Even though the gastrointestinal tract contains variable amounts of gas that precludes ultrasound beam passage, and some parts of the GI tract may be incompletely evaluated, ultrasonography has become an important tool in diagnosing gastrointestinal diseases. Ultrasound of the GI tract offers additional and complementary information to that provided by simple and contrast radiography and other imaging modalities. Ultrasonography may eliminate in many instances the need for performing contrast radiology.

Scanning technique

For optimal conditions animals should be fasted for 12 hours to diminish the amount of gastrointestinal gas. However, non-fasted dogs may show adequate image quality. The administration of 15 ml per kg of bodyweight after removing the stomach gas using an orogastric tube may enhance the visualization of lesions of the upper segments of the digestive tract.

Animal positioning depends upon operator preferences, although changing the position may assist in displacing intraluminal fluid to the areas of interest, such as right lateral recumbency for evaluating the pyloric area, left lateral recumbency for the fundus, and standing position for the body.

A 5 and/or 7.5 MHz (and up to 10 MHz) transducer may be used. High frequency transducers are recommended for evaluating the wall. The stomach, the duodenum, the ileocolic junction, and the colon can be identified by their characteristic anatomic location. Other segments of the small intestine cannot specifically be identified. For that reason, a systematically and thorough examination of the whole abdomen is necessary to completely evaluate the gastrointestinal tract.

Ultrasonographic appearance of the normal stomach and bowel

The gastric and intestinal wall have a layered structure that can be seen with high-frequency transducers as alternating hyper and hypoechoic lines (serosa, muscularis, submucosa, mucosa, and mucosal surface). The muscularis and the mucosa are hypoechoic and the rest of the layers including the mucosal surface are hyperechoic. The mucosa is usually the largest layer. Inability to visualize the wall layers may be due to the presence of gas or to limited resolution of the equipment or transducer.

The stomach is recognized by its location caudal to the liver and by the presence of rugal folds. Without fluid distention the dorsal aspect is often not visualized because of the presence of gas. The rugal folds are thicker and thus more evident in a contracted stomach, and may disappear in a distended stomach. When it is completely empty the body and the fundus appear as a "rosette". The normal thickness of the stomach wall varies with the degree of distention. It usually measures between 3 and 5 mm between rugae. A wall thickness greater than 7 mm is considered abnormal in a noncontracted status. The mean number of gastric peristaltic contractions is 4-5 per minute, but may be slightly enhanced when administering water.

The small and large intestine may vary in appearance according to its degree of distention and the nature of its luminal contents. The descending duodenum is usually the most lateral and ventral bowel loop along the right cranial abdomen. It has a straight course and prominent walls that can measure up to 4-5 mm. The rest of the intestinal wall is usually 2-3 mm thick. The walls are considered thickened when they measure over 6-7 mm for the duodenum and 5 mm for the rest of the intestine. The measurements should be performed in transverse views. In the cat the duodenum is 2.5-3 mm, and the remaining intestine about 2 mm thick. The average number of contractions is 4-5 per minute for the duodenum, and 1-3 for the rest of the bowel. Motility is rarely noted in the descending colon.

There are three ultrasonographic major patterns according to the luminal content of the gut. The mucous pattern is characterized by a hyperechoic lumen without acoustic shadowing. The lumen may be collapsed or variably distended depending on the amount of mucous content in that particular bowel segment. The fluid pattern is observed as an anechoic lumen and enhances the evaluation of the digestive tract and adjacent structures (i.e., the pancreas). The gas pattern appears as intraluminal hyperechoic interfaces with acoustic shadowing and reverberation artifacts, causing a "dirty" shadowing.

Ultrasonographic appearance of gastrointestinal disorders

When evaluating the GI system there are four major criteria to characterize gastrointestinal disease: luminal content, wall thickness, motility, and wall layering. When detecting a lesion, wall symmetry, extension of the lesion, and regional or systemic involvement should also be defined.

Ileus is a failure of transit of the gastrointestinal contents, and may be due to an obstruction (mechanical ileus), or to reduced peristalsis (functional ileus). In patients with mechanical ileus usually a segmental dilatation of the bowel proximal to the obstruction may be observed, whereas in functional ileus commonly the whole intestine appears dilated. Less dilatation is usually noted in patients with functional ileus than in those with mechanical ileus. In both instances the lumen often presents a hypo or anechoic fluid content. In functional and in chronic mechanical ileus the number of peristaltic contractions is commonly diminished or even absent. Functional ileus may be due to severe gastroenteritis, peritonitis, surgery, etc. Mechanical ileus in small animals is often due to the presence of foreign bodies, intussusception, masses, or inflammation/adhesions.

