First and second year — the floor first, then every step
Biphasic CTA: find the cause in the vessels (SMA embolus or thrombosis, SMV thrombosis, low flow), then judge the bowel — wall enhancement, pneumatosis and portal venous gas — because dead bowel changes the operation.
Orient first
Four causes: arterial embolus (sudden, often a few cm beyond the SMA origin, sparing proximal jejunum), arterial thrombosis (at the origin on atherosclerosis), venous thrombosis (thickened oedematous bowel) and non-occlusive ischaemia (low-flow, vasospasm, patent vessels).
The earliest bowel sign is DECREASED wall enhancement; wall thickening is more typical of venous and reperfusion injury.
Pneumatosis with portal venous gas in this setting means transmural necrosis until proven otherwise.
Acquire the study
Arterial and portal venous phase CT (CTA) with no positive oral contrast; thin slices, sagittal MIP of the SMA.
The manoeuvre
Arterial phase, sagittal: SMA origin and course — filling defect, occlusion, dissection; coeliac and IMA; collaterals.
Portal venous phase: SMV and portal vein thrombus.
Bowel wall: enhancement compared with normal loops (reduced or absent = ischaemia), "paper-thin" wall, target thickening in venous ischaemia.
Wide window: pneumatosis intestinalis and portal/mesenteric venous gas.
Mesenteric stranding, ascites, and the distribution of affected bowel against the vascular territory.
What confirms it
A vascular cause plus bowel in its territory with abnormal enhancement, or the non-occlusive pattern with abnormal bowel and patent vessels.
What licenses you to exclude it
A normal CTA makes occlusive mesenteric ischaemia unlikely; non-occlusive ischaemia can have subtle bowel findings — say so in a high-risk patient.
The classic misread
Calling reduced enhancement normal because oral contrast or a single phase hid it.
Missing an SMA embolus a few centimetres beyond the origin by reading axial slices only.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
Normal limits
Superior mesenteric artery · Aortomesenteric angleapproximately 25–60°
Sagittal reformatted CT or MR angiogram. The angle between the long axis of the aorta and the proximal superior mesenteric artery at its origin.
An angle under about 22–25° WITH the compatible clinical picture and duodenal compression supports SMA syndrome — the angle alone diagnoses nothing, and thin asymptomatic patients commonly have narrow angles.
CT · MRI
Superior mesenteric artery · Aortomesenteric distance at the duodenal crossing
approximately 10–28 mm; under about 8–10 mm supports compression
Measured where the third part of the duodenum (or the left renal vein) crosses between aorta and SMA. Both the angle and the distance must agree with the clinical syndrome — incidental narrow measurements are common.
CT · MRI
Diagnostic criteria
Superior mesenteric vein (thrombosis) · Mesenteric-vein thrombosis and bowel viability
a filling defect in the SMV / portal confluence; the bowel-wall and mesenteric-oedema companions decide urgency — not a remembered SMV millimetre (already registered as calibre elsewhere)
Pneumatosis and portal gas change the sentence from “anticoagulate” to “theatre”. Do not invent an SMV-calibre normal here.
CT · USG
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.