Ultrasonography of the gastrointestinal tract: a myriad of disease (Proceedings)

Normal gastrointestinal ultrasound

The patient should be fasted prior to ultrasonography if possible to decrease the amount of gas and ingesta. A high frequency transducer is important (> 7.5 MHz) to maximize resolution and evaluate wall layering. A complete exam of the abdomen is recommended to assess for concurrent disease such as mesenteric lymphadenopathy, pancreatic disease, carcinomatosis, etc. Abdominal radiographs are complimentary to abdominal ultrasound.

Complete ultrasonographic examination of the gastrointestinal tract includes evaluation of wall thickness (from inner mucosal margin to outer serosal margin) and layering, evaluation of luminal contents and determination of peristaltic function. The gastrointestinal tract should be scanned in both the sagittal and transverse planes. For the stomach and duodenum this can be accomplished by scanning the cranial and right abdomen. The colon can also be scanned based on anatomic location. For the small intestines it is best to examine the entire central abdomen in a zigzag pattern. The appearance of the gastrointestinal tract will vary greatly depending on the degree of distention and the luminal contents.

When the stomach is empty it will look like a "flower" especially in the cat. In the normal dog the gastric wall is less than 5 mm in thickness and in the cat less than 3.5 mm. These thickness measurements are taken in between rugal folds. Gastric rugae can be recognized in the fundus and body of the stomach with the visibility and thickness dependent on the degree of gastric distension.

The normal thickness of the jejunum in dogs is less than 5 mm and in cats less than 2.5 mm. The duodenum tends to be the thickest area in dogs measuring up to 6 mm. In cats the ileum can measure up to 3.2 mm and has a prominent bright submucosa. The cat ileum has a distinctive "spoke-wheel" appearance. The large intestine is the thinnest and is usually less than 1.5 mm, but can be up to 3 mm in the dog and 2.5 mm in the cat if non-distended.

Ultrasonography allows for differentiation of the layers of the gastrointestinal tract, which alternate in echogenicity. Under optimal conditions, five separate layers can be identified. These include the luminal-mucosal interface (hyperechoic), mucosa (hypoechoic), submucosa (hyperechoic), muscularis (hypoechoic), and subserosa-serosa (hyperechoic). The submucosa and subserosa-serosa are hyperechoic because of the presence of relatively more fibrous connective tissue. The mean number of peristaltic contractions in the gastrointestinal tract is 4-5 per minute.

The ultrasonographic appearance of the lumen depends on its contents. In a collapsed stated the bowel lumen appears as a hyperechoic core ("mucosal stripe") surrounded by a hypoechoic halo of bowel wall. This hyperechoic core represents mucus and small air bubbles trapped at the mucosal-luminal interface. When fluid is present in the bowel lumen, an anechoic area is present between the walls of the bowel that appears tubular in long axis views, and circular in short axis views. Gas within the lumen of the intestine results in a highly echogenic interface with a distal acoustic shadow. The shadow most often is filled with multiple echoes caused by reverberation ("dirty shadow"), but it may also be anechoic ("clean shadow"). The presence of fluid in the bowel lumen improves the sonographer's ability to evaluate the mucosal and submucosal layers of the GI tract, whereas the presence of gas hinders it.

Abnormal gastrointestinal sonographic findings

Neoplasia

Lymphosarcoma is the most common type of feline gastrointestinal neoplasm and occurs in the dog as well. The most common ultrasonographic features of lymphosarcoma are thickening of the stomach or bowel wall, loss of its normal layered appearance with reduced echogenicity of the wall, decreased motility, and lymphadenopathy. Diffuse disease can also occur with lymphoma.

Carcinomas are the most common gastric neoplasia in the dog. These usually originate in the pylorus, but may occur in any location in the stomach and also within the intestine. If the mass lesion is in the outflow region (pylorus) the stomach may appear severely distended with fluid, fluid and gas or empty post vomiting. Focal changes are often seen with carcinoma. Wall thickening is more often asymmetric, but it can be symmetric. The loss of the normal layered appearance of the gastrointestinal wall reflects infiltration of neoplastic and inflammatory cells, necrosis, edema, and hemorrhage. Carcinomas have been described as having a pseudolayered appearance and more commonly result in mechanical ileus when compared to lymphoma.

Gastrointestinal leiomyosarcomas can be incidental findings during routine abdominal ultrasound examination. They are usually identified as fairly small round masses protruding into the gastric lumen in the area of the cardia if they are within the stomach. In the intestinal tract they are often large, eccentric, heterogenous masses. In many animals gastric leiomyomas are not associated with clinical signs.

Foreign bodies

Ultrasonography can be useful for the identification of a number of different types of gastrointestinal foreign bodies. The ability to detect objects depends on the location, gastrointestinal contents and distention, and the object composition.

One of the most common abnormalities of the gastrointestinal tract is distention. The two broad categories that should be distinguished are mechanical (obstructive) versus functional (paralytic) disease. Determining the degree and extent of dilation of the bowel will usually help with this distinction. Typically mechanical disease has a focal or segmental distribution, where functional disease is more diffuse. Mechanical obstructions usually cause bowel distention that is moderate to severe (this will be duration dependent) where functional disease usually causes mild to moderate distention. There are exceptions to these general rules. Dysautonomia often has a pattern more consistent with mechanical rather than functional disease. Very proximal duodenal obstructions may have very little distention if vomiting occurs. The identification of segmental bowel distension with fluid or gas may signify obstruction and should prompt a careful search for foreign material or mural disease that may be causing the obstruction. Because gas can surround foreign material these can be missed with ultrasound.

