Ultrasonography of the Gastrointestinal Tract: Stomach, Duodenum, and Jejunum (2023)

Localization and Scanning Technique

Typically, an 8 to 10-MHz curved array or 12-MHz linear transducer is used for dogs and cats. Animals are usually scanned in dorsal recumbency; however, right and left lateral recumbency may assist with displacement of gas and fluid in the stomach to better visualize deeper structures. Longitudinal axis and transverse axis views of the different segments of the gastrointestinal tract are necessary for a complete examination.

Maintain a consistent sequence when evaluating the complete gastrointestinal tract; preferably, in the following order: all parts of the stomach (fundus, body, pyloric antrum), pyloroduodenal junction (pyloric sphincter), duodenum, jejunum, ileum, ileocecocolic (cat) or ileocolic (dog) junction, cecum, and parts of the colon (ascending, transverse, descending).

Stomach

The stomach is scanned initially in long axis plane, relative to the patient, which creates a transverse view of the stomach, beginning at the fundic portion located immediately caudal to the left division of the liver. The fundus is located in the left craniolateral quadrant, the body of the stomach is located closer to midline as the transducer is swept to the right of the patient, and the pyloric antrum can extend to the right side of the patient depending on the degree of distension.

The pylorus and pyloroduodenal junction is found closer to midline in most cats. In deep chested dogs, a right dorsal intercostal approach may be needed to better visualize the pyloroduodenal junction. Occasionally, the gastroesophageal junction (cardia) may be visualized.

Duodenum

After imaging the pyloroduodenal junction, the orad portion of the descending duodenum is visualized. The descending duodenum is followed caudally, keeping it in long axis, along the right lateral abdominal body wall in the dog. An intercostal approach may be needed to identify the cranial aspect of the descending duodenum in the dog. When the patient is placed in dorsal recumbency, the right kidney may be used to identify the duodenum as it will be located ventral or ventrolateral to that kidney within the near field of the image.

The descending duodenum in the cat will be either in a midline position or just to the right of midline.

Jejunum

The jejunum is evaluated in its entirety by sweeping the transducer back and forth (side to side) across the abdomen in an overlapping pattern, beginning cranially and slowly progressing caudally.

It may not be possible to trace the jejunum continuously from orad to aborad due to gas interposition or shadowing artifacts from intestinal contents.

Normal Ultrasonographic Features of the Gastrointestinal Tract

Before imaging the gastrointestinal tract, the patient should be fasted, however, this may not be feasible in all circumstances. Ideally, fasting will prevent ultrasound artifacts, such as reverberation artifact and beam attenuation,¹ from impeding the structures either adjacent and dorsal to the gastrointestinal tract or the far wall of the gastrointestinal tract that is being imaged.

Reverberation artifact appears as multiple, equidistantly spaced linear reflections (FIGURE 1). This artifact occurs when multiple echoes are erroneously processed due to a delayed return of the signal.¹

Beam attenuation appears as a reduction of the ultrasound signal at depth in the far field (FIGURE 2). This is due to the attenuation of the ultrasound beam in the near field secondary to gastrointestinal contents.¹

The layering of the walls of the gastrointestinal tract can be assessed using ultrasonography and has a characteristic pattern of alternating hyper-and hypoechoic layers (FIGURE 3); the luminal-mucosal interface, submucosal, and serosal layers are hyperechoic; and the mucosal and muscularis layers are hypoechoic. An easy mnemonic is M&M’s (mucosa and muscularis) are chocolate (dark/hypoechoic).

The gastrointestinal tract layering is as follows from the lumen, centrally, to the serosal margin, peripherally:

1. Interface between lumen and mucosa (hyperechoic)
2. Mucosa (hypoechoic)
3. Submucosa (hyperechoic)
4. Muscularis (hypoechoic)
5. Serosa (hyperechoic)

Canine and feline gastrointestinal wall thicknesses vary depending on the segment assessed (TABLE 1).

Stomach

The normal canine and feline stomach is located caudal to the liver. The different portions of the stomach include the cardia, fundus, body, and pyloric antrum, leading into the pyloroduodenal junction (pyloric sphincter).

In most cases, the cardia is not identified due to its cranial location and interposition of the liver, although, occasionally, it can be identified through dorsal intercostal acoustic windows.

