Basic CT findings and adhesive obstruction
Computed tomography allows for precise planning of conservative treatment or emergency surgery. On scans, obstruction is visualized as dilated (fluid- and gas-filled) proximal loops and collapsed distal bowel loops.
A key landmark is the transition zone (transition zone). This is a sharp boundary between the dilated and narrowed segments. If no visible mass is present in this zone, an adhesive etiology is most likely. Adhesions themselves are not visible on CT, but they are indirectly indicated by fixed or angulated deformations of the loops.
A specific marker of intestinal content stasis is the small-bowel faeces sign (small-bowel faeces sign). It is identified within the dilated loops proximal to the transition zone and appears as a cluster of gas bubbles mixed with dense content resembling feces.
Strangulation and closed-loop sign
Detecting strangulation is a critical task in diagnostic imaging, as this condition requires emergency surgical intervention.
Strangulating obstruction often develops as a closed-loop (closed-loop obstruction), where a segment of the bowel is isolated and occluded at two points. It is characterized by:
- Formation of a sharply dilated U-shaped or V-shaped segment.
- Radial arrangement of fluid-filled loops.
- Convergence of mesenteric vessels toward the point of obstruction.
- The whirl sign (whirl sign) or beak sign (beak sign).
Signs of vascular impairment and developing necrosis include bowel wall thickening, high X-ray attenuation due to hemorrhage, and reduced contrast enhancement (hypoenhancing) compared to healthy areas. The presence of gas in the bowel wall (pneumatosis) and mesentery definitively indicates infarction.
Specific causes of obstruction
In addition to adhesions, small bowel obstruction can be caused by a number of other pathologies with distinct radiological presentations:
- Gallstone ileus (Gallstone Ileus): Occurs due to a biliodigestive fistula. Plain radiography and CT reveal free gas in the biliary tract — pneumobilia (pneumobilia). The cause of obstruction is a gallstone (often with laminated calcification) trapped in the distal ileum.
- Intussusception (Intussusception): Telescoping of one segment of the bowel into another. On CT, mesenteric fat accompanies the entering segment (intussusceptum) within the receiving segment (intussuscipiens). On ultrasound, this presents as the "pseudokidney" sign. In adults, the leading point is often carcinoid tumors, lymphomas, or polyps.
- Phytobezoars: Visualized as a large filling defect within a dilated loop (e.g., in the jejunum).
Acute mesenteric ischemia
Bowel wall ischemia can be either a consequence of strangulation or a primary condition (arterial embolism, venous thrombosis).
CT semiotics heavily depend on the type of lesion. In acute transmural infarction, the wall becomes thinned and fails to enhance with contrast. In venous thrombosis, conversely, prominent wall thickening, mucosal hyperenhancement, mesenteric stranding, and vascular engorgement are observed. Most patients with acute ischemia also present with ascites. The presence of parenchymal organ infarctions should suggest a cardioembolic origin of ischemia (e.g., in the setting of atrial fibrillation).
Briefly: answers to topical questions
What ultrasound signs are characteristic of intussusception?
Intussusception is characterized by the following ultrasound signs:
- Target sign (target sign) or doughnut sign (doughnut sign) in the transverse plane.
- Sandwich appearance (sandwich appearance) in the longitudinal plane, with visualization of multiple layers of the bowel wall and mesenteric fat.
- Crescent-in-doughnut sign (crescent in doughnut sign) — a hyperechoic crescentic structure caused by the drawing of mesenteric fat into the intussusceptum.
What is the normal thickness of the small bowel wall on CT?
Normal bowel wall thickness is less than 3 mm.
What are the anatomical types of intussusception?
The following anatomical types of intussusception are distinguished:
- ileocolic — the most frequent type;
- ileoileocolic;
- ileoileal (small-small bowel);
- colocolic (large-large bowel). The latter three types are significantly less common.
Class and exam questions
How to differentiate adhesive obstruction from tumor obstruction on CT?
The diagnosis is established by the presence of a "transition zone." If no soft-tissue mass is visualized at the site of sharp bowel narrowing, this confirms an adhesive etiology of the obstruction.
What is the small-bowel feces sign?
This is a CT sign of intestinal content stasis in dilated loops proximal to the site of obstruction, where the intraluminal mass has a fecal-like structure (a mixture of gas and dense masses).
What is the danger of a closed-loop obstruction?
An isolated bowel segment occluded at two points undergoes rapid overdistension. This leads to compression of mesenteric vessels, strangulation, wall ischemia, and a sharp spike in mortality up to 37%.
What else to study on this topic
- Large bowel obstruction and the risk of cecal perforation
- Sigmoid and cecal volvulus: differential CT diagnosis
- Radiation enteritis and the "matted loop" sign
- Colonic pseudo-obstruction in elderly patients
- Jejunal diverticula and blind loop syndrome
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