Ch08: Acute Abdomen Imaging（急腹症影像）

# Ch08: Acute Abdomen Imaging（急腹症影像）

Preparatory Mindset

The acute abdomen = a variety of illnesses causing acute abdominal pain from ischemia, infection or inflammation. The imaging approach is algorithmic: Is there pneumoperitoneum? → think perforation. Is there bowel dilation / a transition point? → think obstruction (find the cause). Are there signs of ischemia (pneumatosis, portal venous gas, non-enhancing bowel)? → this is a SURGICAL EMERGENCY. Modalities: abdominal X-ray (rapid screening), ultrasound (children/pregnancy/RUQ/gynecology), CT (gold standard for most non-obstetric acute abdomens).

Core Concepts

Imaging modalities

• Plain radiography (X-ray): rapid screening for pneumoperitoneum, obstruction, foreign bodies. Views: erect CXR (free air) + supine/erect AXR. Limited sensitivity.
• The ABCs of reading an AXR: A — Air (bowel gas pattern, free air, abnormal air collections); B — Bony (spine, ribs, pelvis fractures); C — Calcifications (renal/ureteric/gallstones, calcified structures).
• Ultrasound: first-line in children, pregnant women, right upper quadrant/gynecological pain; excellent for appendicitis (children), cholecystitis, AAA. No radiation, real-time.
• CT: gold standard for most non-obstetric acute abdomens; highest sensitivity/specificity; IV contrast essential; oral/rectal contrast situation-dependent; visualizes entire abdomen + retroperitoneum + abdominal wall.

Normal AXR landmarks

Hepatic angle, splenic angle, renal shadows, psoas muscle, properitoneal fat strip; normal bowel gas: stomach/duodenum gas, colon (mottled stool pattern), few small-bowel gas collections.

Calcifications on AXR

Gallstones (15–20% contain enough calcium to be visible; RUQ; laminated appearance), porcelain gallbladder (wall calcification → chronic obstruction + increased gallbladder carcinoma risk), urinary calculi (85% visible on plain film; staghorn calculus fills the collecting system), calcified adrenal glands (newborn hemorrhage, TB, Addison), pancreatic calcifications (chronic alcohol-induced pancreatitis; coarse punctate; extend across LUQ).

Bowel dilation thresholds

• Small bowel dilated > 2.5–3.0 cm; colon > 5 cm; cecum > 8 cm.

Mechanical small bowel obstruction (SBO)

• Dilated loops of small bowel (>3 cm); air-fluid levels >2.5 cm long; air-fluid levels at differing heights within the same loop (stair-step sign — strong evidence of obstruction); small bubbles of gas trapped between valvulae conniventes; colonic collapse; paucity of air in colon/rectum.
• Causes: adhesions (most common), hernias, tumors, intussusception, Crohn stricture, gallstone ileus, volvulus.
• Erect film: air-fluid levels at different heights; valvulae conniventes extend across the entire lumen diameter.

Mechanical large bowel obstruction (LBO)

• Most colonic obstructions in the sigmoid colon.
• Dilation of colon from cecum to the point of obstruction; paucity/absence of gas distal; air-fluid levels in dilated colon (upright); small bowel distention if ileocecal valve incompetent.
• Causes: carcinoma (commonest), diverticulitis, volvulus, stricture.

Sigmoid volvulus

• Large gas-filled loop (inverted U / coffee-bean shape) without haustra, arising from the pelvis and extending high into the abdomen, often to the diaphragm; no gas in the rectum; barium enema shows beaking sign at the twist; CT: "whirl" of the mesentery at the point of volvulus.

Adynamic (paralytic) ileus

• Reduced peristalsis without mechanical obstruction; decreased/absent peristalsis; less intraluminal fluid; diffuse gaseous distension of small bowel and colon/rectum (gas in the rectum favors ileus over obstruction).

Critical imaging signs

• Pneumoperitoneum (free air): indicates hollow viscus perforation. X-ray: free air under the diaphragm (erect view); CT: free air outlining the abdominal wall or between bowel loops. Common causes: GI perforation, trauma, recent surgery.
• Pneumatosis intestinalis & portal venous gas: critical sign of bowel ischemia/infarction — gas within the bowel wall or portal venous system.

Bowel ischemia (surgical emergency)

Signs: bowel wall thickening/edema; hypoenhancement or non-enhancement; mesenteric fat stranding/fluid; vascular thrombosis (SMV/SMA); pneumatosis intestinalis & portal venous gas; strangulated SBO (adhesions/volvulus); decreased enhanced small bowel in ischemia.

Perforation & acute abdomen summary

• Pneumoperitoneum → PERFORATION.
• Bowel dilation/transition point → OBSTRUCTION (find the cause).
• Pneumatosis, portal gas, non-enhancing bowel → SURGICAL EMERGENCY.

High-Yield Points

• Key Point: Erect CXR is the best plain-film test for free air (under the diaphragm).
• Key Point: SBO: dilated small bowel >3 cm, stair-step air-fluid levels, collapsed colon/rectum.
• Key Point: LBO: dilated colon to the transition point, absent gas distally; sigmoid commonest site.
• Key Point: Sigmoid volvulus = coffee-bean/inverted-U gas loop + no rectal gas + beaking on enema.
• Key Point: Paralytic ileus = gas throughout including rectum, no transition point.
• Key Point: Pneumatosis + portal venous gas = ischemia/infarction = surgical emergency.
• Key Point: CT with IV contrast = gold standard for acute abdomen; US first-line in children/pregnancy.

Topic Summary

The acute abdomen is approached with a three-question algorithm: perforation (pneumoperitoneum on erect CXR/CT), obstruction (dilated loops + transition point + air-fluid levels; SBO vs LBO vs volvulus vs ileus), and ischemia (pneumatosis, portal venous gas, non-enhancing bowel — surgical emergency). AXR uses the ABCs (air, bony, calcifications); US is first-line for children/pregnancy; CT with IV contrast is the gold standard for most non-obstetric acute abdomens.

Illustrations（图解速览）

Case gallery for this chapter — identify the imaging technique, location, features and diagnosis for each: