soft tissue mass might
be causing change to underlying bone.
- requires computer manipulation of images
Nuclear Scintigraphy (nuc med)
- gamma rays used to produce image, emitted from patient
- radioactive nuclide is given IV, per os, per rectum etc.
- radiopharmaceutical will undergo decay and radiation is given off – usually
in the form of gamma rays
- isotopes are bound to a specific agent that located to a specific area such
as bone or liver
- detects abnormal function, metabolic activity or abnormal amount of
uptake
- nuc med is poor with regards to anatomical information
Magnetic Resonance Imaging (MRI)
- uses strong magnetic fields and radiofrequency waves to image structures
- there is no ionizing radiation – only radiofrequency waves
- hydrogen protons in the body are stimulated to align to the direction of the
field then a RF pulse is sent in and then turned off – the image is formed
as the protons go back to a relaxing state
- cross sectional images are obtained (just like CT)
- great for soft tissue – spine, brain etc,
Radiation Therapy
- uses radiation to treat and palliate neoplastic and some benign diseases
- Cobalt and linear accelerators are generally used
- must have special training – they now have their own specialty
All of the modalities listed can be used alone or in combination as needed to aid
in the diagnosis for the patient. Each modality can provide different types of
information about body structures.
What is an X ray?
- form of electromagnetic radiation
- all forms move at the speed of light
- vary in energy and wavelength
Production of X rays
- they can penetrate matter
- they can cause fluorescence of some atoms
- they can expose film
- they can cause biological damage
Production of X rays: X rays are produced by the interaction of a rapidly moving
stream of electrons with the atoms of a target material. When the electrons
interact with the target, they are suddenly decelerated and 1% of their kinetic
energy is converted to X rays (the other 99% is lost as heat).
Three elements are required for the production of X-rays. These are 1) a source
of electrons 2) a target for the electrons to hit and 3) a way to accelerate those
electrons.
THE X-RAY TUBE
The Cathode - the cathode is the electron source.
- The cathode element (filament) is a fine strand of coiled tungsten wire,
which resembles the filament of a light bulb. Tungsten is used because it
can be formed into a thin, strong wire, has a high melting point (3370 C),
and has little tendency to vaporize.
- The cathode assembly includes a negatively charged concave cup
(“focusing cup”) surrounding the filament. The focusing cup functions to
push the electrons into a relatively narrow stream - remember that the
negatively charged electrons will be repelled by each other and will have a
tendency to spread out. The focusing cup will narrow the stream so it is
directed at a small portion of the anode called the “focal spot”. Greater
radiographic detail is achieved with a smaller focal spot. Most X ray units
have 2 focal spots sizes – a small one and a larger one. If a high
exposure is needed to penetrate a part, the larger focal spot is used so as
to not burn up the filament and it will spread the heat out over a larger
area on the anode.
- When current (amperes) is applied to the filament, the wire heats up and
electrons escape from the tungsten atoms = thermonic emission.
- The number of electrons is directly proportional to the strength of the
current, which is applied to the filament.
- The strength of the current is determined by the mA(milliamperes) setting
of the x-ray machine. The higher the mA setting, the greater the current
supplied, the greater the filament heating, the greater number of electrons
produced.
The Anode - the anode is the target, which the electrons strike.
- The anode is composed of a tungsten target - tungsten is used because it
is an efficient producer of x-rays, is able to withstand high temperatures,
and dissipates heat reasonably well. The anode may be stationary or
rotating.
- Stationary anodes have the tungsten target embedded in a large block of
copper. Continuous bombardment of electrons in the relatively small
target area causes significant heat production that is partially dissipated by
the copper. However, because of the problem of overheating these
anodes are limited to X-ray machines with low mA potential. They are
often present in the portable units used in equine radiography.
- Rotating anodes have the tungsten target embedded in a narrow band
around the circumference of a disc, which rotates. The electron stream
bombards the target path but because of rapid rotation of the disc no area
is subjected to continuous bombardment. Because of their ability to
"spread out" the heat load these units have a much greater mA potential.
o Anode Heel Effect: The anode surface is angled. This allows for
better cooling of the anode yet maintains good detail.
 The anode angle will cause the beam intensity to be non
uniform with x rays produced on the cathode side being
more intense and the ones produced on the anode side less
intense.
 The heel effect is used in practice by placing the thicker part
of the structures being imaged on the cathode side of the
tube.
- Electrons produced at the cathode are accelerated to the anode by the
application of a positive charge at the anode (negative flows toward
positive). The positive charge applied at the anode can be varied - it is
controlled by the kVp (kilovoltage peak) setting of the machine. The
higher the kVp setting, the greater the degree of acceleration, the greater
the force with which the electrons strike the anode.
Exit window - X-rays go in all directions, so shielding is present around the tube
except at the exit window (glass).
Housing - Glass tube and vacuum.
Filtration – primarily for radiation safety
- Filtration removes the low energy X rays from the beam which do not
contribute anything useful to the image.
- These low energy x rays get “stuck” in the patient thus increasing dose
- Aluminum
Collimation – beam restricting device, beam limiting device
- control the size and shape of the beam
- decreases scatter radiation thus improving radiation safety to personnel
and improves image quality (less fogging of film)
Options for the X-ray photon once it leaves the tube:
1. Hit the patient and be absorbed.
2. Hit the patient and be scattered in another direction (towards the film,
the holders, or within the patient).
3. Go through the patient, hit the cassette and be absorbed causing
exposure of the film.
4. Go through the patient, through the cassette and table and hit the floor.
What are mAs and kVp and why are they important?
mAs - milliampere - second
- milliampere is current applied to filament, second is time current was
applied
- higher currents generate more electrons which create more x-rays at
target
- longer time generates more electrons which do the same
*** mAs Determines Quantity of X-rays!!***
150mA x 2 sec = 300mAs
300mA x 1 sec = 300mAs
600mA x 1/2 sec = 300mAs
kVp -
Kilovoltage peak
the potential differences in volts between the anode and the cathode
determines the speed the electrons have as they hit the target
higher speed will increase the penetrating power of the X-rays
higher speed also increases the number of interactions that a given
electron can have with target atoms, so increases the number of X-rays.
***kVp Determines Quality of X-rays!!***
kVp also has some effect on quantity of x-rays.
kVp is maximum potential of X-ray photon, and in most machines the
average energy of the beam is 1/3 the kVp