Cummings School of Veterinary Medicine, Tufts University, North Grafton, MA, USA
The place of ultrasonography in the evaluation of the gastrointestinal tract has greatly increased over the past 30 years. The normal ultrasonographic anatomy as well as the main features such as wall thickness, layering, contents, extension/location of the lesion(s) used to evaluate GI diseases are presented.
Technique and Preparation
Animals are scanned in dorsal recumbency, but other positions such as right or left recumbency or standing position are used, if needed, to optimize an acoustic window by displacing the air or by moving intraluminal fluid to the region of interest. Left lateral recumbency helps with the evaluation of the fundus, while right lateral recumbency improves the evaluation of the pylorus. The standing position is most appropriate for evaluating the ventral aspect of the pylorus and body of the stomach. However, the results of these positional studies also depend on the dog's conformation, the degree of stomach dilation and the nature of gastric contents.
High frequency probes are recommended for evaluating the GI wall layers. Transverse and longitudinal views of GI segments are required to fully assess the thickness and the extent of a suspected lesion.
Initially, little patient preparation is required, although a 12-hour fast is recommended to minimize the amount of intraluminal gas. In cases with suspected proximal lesions, fluid administration can be used to create an optimal acoustic window.
Normal GI Tract
The stomach is easily recognized by its location, the presence of rugal folds and the peristaltic activity. The stomach crosses the cranial abdomen perpendicularly in the dog and obliquely in the cat just caudal to the liver. The mean of the observed rate of peristalsis is 3 to 5 contractions per minute in dogs. A wide range of gastric distention can be seen. The gastric wall thickness can be assessed by measuring the distance from the hyperechoic mucosal surface to the outer hyperechoic serosal layer. The normal thickness can reach up to 5 mm.
The appearance of the bowel varies with the degree of distention and the nature of GI contents. The proximal duodenum is identified by its proximity to the pylorus and its superficial, rectilinear course along the right lateral abdominal wall. The jejunum occupies a large portion of the mid abdomen. The ileocecocolic junction can be identified in dogs and cats.
The descending colon can be identified with certainty by its thin wall, its proximity to the bladder and is usually air-filled with a crescent-shaped gas interface.
The luminal contents determine the 4 different types of GI patterns:
1. The alimentary pattern represents irregular or regular food particles within the gastric lumen. It may or may not be associated with imaging artifacts.
2. The mucous pattern is seen as a bright interface in the lumen, which is not associated with acoustic shadowing.
3. The fluid pattern is characterized by anechoic (at times echogenic) luminal contents.
4. The gas pattern appears as an intraluminal, hyperechoic, reflective interface with acoustic shadowing. Gas acts as an acoustic barrier that reflects most of the incident sound, therefore preventing evaluation of deeper structures.
Five ultrasonographic layers can be identified from the lumen to the serosal surface:
1. Mucosal surface, hyperechoic
2. Mucosa, hypoechoic
3. Submucosa, hyperechoic
4. Muscularis, hypoechoic
5. Subserosa/serosa, hyperechoic
While evaluating a suspected abnormal GI segment, one should carefully examine the wall thickness, wall layering, symmetry of the wall at the lesion site, extension of the lesion, GI contents, GI motility and possible regional or systemic involvement.
The most common ultrasonographic sign of GI diseases is wall thickening. However, this finding is nonspecific and is reported in neoplastic as well as in inflammatory changes. Commonly, gastrointestinal lesions are associated with a mild to severe fluid accumulation in part due to motility disturbances. This feature represents a useful landmark for suspecting a GI lesion and should always be investigated further.
A few GI disorders such as intussusception, GI foreign bodies, and mechanical ileus are selected to illustrate the ultrasonographic features and interpretation principles.
Special attention will be placed on features helping to differentiate inflammatory from neoplastic lesions. The ultrasonographic appearance of inflammatory GI diseases varies with the type of the pathological process, the extent of involvement. Wall thickening is the most common finding of inflammatory diseases. Symmetry, the extent of the wall thickening and layer identification are used to distinguish inflammation from neoplasia. Inflammation is usually characterized by extensive and symmetric wall thickening with preserved layering; while neoplasia is often associated with localized, asymmetric wall thickening with disrupted layering.
Finally, US-guided procedures on GI lesions will be discussed.