How does superior mesenteric vein thrombosis appear on imaging?

Imaging Appearance of Superior Mesenteric Vein Thrombosis

Contrast-enhanced CT performed in the portal-venous phase is the diagnostic test of choice and demonstrates a characteristic "target sign"—a high-density vein wall surrounding a central filling defect (thrombus) in the superior mesenteric vein. 1, 2

Optimal Imaging Protocol

• Acquire images during the portal-venous phase (not late arterial phase) to avoid false-positive diagnoses caused by delayed contrast arrival that can mimic a filling defect when no thrombus exists. 3, 4

• Use a comprehensive biphasic CTA protocol that includes pre-contrast scans, arterial phase, venous phase, and multiplanar reconstructions to assess the origin and extent of mesenteric vessel involvement. 1

• Do not administer oral contrast—it is contraindicated and harmful in this setting. 1

• Perform CT without delay even in the presence of acute kidney injury, because every 6 hours of diagnostic delay doubles mortality, and the consequences of missed diagnosis far outweigh contrast nephropathy risk. 1

Primary CT Findings

Direct Thrombus Visualization

• The "target sign" is the most common positive finding: a dense peripheral rim (the vein wall) surrounding a central lucency (the thrombus) on contrast-enhanced scans. 1, 2

• Filling defect in the SMV lumen represents the thrombus itself and confirms the diagnosis. 3, 4

• Thrombus may extend into the portal vein, splenic vein, distal second-order SMV branches, or mesenteric arcades; CT defines the full anatomical extent. 3, 2

Secondary CT Findings Suggesting Bowel Compromise

These findings indicate high risk for intestinal infarction and mandate urgent surgical evaluation:

• Segmental bowel wall thickening (concentric, marked) with zones of different attenuation caused by varying degrees of submucosal edema and hemorrhage. 5

• Abnormal or absent bowel wall enhancement on contrast-enhanced images suggests loss of perfusion. 3, 4

• Bowel dilatation with fluid-filled loops. 3

• Pneumatosis intestinalis (gas within the bowel wall). 1, 6, 5

• Portal venous gas or mesenteric venous gas. 1, 6

• Loss of bowel wall integrity indicating perforation. 6

• Mesenteric fat stranding and vascular engorgement. 3, 5

• Large-volume ascites. 3, 1

• Free intraperitoneal air (pneumoperitoneum) indicating perforation. 6

High-Risk Features Predicting Need for Bowel Resection

• Distal SMV thrombosis involving second-order branches of the superior mesenteric vein. 3, 4

• Homogeneous or heterogeneous hypoattenuating or hyperattenuating bowel wall thickening. 3

• Dense peripheral enhancement of the bowel wall (paradoxical hyperemia). 5

• Combination of SMV thrombus with bowel wall thickening strongly suggests bowel infarction and warrants immediate surgical consultation. 5

Alternative Imaging Modalities (Limited Role)

• Doppler ultrasound may detect absent flow in the portal vein but has lower sensitivity (85–90%) than CT, is operator-dependent, and frequently misses distal arterial emboli or peripheral SMV thrombosis. 3, 1

• MR angiography achieves 95–100% sensitivity and specificity for mesenteric vessel stenosis but is impractical in the emergency setting due to longer acquisition times and limited availability. 1

• Conventional angiography should be reserved for cases where CTA is negative but clinical suspicion remains high, or when simultaneous therapeutic intervention (catheter-directed thrombolysis) is planned. 1

Critical Pitfalls to Avoid

• Do not rely on late arterial-phase images; low portal flow can create a pseudo-filling defect that mimics thrombosis when none exists. 3

• Do not delay CT for oral contrast administration; it is contraindicated and will delay diagnosis. 1

• Do not withhold CT in renal insufficiency; diagnostic delay is far more lethal than contrast exposure. 1