In the setting of acute abdominal pain, US examination should include the GI tract as well as the solid organs of the abdominal cavity. In women, transvaginal US should be added if transabdominal US is not conclusive. Graded compression US is utilized to examine the intestinal tract [2]. The patient is asked to localize the area of maximal pain or tenderness and attention is focused on that area. A linear array 7-to 10-MHz transducer is utilized. In large patients a 5-MHz linear or curved array transducer may be needed. Color Doppler US greatly aids in the detection of inflammation.

The GI tract has a recognizable “gut signature” that identifies bowel during US examination. Although the wall of the intestinal tract has 5 histologic layers, a 3-layer pattern is usually demonstrated by US (Fig. 4.1). The innermost mucosa and submucosa are seen as a distinct echogenic rind that surrounds the gut lumen. The inner circular and outer longitudinal layers of the muscularis propria produce a central hypoechoic muscle layer on US. The outer adventitia/serosa and surrounding fat produce an outer echogenic layer. Thus the usual gut signature shown by US is two echogenic layers separated by a hypoechoic muscle layer that define the wall of the GI tract. The lumen of the gut may contain fluid, food, stool, and air in varying combinations at different locations. Portions of the GI tract are identified by location, appearance, peristaltic activity, and luminal contents.

The stomach is a large sac in the left upper quadrant and epigastrium. It commonly contains fluid and air and, if the patient has recently eaten, food. The antrum extends across the pancreas to the duodenum. The duodenal bulb is commonly just inferior to the gallbladder. The third portion of the duodenum is the only portion of the GI tract that extends posterior to the superior mesenteric artery (SMA) and vein (SMV). The jejunum and ileum occupy the central abdomen. The jejunum is identified by its regular folds, the valvulae conniventes, that produce a “keyboard” pattern seen when fluid is in the lumen (Fig. 4.2). The ileum lacks folds and has a flat featureless wall pattern.

The normal appendix is a narrow tube approximately 10 cm in length (Fig. 4.3). It arises from the tip of the cecum approximately 3 cm below the ileocecal valve. The base of the appendix has a constant relationship to the tip of the cecum, but the remainder of the appendix is free and variably located. It may be behind or below the cecum, behind or in front of the distal ileum, in the pelvis, or occasionally in the left side of the abdomen. The great variability in location influences the clinical presentation of acute appendicitis.

In short axis section the appendix is round, whereas the ileum and colon are oval or elliptical (Fig. 4.4). The colon is identified by its larger size, haustral pattern, and stool content. Stool is heterogeneous in echotexture and liquid, semi-solid, or solid in consistency. Stool commonly contains tiny gas bubbles that produce bright echoes at varying depths.

Doppler routinely shows little or no signal from the gut wall. Increased signal is highly indicative of the increased vascularity of an acute inflammatory process or a neoplasm [3,4].

The normal bowel wall is 2-5 mm in thickness depending upon distention (Fig. 4.1). Wall thickening is nearly always pathologic but is often a non-specific finding. More precise diagnosis is made by detailed examination and correlation with clinical findings [5]. Benign wall thickening produces a target or bull’s-eye appearance and is usually uniform and symmetric (Figs. 4.5, 4.6). Any wall thickening, focal or diffuse, that exceeds 25 mm in thickness is more likely to be malignant. Masses demonstrate focal, asymmetric wall thickening, and may extend intraluminally or be exophytic (Fig. 4.7). Differential diagnosis of thickening of the wall of the small bowel and colon is listed in Boxes 4.1 and 4.2 [5].

The differential diagnosis of vomiting in the first two months of infancy includes hypertrophic pyloric stenosis (HPS), pylorospasm, malrotation with Ladd’s bands or midgut volvulus, duodenal atresia, and gastroesophageal reflux. Barium studies are preferred in the diagnosis of gastroesophageal reflux and confirmation of malrotation. US is the examination of choice for HPS [13].

The US examination is performed with fluid in the stomach and the infant in right posterior oblique position. This position places fluid in the antropyloric region for optimal visualization. The infant can be fed glucose solution if the stomach is empty. However, excessive feeding must be avoided because overdistention of the stomach displaces the pylorus posteriorly and makes examination more difficult [14]. Imaging is performed in the epigastrium along the edge of the liver. The fluid-filled antrum is tracked to the pylorus. The pylorus is carefully evaluated in transverse and longitudinal section for documentation of muscle thickness and length of the pyloric channel, and observation for peristalsis and fluid movement through the pylorus.