Imaging overview: What technology is available (Proceedings)

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.

Use for ultrasound centers around any structure that the probe can be placed on. From eyes, to the heart, to lungs, abdomen and musculoskeletal system, ultrasound can provide diagnostic information. The musculoskeletal ultrasound, though not used in veterinary medicine to the degree as in human medicine, is a useful tool for joint swelling, aggressive bone lesions, and muscle damage, especially when guiding a needle or biopsy to get a sample of the lesion. Ultrasound has virtually eliminated the need for all contrast medium procedures as well. At Michigan State University, we rarely perform barium procedures such as upper gastrointestinal tract examinations, excretory urography and even cystography. The use of ultrasound has replaced these more time consuming and invasive procedures with a rapid, non-invasive modality that provides a large amount of detail centering on soft tissue evaluation and differentiation. By far, the most common use of ultrasound is for evaluation of the abdomen. Being able to examine the internal architecture of organs and imaging through a peritoneal cavity full of fluid to identify a mass has allowed abdominal ultrasound to replace radiography as the routine screening procedure for abdominal pain. One other use of ultrasound worth mentioning is the evaluation of the brain. If MRI and CT are not available to you, imaging through an open fontanel or through the foramen magnum can aid in identification of hydrocephalus as well as increased intracranial pressure with the use of pulse wave Doppler. The applications for ultrasound are quite vast, but it is a rapidly growing, inexpensive and exciting field of diagnostic imaging that can provide a large amount of information regarding a patient.

Nuclear medicine has some uses in small animal medicine mainly centered on portosystemic shunt evaluation. Nuclear medicine works similar to radiography in that radiation is used; however, instead of beaming radiation through a patient, we are administering a radioactive material and observing where it goes. This is done by binding radioactive material to another substance that will allow us to observe bone turn-over and remodeling or how well the lung is being perfused. For portosystemic shunt evaluations, 99m Technetium pertechnetate is administered without binding it to another substance. This is injected either into the rectum or into the spleen and the blood flow to the liver and heart is evaluated. This method provides a qualitative analysis will provide the idea if a macroscopic portosystemic shunt is present, but not the location. Generally ultrasound or more recently CT and MRI have been used to give the surgeons an idea of location. Binding the 99m Technetium pertechnetate to methylene diphosphonate (MDP) allows for evaluation of bone remodeling seen with metastatic neoplasia (like osteosarcoma) or discospondylitis. Though this is very sensitive to detect changes, bone generally remodels for up to 3 years after an injury, so the clinical importance is difficult to assess. 99m Technetium pertechnetate can also be bound to macro-aggregated albumin (MAA) to allow for evaluation of right to left shunts and for pulmonary thromboembolum. By attaching the radioactive Technetium to a large molecule like MAA, the Technetium will stop at the first capillary bed. If that happens to be the left kidney, then there is a right to left shunt (assuming you injected the radiopharmaceutical in a vein). If there is a focal area where no activity is seen in the lung it is considered secondary to an infarcted area likely due to pulmonary thromboemboli.

Computed tomography is an advanced form of radiography. An x-ray tube and detector is used, but the difference is the tube spins around the patient to allow a cross-sectional image of the patient to be generated. This eliminates the superimposition of structures seen with conventional radiography. This method of imaging is fast, easy to perform and provides a large amount of information rapidly. With the newer CT scanners, an entire dog and be imaged in less than 2 minutes allowing for sedated CT examinations rather than placing the patient under general anesthesia. Computed tomography requires specialized equipment and training usually only found at referral hospitals and institutions, but mobile CT facilities and outpatient centers are starting to become more prominent. The use of CT in the veterinary patient centers on musculoskeletal abnormalities, and is considered the gold standard to look for pulmonary metastatic disease due to the lack of superimposition and superior contrast resolution. Other examinations including the brain and soft tissue masses can be performed with administration of intravenous contrast medium. The spinal cord can also be evaluated, but the evaluation is limited to evaluating mineralized disc material and neoplastic masses.

Magnetic resonance imaging has slowly gained acceptance as the modality of choice for neurologic disease. Magnetic resonance imaging works by aligning all the atoms in the patient into one direction, then, using a radio frequency pulse "tuned" into the hydrogen atom, we are able to image various tissues based on their water composition. We call these sequences T1 and T2 weighted based on the time it takes for the atom to return to the resting state. This means that MRI can selectively look at free fluid and alternatively see edema and other soft tissues in an anatomic as well as a physiologic state. Since bone has little water present, it also has little signal on MRI and is difficult to evaluate.

Many options are open to the practitioner involving imaging that can be done to help provide a diagnosis. The goal of this lecture was to familiarize you with the different modalities available to you as a clinician to help guide you with management and diagnostic decisions that you make every day. Not every modality is the best choice in every case, nor is it feasible based on cost of the procedure. Therefore, careful consideration of all the technology at your disposal is necessary in the ever changing world of medicine.