There are many situations in veterinary medicine where diagnostic imaging plays an important and often essential part in the diagnosis of a condition. In general, veterinarians rely on two broad categories of diagnostics: laboratory testing on blood, urine, fluid and tissue samples and diagnostic imaging. Surgery and scoping (of the abdomen, gastrointestinal tract and respiratory tract) play an important third tool in diagnostics, but by their nature tend to be more invasive and usually require anesthesia.
The two readily available imaging options most veterinarians have at their disposal are radiography (commonly referred to as x-rays) and ultrasonography if there is a skilled ultrasonographer in their area. Radiography is relatively straightforward and easy to perform, although there are strict safety guidelines around proper technique to limit exposure of personnel to x-rays and maintain and inspect equipment. Radiography also has certain limitations depending on what you are looking for. Ultrasound requires a level of skill that can only be picked through experience and constant use. As a result, ultrasound is best performed by a veterinarian or technician who is dedicated to performing ultrasounds exclusively. Often, these veterinarians will be internal medicine specialists or radiologists and will provide their services to a number of clinics.
Radiography (a.k.a. x-rays)
When we take a radiograph (this word is the proper term for the image we look at), we place the pet in a specific position on a table over a photosensitive plate and then create a burst of x-rays from an x-ray tube attached to a collimator. The collimator only allows x-rays to pass that are directed to the table and the plate (and the patient), in effect creating a beam and limiting exposure to others in the room and improving the sharpness of the image.
The positions we place pets in to take radiographs are very specific and have been well established through decades of experience with cats and dogs. Part of the reason for this is to make sure we recognize the appearance of normal anatomy and not get confused by variation in position – the organs look very different on a radiograph taken at an angle as compared to one straight on. All veterinarians and registered veterinary technicians have been trained to appropriately position the pet for specific views of practically every part of the body from the head to the toes.
We also make a point of taking a right-angle view to any radiograph taken. The purpose of this is to give a three-dimensional concept of the tissues radiographed and to help prevent artifact (e.g. an object on the skin may look like it is in the middle of the abdomen in one view but not the other).
When most of us think about radiographs, we think about bones. The reason for that is that bones absorb x-rays. In fact, the variation in the absorption of x-rays by different tissues is what provides us with the black and white image. Lungs appear dark on a radiograph because they contain air and x-rays pass through more easily and create the dark colour change on the plate. Fat is also less dense than other tissues, and intra-abdominal fat allows us to visualize the liver, kidneys, bladder, spleen, stomach and intestines in the abdomen.
Unfortunately, radiographs have some inherent limitations. Certain locations, like the back of the nasal cavity and the region of the pancreas in the abdomen, are surrounded by so many other structures that there is no position that will allow x-rays to provide an image. Also, they do not provide the ability to look inside soft tissue structures like the intestines, heart and liver. Radiographs are superior to ultrasound for evaluating the lungs, the bones and the front of the nasal cavity and they provide a simple and relatively quick method to assess the organs in the abdomen as well as the size and shape of the heart. Even in situations where ultrasound may be considered superior, radiographs may provide additional information that will complement and ultimately aid in the diagnosis when done in conjunction.
Most importantly, radiographs are readily available. Most animal hospitals are equipped to take radiographs, and with the advent of digital capture plates, radiographs can be taken quite quickly and with high detail. This can be extremely important in emergency situations.
In general, veterinarians will typically recommend radiographs in any case of lameness (to evaluate for arthritis, fractures or bone tumours), vomiting or anorexia (to evaluate the gastrointestinal tract and abdominal organs) and coughing or respiratory difficulty (to evaluate the lungs and heart). They provide immediate information to give a more complete picture of the pet’s condition. There may be additional uses for radiographs not listed here.
Ultrasound requires the use of powerful, designated computer equipped with software that can send and interpret information from a probe tipped with piezoelectric crystals that release ultrasound waves into tissue and receives reflected ultrasound waves back (conceptually similar to RADAR). It is the different characteristics of the tissues and the degree to which they reflect or transmit the ultrasound that provides the image we see on a sonogram. Most of us associate ultrasound with images of a fetus in a womb, and there are a number of reasons why ultrasound is such a powerful tool in that setting.
Sound waves travel extremely well through liquid, and the more liquid a structure contains, the darker it is on ultrasound. Hence bladders full of urine and amniotic fluid around a fetus appear dark. All tissues contain a certain amount of fluid, and the variation between tissues will reflect differently. Bones and other hard structures do not transmit ultrasound well, and neither do the lungs which contain alveoli full of air. These structures will reflect back the ultrasound waves and block imaging, creating a bright white appearance on the sonogram. Thus, radiography is superior for characterizing the lungs and bones and for evaluating the stomach and intestines when they are filled with gas as is often the case with a gastrointestinal foreign body.
Ultrasound is more effective at evaluating the heart and the internal structures of the organs in the abdomen, especially in the hands of a skilled and experienced ultrasonographer. The heart can be evaluated for muscle wall thickness, anomalies of the heart valves, and blood flow abnormalities. The ultrasound can also evaluate the internal structures of the liver, kidneys, bladder, pancreas, prostate, spleen and to some extent the lining of stomach and intestines. This is an excellent, non-invasive way to look for tumours, obstructions, inflammatory lesions, as well as abdominal fluid. Ultrasound can also be used to guide a small biopsy punch (the thickness of a Q-tip handle) to obtain a tissue sample from a suspicious mass or abnormal appearing liver to further the diagnosis.
Most clinics will need to call on a mobile ultrasonographer to have an ultrasound performed, and scheduling is usually within 1-2 days. Cost is usually more than twice that of x-rays, which reflects the cost of the machine and the skill of the ultrasonographer. There are many times when radiography and laboratory work fail to give us the diagnosis or enough information to direct our therapy and ultrasound is needed.
Veterinarians will typically recommend ultrasonography for any pet with a heart murmur as well as pets with fluid in the abdomen or chest. They will also recommend ultrasonography to evaluate abdominal masses and for non-specific liver or kidney anomalies on bloodwork. Chronic weight loss with no laboratory anomalies is another situation where ultrasound may provide a non-invasive way to evaluate the stomach wall and intestinal tract. Finally, tumours in the bladder wall and anomalies of the prostate are best evaluated by ultrasound.
Veterinarians (and owners) prefer a diagnosis to be made with as few steps as possible to mitigate cost and invasiveness on the pet. However, some pets are unlucky enough to have a condition that requires more extensive testing, and one should be aware of the pit-falls of tunnel vision when making a diagnosis (in other words, missing the whole clinical picture by focusing on one condition). Imaging gives us extra information that may be required for the diagnosis or can help us determine the extent of the illness. That, in turn, allows us to make appropriate decisions on treatment. The owner is the primary decision maker for any testing. I hope this summary helps our owners understand imaging and helps in making those decisions.
Written by Dr. Gawen Thompson DVM