Gastrointestinal and Hepatobiliary · Bowel
Bowel Ischaemia, Obstruction and Perforation Essentials
Core imaging signs of mechanical bowel obstruction, mesenteric ischaemia, and gastrointestinal perforation on radiography and CT.
Acute abdominal emergencies require rapid, accurate imaging to differentiate mechanical obstruction, mesenteric ischaemia, and perforation. CT with intravenous contrast is the modality of choice, providing critical information regarding the aetiology, viability of the bowel, and need for urgent surgical intervention.
- Small bowel obstruction (SBO) radiograph signs
- Dilated small bowel loops (>3 cm) with valvulae conniventes crossing the entire width of the bowel, typically centrally located, with a paucity of distal gas.Radiography has low sensitivity for partial or early obstruction.
- Large bowel obstruction (LBO) radiograph signs
- Dilated colon (>6 cm, caecum >9 cm) with haustral folds that do not cross the entire width, typically peripherally located; the ileocaecal valve competence determines caecal pressure.A competent ileocaecal valve creates a closed-loop obstruction, risking caecal perforation.
- CT transition zone identification
- The precise point where dilated, fluid-filled proximal bowel transitions to collapsed, empty distal bowel; identifying the aetiology (e.g., adhesion, hernia, mass) at this site is critical.Multiplanar coronal and sagittal reformats significantly improve transition zone detection.
- Closed-loop obstruction signs
- Occurs when a segment of bowel is obstructed at two points (e.g., volvulus, internal hernia), forming a U- or C-shaped dilated loop with radial distribution of mesenteric vessels.High risk of rapid strangulation and ischaemia; requires urgent surgery.
- Whirl sign
- A swirling pattern of mesenteric fat and vessels, indicating a volvulus (midgut or sigmoid) or internal hernia; the degree of twisting correlates with the risk of vascular compromise.Classic for small bowel volvulus or sigmoid volvulus.
- Early CT signs of bowel ischaemia
- Bowel wall thickening (>3 mm), submucosal oedema (target sign), mesenteric stranding, and ascites; hyperenhancement may occur in venous congestion.These signs are reversible if perfusion is restored promptly.
- Late CT signs of ischaemia (irreversible)
- Pneumatosis intestinalis (gas within the bowel wall), portal venous gas, lack of bowel wall enhancement, and bowel wall thinning indicate transmural infarction and necrosis.Portal venous gas in the setting of ischaemia carries a very high mortality rate.
- Lack of bowel wall enhancement
- Failure of the bowel wall to enhance following intravenous contrast administration is the most specific CT sign of transmural infarction.Must be differentiated from unopacified bowel due to poor contrast timing.
- Mesenteric stranding and ascites
- Increased attenuation of mesenteric fat and free fluid are sensitive but non-specific signs of venous congestion, inflammation, or early ischaemia.Haemorrhagic ascites suggests advanced infarction or mesenteric venous thrombosis.
- SMA embolus versus thrombosis
- Emboli typically lodge distal to the origin of the middle colic artery (sparing the proximal jejunum), whereas thrombosis usually occurs at the ostium in patients with severe atherosclerosis.Embolic ischaemia often presents with sudden, severe pain out of proportion to exam.
- Superior mesenteric vein (SMV) thrombosis
- Presents with bowel wall thickening, mesenteric oedema, and a filling defect within the SMV; often subacute and associated with hypercoagulable states or intra-abdominal infection.Bowel infarction occurs later in venous thrombosis compared to arterial occlusion.
- Non-occlusive mesenteric ischaemia (NOMI)
- Occurs in low-flow states (e.g., shock, heart failure, vasopressors) without macroscopic vascular occlusion; shows segmental spasms and alternating dilation on angiography.Diagnosis is often delayed; mortality remains high despite treatment.
- Perforation free gas locations
- Pneumoperitoneum is best detected on CT anterior to the liver or over the non-dependent right lateral abdominal wall; radiographs require upright or decubitus positioning.Microperforations may only show localised extraluminal gas bubbles and fat stranding.
- Small bowel faeces sign
- Particulate faecal-like material with gas bubbles within dilated small bowel loops, typically located just proximal to the transition zone in subacute or chronic SBO.Indicates stasis and helps localise the obstruction site.
- Intussusception target sign
- A bowel-within-bowel configuration appearing as a target or sausage-shaped mass on cross-sectional imaging; in adults, it is almost always secondary to a pathological lead point (e.g., tumour).Requires surgical resection in adults, unlike paediatric cases.
- Volvulus coffee bean sign
- A classic radiographic sign of sigmoid volvulus, showing a massively dilated, inverted U-shaped loop arising from the pelvis; CT shows the whirl sign and a beak-like tapering.Caecal volvulus typically appears as a dilated, gas-filled structure in the left upper quadrant.
- Epiploic appendagitis
- Torsion or thrombosis of an epiploic appendage, presenting as a small, fat-density oval lesion with a hyperattenuating ring and adjacent inflammatory stranding, mimicking diverticulitis.A self-limiting condition managed conservatively with NSAIDs.
- Mesenteric panniculitis
- Idiopathic inflammation of mesenteric fat presenting as a hazy, increased attenuation mass encasing mesenteric vessels, often with a 'fat ring sign' sparing the perivascular fat.Must be differentiated from lymphoma or carcinoid tumour.
Caution
The presence of portal venous gas in the setting of acute bowel ischaemia indicates transmural infarction and carries a high mortality rate; it mandates immediate surgical consultation.
Exam pearl
A competent ileocaecal valve in the setting of large bowel obstruction creates a closed-loop system, placing the caecum at highest risk for perforation due to the Law of Laplace.
Viva questions
- What is the 'small bowel faeces sign' and what does it indicate?
- The small bowel faeces sign refers to particulate faecal-like material with gas bubbles within dilated small bowel loops, indicating stasis and helping to localise the transition zone in subacute obstruction.
- How do you differentiate an SMA embolus from an SMA thrombosis on CT angiography?
- An embolus typically lodges distal to the origin of the middle colic artery, whereas thrombosis usually occurs at the vessel ostium in the setting of severe atherosclerotic disease.
- What are the late, irreversible CT signs of bowel ischaemia?
- Late signs include pneumatosis intestinalis, portal venous gas, lack of bowel wall enhancement, and bowel wall thinning, all indicating transmural infarction.
- Why is a competent ileocaecal valve dangerous in large bowel obstruction?
- A competent valve creates a closed-loop obstruction, preventing retrograde decompression and placing the caecum at high risk for perforation due to its large resting diameter (Law of Laplace).