The fundus, located in the left cranial abdominal cavity, is scanned in longitudinal and transverse axes. Next, the transducer is moved medially towards midline to scan the body of the stomach.

In the feline patient, the body of the stomach can be found on the left of midline; the canine gastric body can be located right of midline if ingesta, gas, and/or fluid are present within the lumen of the stomach (FIGURE 4).

The pyloric sphincter can be recognized due to its hyperechoic mucosa in contrast to the pyloric antral mucosa and duodenal mucosa, which are hypoechoic.

The transverse section of the empty feline stomach has a characteristic wagon wheel appearance, often with a thick, hyperechoic submucosal layer due to fat deposition (FIGURE 5).

Duodenum and Jejunum

The duodenum in the dog is the thickest segment of the small intestinal tract and contains the thickest mucosal layer, representing 63% of the total wall thickness. At times, Peyer’s patches, or pseudoulcers, can be seen when using ultrasonography, forming focal depressions of the mucosal surface (FIGURE 6).

The feline duodenum has a similar thickness and appearance to the jejunum; the mucosa is not as apparent as in the dog (FIGURE 7).

Within the cranial aspect of the descending duodenum, the major duodenal papilla can be seen (FIGURE 8), particularly when using a high resolution, high frequency, linear or curved array transducer. The major duodenal papilla in the cat varies from 2.9 to 5.5 mm in width and has a maximum thickness of 4 mm on the transverse view.¹³

In normal dogs and cats, the small intestines are relatively uniform in distribution. Depending on the segment of small intestine, some layers may be thicker than others. This can be used to identify the different segments of intestines. For example, in the dog, the mucosal layer of the duodenum is thicker than the mucosal layer of the jejunum.

Stomach Abnormalities

Dilation

If the stomach becomes progressively distended or dilated, the stomach wall will become thinner, the wall layering will be difficult to distinguish, and the rugal folds will be less distinct.

Depending upon its composition, gastric content may be hypoechoic to hyperechoic.

A gas dilated stomach may contain reverberation artifact within the far field of the image, resulting in the inability to visualize abnormalities of the dorsal aspect of the gastric wall or lumen.

Pyloric Outflow Obstruction

Causes of pyloric outflow obstruction include pyloric stenosis, foreign bodies, inflammatory disease, and neoplasia.

Congenital hypertrophic pyloric stenosis causes circumferential thickening of the pylorus and is more common in dogs than cats. Ultrasonographically, gastric wall thickness > 6 to 7 mm and muscular layer thickness > 4 mm is considered pathologic (FIGURE 9).²

Foreign bodies lodged in the pyloric region typically have an irregular or geometric shape and strong acoustic shadowing.

Chronic hypertrophic pyloric gastropathy also causes muscular or mucosal hypertrophy; pyloric wall thicknesses for affected dogs ranges from 9 to 15.3 mm, and the thickness of the muscular layer ranges from 3 to 5.4 mm.⁹

Gastric Foreign Bodies

Gastric foreign bodies are often diagnosed on survey radiographs and can sometimes be diagnosed using ultrasound, depending on the contents in the gastric lumen. Often, foreign material has a hyperechoic interface with intense distal acoustic shadowing. If the foreign body is surrounded by fluid, it can be readily seen (FIGURE 10). The shape and size of the foreign material varies.

Gastric Wall Thickening

Non-neoplastic lesions, such as gastritis, can produce a diffuse, mild to moderate thickening with preservation of the wall layering (FIGURE 10).

Neoplastic lesions usually cause focal loss of wall layering along with varying degrees of wall thickening (FIGURE 11).^8,14,15

Malignant Gastric Neoplasms

Adenocarcinoma is the most common gastric neoplasm in the dog. This neoplasm is extremely rare in the cat.

Most carcinomas are located in the lesser curvature and pylorus in the dog.

Features of gastric adenocarcinoma include a pseudo-layered pattern, asymmetrical transmural thickening, and altered wall layering with a poorly echogenic lining to the mucosal and/or serosal wall layers.^16-18

A pseudo-layered pattern has been described in some canine gastric epithelial neoplasia, such as carcinoma.¹⁷ In that study, pseudo-layering was characterized by transmural thickening with altered wall layering, as well as a poorly echogenic lining along the innermost and/or outermost portions of the gastric wall, separated by a more echogenic central region.¹⁷

Leiomyosarcomas produce focal masses, often involving the gastric antrum, and thickening of the muscular layer of the gastric wall.¹⁹ These neoplasms are usually small, rounded masses that protrude into the gastric lumen at the level of the cardia. The luminal surface of these lesions is usually smooth, due to their origin in the muscularis layer.

