Abdominal ultrasound: What we really see (Proceedings)
The use of radiography to examine the abdomen is full of complications. Radiographs are very good at determining the difference between bone and gas, but soft tissue and fluid are the same opacity. When dealing with intra-abdominal lesions, the main goal is to differentiate one soft tissue mass from a normal soft tissue structure from abdominal fluid. Ultrasound uses high frequency sound waves to accomplish what radiographs cannot. With ultrasound, fluid and soft tissue can be clearly distinguished from one another, where bone and gas cannot. The purpose of this proceeding is to describe the benefits and uses of abdominal ultrasound to the general practitioner.
Abdominal ultrasound is a unique diagnostic test in veterinary imaging. Unlike blood work, radiographs, computed tomography or magnetic resonance imaging, ultrasound requires the sonographer to both acquire images as well as interpret them. This unique combination is why physicians have allowed technicians to acquire the ultrasound images and that leaves radiologists free to perform other studies and interpret the images acquired. This model has not been accepted in veterinary medicine as yet.
So the first stage to abdominal ultrasound is gaining the technical skill to acquire images. This requires patience and time, but is relatively easy with practice. Where ultrasound skill comes into play is with adaptation for disease processes. It is necessary to know that if you suspect portal hypertension, you need to look behind the left kidney for acquired portosystemic shunts. If you see a thrombus in the splenic vein, you need to evaluate the portal vein for thrombosis as well. This is the degree of medicine that keeps the ultrasound probe in the hands of the veterinarian.Ultrasound examinations have nearly replaced abdominal radiographs at Michigan State University. As an example, on 9/8/10, we performed 14 ultrasound examinations and 5 abdominal radiographic series. This replacement has occurred because ultrasound provides better detail and more information about the abdomen compared to plain radiographs. Though we have virtually replaced radiography, radiography is more rapid and gives a better overview of the abdomen compared to ultrasound. For example, a gastric dilation with volvulus can be diagnosed with ultrasound, but it would be easier and more accurate to use radiography to identify the gas filled pylorus displaced dorsally and to the right.
Once the images have been acquired, the next step is interpretation. When ultrasound was first used, it was the first non-invasive, cross-sectional imaging modality. This means that rather than just seeing the outline of an organ, you can now see the portal vein within the liver and the medulla within the kidney. Ultrasound images were compared to gross necropsy examination, but done in a much less invasive manner. Since the image generated can see into the organ, it is very sensitive to find morphologic changes such as masses, cysts, abscesses and tumors. However, unlike gross pathology, you no longer have color and smell to aid in your diagnosis. For this reason, an abscess can look just like a tumor which can look just like a blood clot. This is why we considered ultrasound very sensitive for disease, but not very specific. The benefit of ultrasound is the ability to identify a lesion in an organ of interest as well as aid in obtaining a sample, either with fine needle aspiration or with a biopsy to help determine the true nature of the lesion.