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.

Common lesions identified using ultrasound include: foreign body obstruction, mucocele formation, splenic hemangiosarcoma and urinary tract disease will be shown for the purpose of illustration. Previously, a foreign body obstruction could only be identified if it was completely obstructive, was radiopaque or radiolucent and if there was marked dilation orad from the lesion. With the superimposition of other organ structures, sometimes barium was used to evaluate wall thickness and motility. Ultrasound has virtually eliminated the need for barium studies and allows the evaluation and identification of foreign material, whether completely or partially obstructed, within the gastrointestinal tract. This is because that any foreign material, whether it is made from wood, cloth or metal, will absorb sound and cast a dark shadow deep to the lesion. That coupled with the increased ability to identify small intestinal distension and wall layering, makes the determination between a foreign body obstruction and a neoplastic mass easily distinguished.

A mucocele is a chronic form of cholecystitis. Generally, a patient presents with a chronic history of intermittent vomiting followed by an acute onset of collapse or severe, unrelenting vomiting. Ultrasound is the only method available to non-invasively examine the gallbladder and bile duct for evidence of obstruction or mucocele formation. A mucocele has the unique appearance of linear striations that radiate from the center of the lumen. These radiations are thought to be bile salts trapped within a thick, hypoechoic (dark) mucosal wall. At this stage, the gallbladder is considered a nidis for infection and a surgical emergency since it has a high risk of rupture if left in place.

Large splenic masses are generally easily identified on radiographs or ultrasound (as well as physical examination). The difference is in the dog that presents with acute collapse and a hemoabdomen. It is true that with a hemoabdomen and no history of trauma, you can perform an exploratory surgery to find the source of the bleeding, but this is usually difficult to do. Instead, ultrasound evaluation of the abdomen to look for a mass as well as metastatic disease is considered the non-invasive method of choice to help with the surgical planning.

Lastly, urinary tract abnormalities such as hydronephrosis, perinephric pseudocysts, transitional cell carcinoma and cystitis can all be evaluated without the use for contrast medium or general anesthesia in a rapid non-invasive way using ultrasound. Examination of the kidneys will show if the renal pelvis is dilated or the kidney is surrounded by fluid. With radiographs, since fluid and soft tissue are the same opacity, it is not possible to make this determination without contrast medium, which is considered nephrotoxic. Instead, ultrasound can show the architecture of the kidney as well as help find a distended ureter if one is present. The wall of the urinary bladder is also a dilemma with ultrasound since the urine will obscure the luminal margin. With ultrasound, small areas of mineralization within the mass as well as proliferation of the wall seen with cystitis can be quickly and accurately identified, though differentiating tumor versus inflammation is difficult without obtaining a sample with traumatic catheterization.

Abdominal ultrasound in the general practice has the potential to provide a practitioner with rapid information to help facilitate referral or further diagnostic tests especially in the vague, chronically ill patient. With practice, guidance and perseverance, it is possible to use this modality as a triage tool as well as method to determine the progression and regression of disease.