Radiology of the Rabbit Thorax
Radiology is a frequently underutilized diagnostic tool in the medical and surgical care of rabbits. Following is an excerpt from Dr. Sam Silverman’s radiology lecture at the HRS Veterinary Conference last March, discussing radiographic interpretation of some common clinical views of the rabbit thorax.
When thinking about a rabbit’s lung capacity, I remember when I was in the mountains flying a helicopter, I’d see these tracks in the snow, rabbits running and running and doing figure eights– a coyote or something was chasing them. I was amazed at how aerobically fit these guys must be. But when you look at their thorax, it’s absolutely amazing, because there is very little aerated lung for the size of the animal.
Notice there’s no retrosternal lucency of cranial lung lobes inflated with gas, and the cardiac borders are often relatively indistinct, especially if a large amount of fat is present. It is essential that the front legs be fully extended cranially on the lateral projection to obtain optimum radiographic detail of the cranial thorax. You do not see large caudal lung fields; the scapulas are superimposed on the dorsal thorax, so if you were to read this out as a dog or a cat, you would come out with a much different diagnosis than you would as a rabbit.
The Cat Thorax
A cat is about the same size as a rabbit. But in a rabbit the heart is located cranially, very close to the thoracic inlet (compare a cat thorax to a rabbit thorax on the opposite page). You do not see the cranial mediastinum in the rabbit, because the heart is so far cranial. You do not see distinct bronchial markings. You do not have tremendous inflation of lungs in the normal rabbit either. You can’t think of a rabbit as a ferret. You can’t think of a rabbit as a guinea pig. A rabbit is a completely different species in regard to radiographic anatomy.
A frontal view is a little better, but don’t forget, when we make a measurement on the chest, our technique charts are set up on the basis of soft tissue to aerated lungs. If you were to take a 10-centimeter cat and a 10-centimeter rabbit, and use the same exposures, the rabbit would be underexposed. Notice again that there is no cranial mediastinum, because the heart is located far forward.
Obese Rabbit Thorax
When you look at a radiograph of an obese rabbit you might ask: Where’s the chest wall? Where is the heart? Where are the lungs? You could go on and on describing the abnormalities in this rabbit. This is a clinically normal rabbit. When I read this out as “obesity,” a client got really upset with me. So now I read these as, “There is a copious amount of subcutaneous and intracavitary fat.” When radiographing an obese rabbit, you have to realize that your ability to make a diagnosis is affected seriously by the amount of fat. Is this a bronchial pattern or an interstitial pattern or a mixed pattern? The problem is that the density, outside and inside the thorax, obscures pattern detail. The only thing you can do on these animals is to go to a grid technique, even if they’re not thick enough, and that will clear up some of the scatter. Low kvp, 40-65 kvp and short exposure time, 1/60 sec. or faster, techniques will help increase radiographic detail.
Bezoar in Thoracic Radiograph
This is an abdominal structure that we often incidentally see on thoracic radiographs. This is the halo sign. The halo is a discrete radiolucency in the stomach, that’s often indicative of bezoars within the stomach. But bezoars are relatively common and possibly normal.
Chronic Respiratory Disease
Chronic pulmonary consolidative disease is common, and radiographs are helpful in documenting the changes characteristic of chronic pulmonary consolidation. This is important since the radiographic information will help differentiate upper respiratory diseases from the chronic and acute pneumonia. It is often difficult to determine the chronicity of pulmonary disease from the rabbit’s history of clinical signs. In some rabbits with chronic respiratory disease we will tend to see indistinct bronchial pulmonary markings throughout the lungs. We don’t read the cranial fields because we only see cranial mediastinum and fat. We don’t see the amount of middle lung lobe atelectasis that we see in dogs and cats. You will see an increased indistinct interstitial pattern. In a more severe case of chronic respiratory disease, we see focal areas of pulmonary consolidation. Sometimes the VD view is not as helpful as the lateral, sometimes the opposite will be true. In chronic pleuritis you will see adhesive disease and scarring of the lungs with decreased aeration, which can be hard to distinguish from large amounts of intrathoracic fat.
Although our ability to closely describe patterns and abnormalities in the rabbit thorax is very limited compared to our abilities in cats, radiography is still a very useful tool.
Radiographs by the Author
by Sam Silverman, DVM, University of California/Davis, School of Veterinary Medicine
House Rabbit Journal Radiology of the Rabbit Thorax