Objects that transmit sound are more accurately represented by their ultrasonographic image than are objects that attenuate sound. All but the near margin of strongly attenuating objects are obscured by the acoustic shadow that they produce. Although this shadow prevents full visualization of the object, its presence can be an indicator that foreign material is present. Objects that attenuate sound produce a highly echogenic linear interface at their near surface, followed by an acoustic shadow that may have either a "clean" or "dirty" appearance. The shape of the echogenic line may help to identify the type of foreign material present. Food/ingesta can shadow to various degrees and should not be mistaken for foreign material. Similarly feces within the colon can have a similar appearance to foreign material within the small intestine. It is very important to recognize large intestine. The large intestine can often be followed starting at the distal descending colon dorsal to the urinary bladder.

With ultrasound linear foreign objects are often associated with bowel wall thickening and plication. The foreign material is often hyperechoic with variable degrees of shadowing. Gastrointestinal parasites can sometimes mimic linear foreign material. Secondary changes can be seen if perforation of the wall has occurred as described below.

Inflammatory Disease

Inflammatory bowel disease has a broad spectrum of changes, which are relatively non-specific (animals with disease may have normal ultrasound examinations). Changes that have been reported are focal to diffuse thickening, altered echogenicity of the wall, poor intestinal wall layer definition and enlargement of adjacent lymph nodes. The most common small intestinal finding would be mild, diffuse wall thickening with intact wall layering. In comparison neoplasia is more often focal, with greater thickness of the wall and loss of the normal layering. These categories can overlap; therefore, cytology or histopathology is required for definitive diagnosis.

Lymphocytic-plasmacytic enteritis is highly variable in appearance. In cats increased thickness of the muscular layer can be seen with chronic disease.

Gastritis can be diffuse or focal wall thickening and is often associated with decreased motility. Gastric wall edema is generally diffuse with thickening of the wall and altered appearance of the layering. Mineralization of the mucosa (hyperechoic, shadowing area) is seen occasionally with chronic uremia.

Intussusception

The sonographic diagnosis of an intussusception is generally straightforward. An intussusception in the transverse plane is that of concentric layers of bowel wall within the intussuscepted segment (target or bulls eye). On the longitudinal scan an intussusception has the appearance of a thickened segment of bowel with an excessive number of layers that alternate in echogenicity. Hyperechoic mesenteric fat is generally seen associated within the intussusceptum. Although not common it is important to search for associated findings such as tumors or foreign bodies.

Lymphangiectasia

Thin linear bands, oriented perpendicular to the lumen, within the normally hypoechoic mucosa are thought to occur secondary to dilation of lacteals. This finding is seen in dogs with protein losing enteropathies secondary to lymphangiectasia.

Pyloric hypertrophy

Chronic hypertrophy of the pylorus has sonographic changes that include gastric distension and thickening of the pyloric wall. Examination of the pylorus in a transverse plane shows an evenly thick hypoechoic ring (representing the muscularis) surrounding the pyloric lumen. In cases of hypertrophy the total thickness of the pylorus is > 9 mm, with the muscularis layer > 4 mm.

Ulcer

Gastric ulceration can be either a primary disorder or complication secondary to other diseases. Gastric ulcers appear as hyperechoic linear foci within the gastric wall. These foci represent small gas bubbles trapped at the luminal surface of the ulcer crater. Occasionally, a focal loss of the gastric mucosa and submucosa may be apparent. The inflammation and edema associated with ulceration may cause a loss of the layered appearance and thickening of the wall. Ultrasound is insensitive for the identification of superficial ulceration, which is better evaluated via endoscopy.

Perforation

Findings with gastrointestinal perforation or dehiscence include focal wall thickening and loss of layering, focal or diffuse fluid accumulation and increased brightness of the mesentery. Free abdominal gas is best seen at the non-dependent portion of the abdomen, which will change with changing patient positioning. Free gas is seen as bright interfaces not associated with the gastrointestinal lumen.

Miscellaneous

Corrugation of the intestinal tract has been described with many disease processes. These include inflammation (enteritis, pancreatitis, focal peritonitis), neoplasia, and ischemia. Regional extension from pancreatitis often affects the stomach, duodenum, or colon.

Methods of diagnosis

When considering methods of diagnosis the two main considerations are the location and size of the lesion. With a mural lesion of the stomach or proximal duodenum, endoscopy is the preferred method of sampling. If the lesion is located in the small intestine or involves the entire stomach wall and is greater than 1cm in thickness then ultrasound guided fine needle aspirates can be obtained. For fine needle aspirates a 1 to 1 ½ inch 22-gauge needle should be used. If the lesion is thicker than the throw of the biopsy gun (usually 1 ½ to 2 ½ cm) a biopsy can be safely performed. It is important to avoid the lumen during aspiration and biopsy. Commonly, adjacent enlarged lymph nodes may be easier to sample than the affected bowel wall and if possible it is recommended that both areas be aspirated to increase the likelihood of an accurate diagnosis. Diffuse, mild wall thickening (especially if the thickened layer is the muscularis) is best diagnosed with full-thickness surgical biopsies.