Lymphoma is the most common gastric neoplasm in the cat. It appears as a focal mass, multiple masses, or diffuse infiltrative neoplasia, characterized by thickening and/or loss of normal layered appearance to the wall (FIGURE 11).^20-22

Features of malignant histiocytosis include a single, well circumscribed, hypoechoic mass with well-defined borders and an abnormal loss of wall layering in the dog.²³

Benign Gastric Neoplasms

Adenomas can occur in dogs and cat and can appear flat or polypoid.¹⁸

Gastric leiomyomas are the second most common neoplasm in the stomach of a dog. They form single or multiple, sessile, round polyps protruding into the lumen.²⁴ The most common locations include the gastric cardia or gastroesophageal junction.²⁵ Leiomyomas cannot be differentiated from leiomyosarcomas using ultrasonography alone; cytology or histopathology are required for definitive diagnosis.

Other Causes of Gastric Wall Masses or Abnormal Wall Layering

Chronic hypertrophic gastritis can cause severe gastric wall thickening without a loss of wall layering, or a thickened, hypoechoic layer can be seen surrounding the pyloric lumen (interpreted as a thickened muscularis layer, histologically).⁹ In particular, the rugal folds of the mucosal layer become severely thickened and project into the lumen.^26,27

Eosinophilic sclerosing fibroplasia occurs in cats. Ultrasonographically, they are focal mass lesions or mural thickening at the pyloric antrum with a loss of wall layering (FIGURE 12).^28,29

Pythiosis is a chronic, pyogranulomatous infection caused by the water mold Pythium insidiosum. This can cause focal thickening of the gastric wall, with partial or complete obliteration of the wall layers (FIGURE 13).^30,31

The site of a gastropexy may have a focal thickening and alteration of normal wall layers.^6,32

Uremic gastritis can be seen in patients with chronic uremia. Findings include thickening of the gastric wall and a hyperechoic line in the mucosal or submucosal layer, representing mineralization.^33,34

Nonspecific gastritis, such as infectious or toxic (eg, peroxide toxicity), can cause wall edema and wall thickening without a total loss of wall layering. Absence of visible abnormalities on ultrasound does not rule out gastritis. Occasionally, perigastric hyperechoic fat is noted in association with gastritis.

Duodenum and Jejunum Abnormalities

Foreign Body

A large foreign body can cause a distinct hyperechoic interface with strong distal acoustic shadowing.^8,35-37

Linear foreign bodies have a characteristic appearance as a result of the plication of the small intestines. Oftentimes, the linear foreign body itself is identified, forming a focal hyperechoic linear band seen centrally within the affected small intestinal lumen (FIGURE 14).^36,38

The bowel proximal to an obstructive foreign body is typically dilated with fluid, gas, and possible food material, whereas the bowel distal to the obstruction is empty or normal. If a foreign body is suspected in a dog with dilated segments of small intestine, following the dilated loops of small intestine will facilitate detection of the foreign body.

Circumferential loss of the normal, hyperechoic submucosal layer has been shown to represent extensive submucosal ulceration and necrosis, correlated with a greater likelihood of perforation, in humans with appendicitis.^39,40

Intussusception

Most intussusceptions occur in young dogs and are secondary to viral, bacterial, and parasitic etiologies. In older dogs or cats, intussusceptions are often triggered by focal infiltrative disease of the intestine, such as neoplasia, and the intestinal wall in the vicinity of the intussusception should be carefully scrutinized to rule out such conditions (FIGURE 15).

Intussusceptions are named according to the segments involved. Jejuno-jenunal, ileocolic, and cecocolic (cecal inversion) intussusceptions are the most common types.

An intussusception has a multilayered appearance in longitudinal axis and a concentric ring appearance (“bullseye pattern”) in transverse axis (FIGURE 16).^{8,35,37,41,42}

Lymphangiectasia

Lymphangiectasia is pathologic dilation and rupture of lymphatic vessels with leakage of lymphatic contents.

