15.3 GI Tract, Adrenals, Lymph Nodes, Hernias & Peritoneal Cavity

Bowel Sonography and the Gut Signature

Although gas limits sonography, bowel can be assessed with a high-frequency linear transducer and graded compression - slow, steady pressure that displaces or compresses normal bowel. Normal bowel demonstrates the gut signature: alternating echogenic mucosa and hypoechoic muscularis layers surrounding a central echogenic interface, with observable peristalsis in real time. Intraluminal gas casts dirty shadowing and reverberation, while fluid-filled loops are easy to see through. Aperistaltic, dilated, thick-walled bowel is always suspicious.

Appendicitis

Acute appendicitis is the classic application of graded compression. Using the linear transducer over the point of maximal tenderness in the right lower quadrant, the inflamed appendix appears as a noncompressible, blind-ending, aperistaltic tubular structure arising from the cecum. An outer diameter greater than 6 mm is abnormal. Supporting findings include:

• Appendicolith - echogenic focus with clean shadowing within the lumen
• Periappendiceal echogenic fat - inflamed mesenteric fat surrounding the appendix
• Increased color Doppler flow - mural hyperemia
• Focal tenderness directly over the appendix, the sonographic McBurney sign
• Periappendiceal fluid or a complex mass suggesting perforation and abscess

Intussusception and Diverticulitis

Intussusception is telescoping of one bowel segment into the next, most often ileocolic in young children presenting with colicky pain and currant-jelly stools. In transverse it creates a target or doughnut sign of concentric rings; in longitudinal planes it produces the pseudokidney (sandwich) sign. In children it is often idiopathic and may be reduced with an air or contrast enema, while in adults a lead point such as a polyp or tumor must be excluded. Diverticulitis, usually sigmoid, shows a hypoechoic inflamed outpouching from the colonic wall with surrounding echogenic inflamed fat and segmental wall thickening; graded compression over the left lower quadrant reproduces focal tenderness, and complicated disease may form a pericolic abscess.

Adrenal Glands

Adult adrenal glands are thin, inverted Y- or V-shaped structures that are difficult to image unless enlarged. The right adrenal is easier: use the liver as a window and look above the right kidney between the liver, inferior vena cava (IVC), and right crus of the diaphragm. The left is obscured by stomach gas; search between the aorta, splenic vessels, and upper pole of the left kidney. In neonates the adrenal is proportionally large - about one third the size of the kidney - with a thin echogenic medulla and thick hypoechoic cortex, and adrenal hemorrhage from birth stress appears as a suprarenal mass that involutes over weeks.

Lymph Nodes

Normal and reactive lymph nodes are oval or flat, under about 1 cm in short-axis diameter, with a preserved echogenic fatty hilum and hilar blood flow. Malignant nodes - from lymphoma or metastatic disease - become round (a long-to-short axis ratio under 2), markedly hypoechoic, and lose the fatty hilum; they may show eccentric cortical thickening and peripheral or mixed vascularity. Systematically survey the para-aortic and paracaval regions, mesentery, and inguinal chains when staging abdominal malignancy.

Hernias

Abdominal wall and inguinal hernias are imaged with a linear transducer while the patient performs a Valsalva maneuver or stands upright, which drives contents into the defect. Common sites include the inguinal canal (indirect hernias pass through the internal ring lateral to the inferior epigastric vessels, direct hernias medial to them), the umbilicus, and prior surgical incisions. Hernia contents may be bowel, recognized by the gut signature and peristalsis, or omentum, which is echogenic and nonperistaltic. Document the fascial defect size, reducibility, and content. Complications include incarceration, where contents cannot be reduced, and strangulation, where compromised blood supply produces aperistaltic dilated bowel, absent Doppler flow, and surrounding fluid.

The Peritoneal Cavity

Free intraperitoneal fluid (ascites) obeys gravity and collects in dependent recesses. In the supine patient, Morison's pouch (the hepatorenal recess between the liver and right kidney) is the most dependent site in the upper abdomen, while the pelvis - the pouch of Douglas in women or the rectovesical space in men - is the most dependent overall. Also check the paracolic gutters and perihepatic and perisplenic spaces. Simple ascites is anechoic and shifts with repositioning; loculated fluid is trapped by adhesions, fails to move, and may contain septations or debris, suggesting infection, hemorrhage, or malignancy. Peritoneal carcinomatosis combines ascites with omental caking - a sheet-like echogenic omental mass floating in fluid - and discrete peritoneal implants, most often from ovarian, gastric, or colon primaries. When carcinomatosis is suspected, scrutinize the omentum anterior to the bowel, the undersurface of the diaphragm, and the serosal surfaces of the liver and spleen for soft-tissue nodules.