In a known or suspected AAA with pain or shock, the question is blood outside the wall: retroperitoneal haematoma beside the sac, active extravasation, and the signs of impending rupture — then the neck anatomy the vascular surgeon needs for EVAR.
Orient first
- Rupture is usually into the LEFT retroperitoneum (posterolateral wall); free intraperitoneal rupture is rarer and often fatal before imaging.
- Impending-rupture signs matter even without haematoma: the hyperattenuating crescent in the mural thrombus or wall, a draped aorta over the spine, and focal wall discontinuity.
- The unenhanced series shows the crescent and fresh haematoma best; the arterial series shows extravasation and gives EVAR planning measurements.
Acquire the study
- Unenhanced CT abdomen, then CT angiography (arterial phase, bolus-tracked) from the thoracic inlet to the common femoral arteries; 1–1.25 mm reconstructions with coronal and sagittal reformats. An unstable patient goes to theatre, not to CT — the scan is for the patient stable enough to lie on the table.
The manoeuvre
- Measure the maximal outer-wall-to-outer-wall diameter perpendicular to the centre line (double-oblique reformat), not an axial oblique cut through a tortuous aorta.
- Unenhanced series: look for the hyperattenuating crescent within the thrombus or wall (fresh blood dissecting in, higher than the lumen on non-contrast images).
- Retroperitoneum: haematoma in the perirenal and pararenal spaces, effacing the aortic wall and extending along the psoas; note if it crosses into the peritoneum.
- Arterial phase: contrast extravasation beyond the wall = active bleeding; report the site on the circumference.
- Draped aorta sign on axial images: the posterior wall moulds to the vertebral body without a fat plane — a contained leak.
- For EVAR: neck length (lowest renal artery to the sac, mm), neck diameter and angulation, iliac diameters and access vessel calibre.
What confirms it
- Retroperitoneal haematoma contiguous with an aneurysm wall — with or without active extravasation — is rupture until proven otherwise; say it in the first line and phone it.
What licenses you to exclude it
- An aneurysm with intact fat planes, no crescent, no draped aorta and no periaortic blood excludes rupture on this study — but a symptomatic intact aneurysm is still urgent; say "no CT evidence of rupture" rather than "normal".
The classic misread
- Reading a retroperitoneal haematoma as a horseshoe kidney or lymph node mass because contrast was given without an unenhanced series.
- Mistaking periaortic inflammatory soft tissue (inflammatory aneurysm, sparing the posterior wall) for a leak.
- Measuring an axial slice through a tortuous sac — overcalls diameter by several mm.