Diagnostic imaging has seen a huge technology shift in the last 10 years. Modalities that were not accessible to the small
animal patient, such as magnetic resonance imaging, are now considered the modality of choice for neurologic examinations.
This technology shift has caused a lot of confusion as well as questions about what modalities are used for which diseases
and why. The purpose of this article is to explain the different modalities including conventional radiography, ultrasound,
nuclear medicine, computed tomography and magnetic resonance imaging, their uses and the pros and cons of each.
Radiography is the oldest and widest used diagnostic imaging modality available. Since its discovery by Wilhelm Conrad Roentgen
on November 8, 1895, several changes have been made. These changes include the use of screens to minimize patient dose while
increasing the efficiency of information transfer from the x-rays to film. In addition, automatic processors were invented
to speed the development of the film to generate an image. Computed radiography (CR) and digital radiography (DX) have been
created to optimize contrast resolution and create a virtual image that can be stored in a computer, rather than on a shelf
by creating a digital image. These modalities can be further divided into direct and indirect imaging. Direct imaging occurs
when the x-ray photon directly strikes a detector to create an image. This will provide the greatest spatial resolution for
digital images, but it is still less than screen-film combinations. Indirect imaging is when the x-ray photon interacts with
a phosphor in the screen to transform the x-ray photon into light. The light can then expose the imaging plate with greater
efficiency and minimal loss of resolution.
The choice of which system to buy will be guided by your needs as a practitioner. Digital, indirect radiography such as a
charged coupling device, is inexpensive but provides a rapid digital image. This system generally comes with an x-ray table
and a large device that works similar to a digital camera. Other forms of indirect and direct digital radiographic systems
may have an imaging plate but are considerably more expensive. In exchange, for the added expense more detail and better
imaging quality is obtained. Computed radiography is an indirect cassette based system much like conventional radiographs.
When the cassette is exposed it is placed in a reader to generate the image. This can take around 45-60 seconds, but is mildly
less expensive (depending on the number of cassettes required) and more versatile than most DX systems.
The main thing to avoid is the high pressure salesperson talking of resolution. People will use the terms megapixels, pixel
depth, and even line pairs per millimeter. The thing to remember is that all digital systems (with the exception of digital
mammography) will have less spatial resolution than most film screen combinations. That said, it is not the spatial resolution
we care about. Spatial resolution, the ability to see to objects of similar opacity next to each other, is not as importance
as contrast resolution. Contrast resolution is the ability to see two structures of slightly different opacities next to
each other. This is where digital imaging (direct and indirect) is superior. Because it is possible to adjust the grey scale
on the images after exposure, the ability to identify small fragments, areas of mineralization or nodules within the lungs,
is far greater with digital imaging modalities compared to conventional film. The choice of which vendor and technology is
right for your clinic is difficult and it is recommended that you seek help from a board certified radiologist or advice from
colleagues who have the system you are interested in, to guide your purchase choice.
Radiography is the method of choice for rapid evaluation of the skeletal system and the thorax. Pulmonary edema can only
be evaluated with radiography (be it computed tomography, digital radiography or conventional) and fractures, though seen
with ultrasound and nuclear medicine, can best be evaluated with some form of radiographic technique. In addition, radiography
can be used to give an overview of the abdomen. Unlike ultrasound, which will be discussed next, radiographs can help look
at large gas filled structures that are not easily evaluated with ultrasound. Examples include gastric dilation with volvulus
and mechanical obstructions. It is possible to identify these with ultrasound as well, but radiography remains faster and
easier to make the diagnosis.
Ultrasound is a rapidly growing, non-invasive method to evaluate any portion of the body. Ultrasound involves sound waves
that enter the body and are reflected in various degrees that allow the generation of an image. The main strength of ultrasound
is to be able to differentiate soft tissue compared to fluid. With radiography, soft tissue and fluid have the same opacity.
With ultrasound, it is possible to see the cortex compared to the medulla of the kidney as well as tell the difference between
the portal vein and the hepatic veins of the liver. The main drawback to ultrasound is that it is a technical skill that
requires practice and patience as well as guidance to perform a good quality examination.