The study that names the transition point and answers ischaemia — read on coronals, and re-window for pneumatosis.
Orient first
- Obstruction is a mechanical diagnosis: dilated bowel PROXIMAL to a point and collapsed bowel DISTAL to it. Dilated bowel everywhere with no collapsed segment is ileus, not obstruction.
- The transition point is the whole reason a CT is being done — it names the cause and decides the operation.
- Small versus large bowel is read off the mucosal pattern: valvulae conniventes cross the whole lumen; colonic haustra do not.
- Closed-loop obstruction and ischaemia are the findings that convert this from an admission into an emergency laparotomy.
Acquire the study
- CT abdomen and pelvis, portal venous phase. Positive oral contrast is usually unhelpful and can obscure mural enhancement.
- Read on CORONALS as the primary plane for following bowel — axials alone make loop-tracing far harder.
- Review on a soft-tissue window, then re-look on a WIDER window for pneumatosis and portal venous gas, which a narrow window hides.
- A non-contrast study can show obstruction but cannot properly assess ischaemia — say so if contrast was withheld.
The manoeuvre
- Confirm dilatation and measure it (small bowel over 2.5 cm; colon over 6 cm, caecum over 9 cm).
- Establish small versus large bowel from the mucosal pattern and the position of the loops.
- FOLLOW THE BOWEL from the collapsed segment BACKWARDS to the dilated one — tracing retrogradely from collapsed bowel is far easier than forwards, and this is the technique that finds the transition point.
- At the transition, name the cause: adhesion (an abrupt change with no mass), hernia (check every orifice — inguinal, femoral, obturator, umbilical, incisional, internal), tumour, volvulus, gallstone ileus, intussusception, stricture.
- Look for a CLOSED LOOP: a C- or U-shaped fluid-filled loop with two adjacent transition points and radially converging mesenteric vessels.
- Assess bowel viability: reduced or absent mural enhancement, mural thickening, pneumatosis intestinalis, portal venous gas, mesenteric fat stranding, ascites.
- Check for the whirl sign of a volvulus around a mesenteric axis.
- Look for free gas and free fluid.
What confirms it
- Dilated proximal bowel, a transition point, and collapsed distal bowel.
- Ischaemia is a separate finding and must be stated separately — reduced mural enhancement plus mesenteric oedema in a closed loop is the urgent combination.
What licenses you to exclude it
- Uniformly distended bowel throughout, including the rectum, with no transition point and no collapsed distal segment, is ileus rather than obstruction.
- ⚠️ An early or partial obstruction can have a subtle or absent calibre change. If the clinical picture is convincing, say the study does not exclude a low-grade obstruction.
The classic misread
- Reporting "dilated loops" without hunting for the transition point — the single most common shortfall in this report.
- Missing an obturator or femoral hernia because only the inguinal region was checked.
- Missing pneumatosis on a narrow window.
- Calling ischaemia on mural thickening alone — enhancement is the discriminator.