Intestinal changes consistent with lymphangiectasia include a combination of intestinal wall thickening, linear areas of striated hyperechogenicity of the small intestinal mucosal layer that are perpendicular to the long axis of the intestine, small intestinal wall corrugation, indistinct small intestinal wall layering, and small intestinal hypermotility (FIGURE 17).^43,44

Concurrent anechoic peritoneal effusion may be present due to hypoproteinemia secondary to protein-losing enteropathy.

Duodenal and Jejunal Wall Thickening

Focal wall thickening with a loss of wall layering are commonly seen with intestinal focal neoplasia (FIGURE 18).^14,35,45,46 The most common intestinal tumors of dogs are leiomyosarcoma, lymphoma, and adenocarcinoma.^47-50 Smooth muscle tumors of the intestines, such as leiomyosarcoma, often appear as large masses, eccentrically projecting from the intestinal wall, containing single or multiple hypo- or anechoic regions.⁵⁰

Carcinoma is a localized, irregular, often mixed echogenicity thickening of bowel wall with a loss of layering;⁵¹ it can also present as an annular, constrictive lesion (FIGURE 19) that might be difficult to see on ultrasound, due to the gravel sign surrounding the area caused by the chronic partial obstruction.

Although more common in the large intestine, gastrointestinal stromal tumors can also be seen in the small intestine.⁵² No unique features of gastrointestinal stromal tumors have been recorded to differentiate from other gastrointestinal spindle cell tumors.⁵²

In cats, common intestinal tumors include lymphoma and adenocarcinoma. Mast cell tumor and hemangiosarcoma have also been reported in the cat.^44-47

Multicentric small intestinal neoplasia, particularly lymphoma, has moderately to severely thickened walls⁵³ with muscularis layer thickening (FIGURE 20).⁵⁴ Muscularis layer thickening is not definitive in diagnosing infiltrative neoplasia as it is also seen in cats with inflammatory bowel disease; however, it has been determined that the odds are high for a cat with muscularis thickening to have lymphoma.⁵⁴ Thickening of the muscularis may be explained by the fact that lymphoma commonly occurs in conjunction with inflammatory bowel disease in cats,⁵⁴ as chronic gastrointestinal inflammatory processes in cats can transform to develop subsequent gastrointestinal lymphoma.⁵⁵

Additional imaging findings of lymphoma include circumferential, homogeneous, hypoechoic thickening of the small intestinal walls with a loss of normal wall layering;⁴⁵ regional, moderate, hypoechoic lymphadenopathy is generally present (FIGURE 21). A complication of infiltrative intestinal neoplasia includes mechanical obstruction due to intraluminal narrowing.

Common inflammatory bowel diseases, such as lymphocytic-plasmacytic enteritis, are usually associated with mild to moderate wall thickening affecting several or all intestinal segments with variable severity. Other ultrasonographic features of intestinal inflammatory diseases include circumferential, mild to moderate wall thickening affecting primarily the mucosa, submucosa, and/or muscularis layers (FIGURE 22);⁵⁴ diffuse increased echogenicity of the mucosa; or the presence of bright mucosal speckles.^45,56 The bright mucosal speckling has been postulated to represent a section through dilated lacteals or focal accumulation of mucus, cellular debris, proteins, and/or gas within the mucosal crypts.⁵⁷

There is significant overlap between the ultrasonographic appearance of inflammatory bowel disease and small cell lymphoma in cats, and the differentiation between these two entities is often impossible based on ultrasound findings alone.^53,54

The presence of large, rounded hypoechoic mesenteric lymph nodes should be evaluated for potential multicentric disease (lymphoma or pythiosis) or metastatic disease (adenocarcinoma).

In dogs, intestinal tumors have significantly greater wall thickness, loss of wall layering, and more focal lesions than seen with enteritis.⁴⁵ However, the absence of wall thickening does not completely rule out inflammatory disease as the correlation between wall thickness and histopathological diagnosis of inflammatory bowel disease in dogs have not been seen.⁵⁸ To obtain a definitive diagnosis, histopathology of the affected area is required.⁵⁹

Summary

A systematic examination of the gastrointestinal tract is a routine part of the complete ultrasonographic abdominal evaluation. In Part 1 of the gastrointestinal tract, the normal and common abnormal ultrasound findings of the stomach, duodenum, and jejunum have been presented. The ileum, cecum, and colon will be addressed in Part 2.

References