Gastrointestinal ultrasonography: Whats new? (Proceedings)

In the dark ages of ultrasound, small intestinal loops were merely black rings on the ultrasound screen. It was only within the last 20 years that we were able to discern the various layers of all parts of the gastrointestinal tract. Within that short period of time we defined the appearance of many diseases and have advanced the technology necessary to perform minimally invasive ultrasound guided procedures. In this talk we will discuss the appearance of the normal gastrointestinal tract and cats and dogs and the ultrasonographic appearance of the surgical and medical diseases.

In both dogs and cats, all portions of the gastrointestinal tract have 5 acoustic layers. The outermost layer is the serosa, followed by an inner black layer of muscularis, submucosa, thicker black mucosa and finally the inner mucosal interface. The serosa and mucosal interface are not real histologic layers, but represent interfaces between the adjacent fat (serosa) and luminal contents (mucosal interface) of that portion of the intestinal tract.

Measurement should not be made of these layers. The remaining 3 layers represent histologic subdivisions within the gastrointestinal tract and are variable in thickness and proportion depending on the species and portions of the tract being evaluated.

Thickness (mm)

Dog

Cat

Stomach

5

3

Duodenum

5.1 – 5.7

3

Jejunum

4.1 – 4.7

2.5

Ileum

4

3

Colon

1

1

When scanning the gastrointestinal tract, measurement should be made of all these layers as part of a normal scan. I only measure total wall thickness, although I always subjectively evaluate the proportion of each sublayer. The measurement of the stomach assumes a mild amount of luminal contents, since and completely empty stomach is almost impossible to measure the wall thickness because of accumulation of exaggerated rugal folds. Similarly, I assume modern moderate contents in all patients and assume that the wall will be thin because of this effect. You may read thicker measurements of the colon assuming patients that have no luminal volume.

During the remaining portion of this presentation, we will discuss causes of thickness and loss of layering. We'll discuss surgical and nonsurgical conditions in each of the sub sections of the gastrointestinal tract. We will discuss the major differences between cats and dogs. It will discuss the necessity for imaging adjacent structures and affiliated structures associated with malignancy.

Of special note are certain surgical conditions that should not be missed during an ultrasound examination of a dog or cat. These include mechanical obstruction. To reach a diagnosis of mechanical obstruction you need 3 components; 1) fluid distended segment, 2) empty segment and, 3) transition point with lesion identified. Lacking any one of these 3 components should leave you in doubt as to whether the patient has a surgical condition. Possible lesions at the transition point include foreign body, neoplastic mass or intussusception. Most foreign bodies have a very hyperechoic interface and are very hyper attenuation (shadowing).

Linear foreign bodies are another very important surgical condition. The exaggerated plicated appearing folds of intestines form the basis of this diagnosis. Often the linear form body itself can be identified by hyperechoic interface and shadow artifact. This assumes a larger foreign body, but very thin foreign bodies may not have a shadow. The plicated appearance needs to be differentiated from corrugations and examples will be given in the lecture. Corrugation is intestinal wall spasticity and is caused by medical conditions, whereas plication is a surgical condition.

Finally regardless of the location of the gastrointestinal lesion, we must always be on the lookout for metastatic disease. A complete evaluation of the draining lymph nodes is paramount for evaluation of metastatic disease. For gastric lesions we evaluate the gastric lymph nodes which live between the liver and stomach. For duodenal and jejunal lesions we evaluate the lymph nodes at the root of the mesentery (jejunal lymph nodes). At the ileocolic junction cats we evaluate the right colic lymph nodes. For all lesions of the gastrointestinal tract you should consider evaluating the sternal lymph nodes. These lymph nodes drain the cranial abdomen and are involved with both neoplastic and reactive diseases. These are easily evaluated and only add a few seconds to a complete a abdominal ultrasound scan. They are often the easiest site to perform a fine needle aspirate upon, in cases of a deep cranial abdominal primary mass lesion.

Gastrointestinal disease is extremely common. In our clinical caseload, primary gastrointestinal disease forms at least 25% of all cases. Diseases such as lymphoma and inflammatory bowel disease are extremely common in cats need to be recognized in clinical cases. In both cats and dogs surgical conditions need to be recognized so that patients are promptly taken the surgery. Ultrasound has the advantage of prompt, real-time imaging and ability to take samples within a relatively short period of time. The modality has very little overhead costs associated with it with the exception of clinical experience and skill of the ultrasonography.