Intussusceptions are seen ultrasonographically as a target-like lesion with multiple concentric hyper and hypoechoic rings that represent the wall layers of both bowel segments involved. The outer bowel segment is usually edematous and may appear hypoechoic. Commonly the presence of hyperechoic tissue and anechoic tubes that represent mesentery, fat and vascular structures are observed adjacent to the inner bowel segment.

Gastric or intestinal masses appear ultrasonographically as a focal wall thickening of variable size and extension. Usually a central hyperechoic region that casts an acoustic shadow due to the presence of gas in the affected segment is also identified. The mass may be due to a neoplasm, but other diseases may produce a similar appearance, such as granulomas, mucosal hyperplasias, or severe ulcerative or necrotic inflammatory disease. Morphology of the wall layers is more often preserved in inflammatory disease than in tumors, but in some inflammatory processes the layered appearance may be no longer identified.

The ultrasonographic appearance of foreign bodies depends on their acoustic properties. Most of the foreign bodies do not transmit ultrasound waves and cast a "clean" acoustic shadow, that may be distinguished from the gas "dirty" shadowing, that is due to the superimposition of reverberation artifacts. The detection of a segmental dilatation of a bowel loop, an abrupt change in bowel diameter, and a consistent structure that casts a clean acoustic shadow should be considered suspicious of a foreign body causing a mechanical ileus. Some foreign bodies that transmit the ultrasound beam, such as some type of balls, may be easier to identify. Linear foreign bodies are often but not always associated with a pleating appearance of the affected bowel segment.

The ultrasonographic appearance of diffuse inflammatory disease of the digestive tract depends on the cause and severity of the process. Inflammatory processes may produce a functional ileus with gastrointestinal hypomotility, with or without thickening of the wall. Other signs that may be observed in inflammatory disease are an altered echogenicity of the layers, such as a hyperechogenicity of the internal aspect of the mucosa, poor wall layer definition, and/or pleating of the intestinal wall. Associated mesenteric lymphadenopathy may also be identified.

REFERENCES

1.  Baez JL, Hendrick MJ, Walker LM, Washabau RJ. Radiographic, ultrasonographic, and endoscopic findings in cats with inflammatory bowel disease of the stomach and small intestine: 33 cases (1990-1997). J Am Vet Med Assoc 1999;215:349-354.

2.  Biller DS, Partington BP, Miyabayashi T, Leveille R. Ultrasonographic appearance of chronic hypertrophic pyloric gastropathy in the dog. Vet Radiol and Ultrasound 1994;35:30-33.

3.  Cristal MA, Penninck DG, Matz ME et al. Use of ultrasound-guided fine-needle aspiration biopsy and automated core biopsy for the diagnosis of gastrointestinal diseases in small animals. Vet Radiol and Ultrasound 1993;34:438

4.  Ultrasonographic measurements of gastrointestinal wall thickness and the ultrasonographic appearance of the ileocolic region in healthy cats. Goggin JM, Biller DS, Debey BM, Pickar JG, Mason D. J Am Anim Hosp Assoc 2000;36:224-228.

5.  Lamb CR, Grierson J. Ultrasonographic appearance of primary gastric neoplasia in 21 dogs. J Sm Anim Pract 1999;40:211-215.

6.  Lamb CR. Recent developments in diagnostic imaging of the gastrointestinal tract. Vet Cl North Am 1999;29:307-342.

7.  Penninck DG. Gastrointestinal Tract. In: Nyland TG, Mattoon JS (eds). Small Animal Diagnostic Ultrasound, 2nd edition. W.B. Saunders, Philadelphia 2002; pp 207-230.

8.  Penninck DG, Moore AS, Gliatto J. Ultrasonography of canine gastric epithelial neoplasia. Vet Radiol Ultrasound 1998;39:342-348.

9.  Tidwell AS, Penninck DG. Ultrasonography of gastrointestinal foreign bodies. Vet Radiol and Ultrasound 1992;33:160-169.