Imaging in veterinary medicine has advanced greatly since the first radiographs (x-rays) were taken of pets just decades ago. Today several imaging tests are available to help diagnose and treat diseases in my practice at the Manchester Animal Hospital. These tests include radiography (x-rays) and ultrasound. However, in some cases, such as CT and MRI scans, I will refer to major teaching hospitals or referral clinics, where they are available and performed by specialists. As with all tests, none of them is perfect for every situation. The following is a brief description of different types of imaging.
A radiograph, commonly called an x-ray, is a black and white two-dimensional image of part of the interior of a body. An image is generated by passing radiation through a particular structure or area, such as the chest or a limb, and the image is then captured. The traditional way of recording the image is on specific x-ray film that senses how much radiation passes through the structure and reaches the film, much like photographic film captures light. The denser a tissue is (such as bone), the whiter the image is on the film. Less dense structures, such as air in the lungs, allow almost all of the x-ray energy to pass through to the film, turning that area black.
In the past 10-12 years, many practices have upgraded to digital radiography. The principles are similar, but the images are captured on a digital recording device and displayed on a computer screen. No x-ray film is used. These images are easy to store as well as to transmit to other hospitals, or to copy to send home with pet owners.
Regardless of whether the images are on film or digital, radiography is the most common and readily available imaging test in veterinary practice. It is used to evaluate the size and shape of organs, such as the heart and lungs, as well as to demonstrate broken bones, some foreign objects, fluid accumulations, and many more abnormalities that may aid in diagnosis. It is also the most affordable imaging test and is most often done prior to any of the other imaging options.
There is a subcategory of x-ray studies that use contrast dyes that show up on radiographs to highlight certain structures. The most familiar of these is the barium series, in which either a liquid or a paste containing barium is given orally or by enema to a patient to highlight a part of the gastrointestinal (GI) tract. Because some objects do not show up on radiographs (such as plastic, cloth, toys, rubber, etc.), barium can help diagnose obstructions or blockages. Barium shows as bright white on radiographs, so if it reaches a certain point in the GI tract and stops abruptly, we can infer that there is something blocking its progress. Sometimes we can also see a foreign object outlined by the barium trying to get around it.
The reason it is called a barium “series” is because it’s necessary to take a series of x-rays at timed intervals as the barium goes through the stomach and intestines to the large intestines. The amount of time the series must be continued depends on what is found, but it can take up to 24 hours to complete in some cases, so usually the pet is hospitalized for the day. Some patients may even need to return to the hospital the next day for follow-up.
A barium study can be done in any clinic that has radiographic equipment and liquid barium. I am frequently grateful for the digital equipment we now employ. I now have the ability to adjust the exposure of a study on a computer and not be limited to a certain number of films I may wish to take in any study.
Unlike radiographs, no radiation is used in an ultrasound study. An ultrasound machine uses sound waves. The ultrasound waves move out from the wand and either become absorbed into organs, pass through them, or are reflected (echo) back. Depending on how many sound waves are absorbed or reflected, an image of the internal organs is formed by a sophisticated integrated computer, and the image is then displayed on a monitor. Real-time moving images are displayed, and still images can be captured as well.
Ultrasound is painless and does not require anesthesia or even sedation in most cases. For an ultrasound evaluation to be done, the pet does need to have the hair shaved from the evaluation area because it will interfere with the images.
This test is typically done after blood tests, x-rays, or a physical examination indicates a possible problem. It is useful for evaluating abdominal organs, eyes, and the reproductive system. As with people, it can be used during pregnancies. A specific ultrasound called an echocardiogram is used to visualize the heart and blood vessels as well as its valves. However, I prefer to refer an “Echo” to a board-certified cardiologist as he will provide expert analysis of the heart study.
Ultrasound can “see” some things that can’t be visualized on radiographs. For example, if the abdomen is filled with fluid, the organs can’t be distinguished on traditional x-rays because fluid and tissue have the same density. However, they appear quite different from each other on an ultrasound image, so we can see through the fluid. It is also useful, for the same reason, for seeing inside an organ such as the heart or liver.
On the other hand, it is not as good at seeing through air or bone, so it does not replace radiography but rather is complementary to the information we can get from radiographs. It is common to do both x-rays and ultrasound in order to get a good picture of what is going on.
As with radiography, there is also a subset of ultrasound imaging tests called contrast ultrasonography in which a material that is visible to ultrasound waves is injected as the image is being watched on the screen. These procedures are usually performed by a specialist.
Computed Axial Tomography (CAT or CT scan)
These images are white, black, and various levels of gray. Depending on how dark or light the gray is, a radiologist can see how well tissue absorbs the x-ray beam and can thus identify abnormal tissue. It can differentiate tissue and display smaller structures, such as lung nodules, better than a traditional radiograph. CT scans are helpful in a variety of areas. Some of the most common indications for a CT scan are nasal disease, brain and spinal cord disease and injury, lung disease, and urinary tract abnormalities.
Due to the high cost of the equipment, CT scans are more expensive than radiography or ultrasonography, and are usually available only in referral centers or large practices. Your veterinarian will refer a pet for CT scans when x-rays and ultrasound cannot provide a diagnostic result.