A number of imaging procedures have been developed to help diagnose diseases in humans, and many of these have been adapted for use in animals. Most imaging methods provide a lot of information by noninvasive and economical means and, at the same time, do not change the disease process or cause unacceptable discomfort to the pet. However, because of the complexity and expense of the equipment and instruments, some of these procedures are carried out in facilities designed especially for their use.
X-ray Imaging (Radiography)
X-rays have been in use for many decades to create images called radiographs (black, white, and gray images). Radiography is the most common imaging procedure used in veterinary practices. The x-ray images (radiographs) are produced with the same processes used in human medicine except that the equipment is sized for use with dogs, cats, and other small animals. Portable equipment may be used in a large animal clinic that treats horses and other large animals. Although the procedure is painless, in some cases pets are sedated to reduce the anxiety and stress associated with the procedure, to position the animal, and to help the animal stay still while the images are taken.
Radiographs are often used to identify foreign objects that have been swallowed and are lodged in the gastrointestinal tract.
X-rays work well for creating images of bones, foreign objects, and large body cavities. They are often used to help detect fractures, tumors, injuries, infections, and deformities. Although radiographs may not give enough information to determine the exact cause of your pet’s problem, they can help your veterinarian determine which other tests may be needed to make a diagnosis.
The body’s soft tissues do not absorb x‑rays well and can be difficult to see using this technology alone. Specialized x‑ray techniques, called contrast procedures, are used to help provide more detailed images of body organs. In these procedures the animal is given a dye that will block x‑rays. This can be given intravenously to examine organs like the kidneys or heart, or by mouth to examine the digestive tract. A series of x-rays is taken after the dye is given, which will outline the organs where the dye collects. This makes it easier to spot any abnormalities.
The x-ray machine is positioned so that x-rays are focused on the area to be examined. Exposure to x-rays lasts only a fraction of a second. However, the greater the exposure, the greater the risk that radiation may damage cells. For this reason, a very low dose is used, and lead shields may be used to protect areas that are not being x-rayed and the people taking the radiograph.
Traditionally, radiographs were captured on film. However, recent advances in technology have made it possible for these images to be stored digitally on computers.
There are many advantages to storing radiographs on computers. One of the most important is the ability to rapidly and economically transmit copies of the images to specialists or other clinics. Specialists (board-certified radiologists or surgeons) or individuals at other clinics can study the images of your pet and help your veterinarian accurately diagnose and treat your pet’s condition.
Ultrasonography (commonly called ultrasound) is the second most commonly used imaging procedure in veterinary practices. It uses ultrasonic sound waves to create images of body structures based on the pattern of echoes reflected from the tissues and organs. Ultrasound is much better than x-rays at showing the soft tissues within the body.
The veterinarian or technician usually performs an ultrasound scan by pressing a small probe against the animal’s body, most frequently the abdominal wall. The sound waves are directed to various parts of the abdomen by moving the probe. Echoes occur as the sound beam changes velocity while passing through tissues of varying density. The echoes are converted into electrical impulses that are then converted into an image that represents the appearance of the tissues. In modern scanning systems, the sound beam is swept through the body many times per second, producing a dynamic, real-time image that changes as the probe moves across the body.
To check whether a mare is pregnant, a veterinarian can perform ultrasonography by inserting the wand into the horse’s rectum.
An ultrasound scan can show the size and shape of many organs and can also show abnormalities within them. An ultrasound scan is both painless and noninvasive, and poses no risk of complications. This is the same diagnostic tool used in many medical facilities to check the condition of a human fetus in the womb.
Although ultrasound can be used to evaluate most soft tissues, the heart and abdominal organs are the most frequently scanned in veterinary clinics. Ultrasonic imaging of the heart is termed echocardiography. The structure and function of the heart and its valves can be evaluated by this procedure. There are limitations to ultrasonography, because it cannot be used to scan gas-filled (lungs, intestine) or bony tissues.
Computed tomography (CT) is a computer-enhanced x-ray procedure used to detect abnormalities in various body organs. Because of the expense and size of the equipment, and the need for specially trained technicians, this procedure is not used for pets as often as other imaging types. However, it may be available in some locations (such as hospitals associated with veterinary schools or large specialty practices) and may occasionally be recommended.
In this procedure, the animal is placed on a motorized bed inside a CT scanner, which takes a series of x-rays from different angles. When one series, or scan, is completed, the bed is moved forward, and another scan is taken. CT scans differ from ordinary x-rays because they show different levels of tissue density and produce more detailed images. From these scans, a computer creates cross-sectional images of the body part under investigation and displays the images on a monitor. A dye that can be seen on x-rays may be injected intravenously to make it easier to see abnormalities in the images. By sequentially scanning a body area, an entire organ or other structure can be imaged without interference from neighboring or overlying structures. These scans can be used by the veterinarian to detect structural changes deep within the body, including tumors, abscesses, changes in blood vessels, and fractures.
Because of the need to remain still for a relatively long time while scanning is completed, animals undergoing a CT scan are anesthetized.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is another form of imaging in use today. It is an alternative to computed tomography. With this procedure, a very powerful magnetic field generates detailed anatomic images. No x-rays are involved and it is extremely safe. However, because of the expense and size of the equipment and the need for specially trained technicians, this procedure is not often used for pets. And, due to size constraints, it is not practical for use with horses or other large animals.
For the procedure, the animal is placed in a tubular electromagnetic chamber and pulsed with radio waves, causing tissues in the body to emit radio frequency waves that can be detected. Many repetitions of these pulses and subsequent emissions are required. The emitted waves are then converted into images that are displayed on a computer screen. The images can also be saved for additional study. Sequential examination of slices through the body is done in the same way as for computed tomography. Because the procedure is rather lengthy and the animal must not move throughout the procedure, general anesthesia is used in most cases.
Nuclear Medicine Imaging
Nuclear medicine imaging, also known as radionuclide imaging or scintigraphy, involves dosing the animal with an element that emits a type of radiation known as gamma rays. This element is then detected within the body by means of a special camera attached to a computer, which generates the image. The element is attached to a molecule that has an affinity for the organ or tissue of interest. If the molecule is metabolized by the organ or tissue or stays in the tissue for only a short time, consecutive camera images can be used to evaluate the function of the organ or tissue. Veterinarians most frequently use nuclear medicine imaging to analyze the lungs, kidneys, liver, thyroid, and heart, although other portions of a pet’s body may also be studied with this technique.