First and second year — the floor first, then every step
Measure the maximum diameter outer wall to outer wall perpendicular to the centreline, then look for rupture (retroperitoneal haematoma) and impending-rupture signs, and give the anatomy an endovascular repair needs.
Orient first
Aneurysm is commonly defined as an infrarenal diameter of 3 cm or more; repair is typically considered around 5.5 cm in men and lower in women (verify against the vascular guideline in use).
Measurement convention changes the number: outer-to-outer versus inner-to-inner and axial versus centreline differ by several millimetres — state the method.
Rupture is usually into the left retroperitoneum; a contained leak can look like a periaortic mass.
Acquire the study
Curvilinear probe, fasting; transverse and longitudinal views from the diaphragm to the bifurcation.
The manoeuvre
Maximum anteroposterior diameter outer-to-outer in a true transverse plane (perpendicular to the aorta).
Relation to the renal arteries (neck) and extension to the iliac arteries.
Mural thrombus and the flow lumen on colour Doppler.
What confirms it
Diameter at or above the threshold; rupture by retroperitoneal haematoma or extravasation.
What licenses you to exclude it
Ultrasound cannot exclude rupture reliably — a symptomatic patient with a known aneurysm needs CT if stable.
The classic misread
Measuring an oblique section of a tortuous aorta — it over-reads.
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
Abdominal aorta (calibre) · Outer-to-outer AP — same 30 mm floor as the AAA entry
under 30 mm outer-to-outer AP is the same adult screening normal as the already-registered abdominal-aorta entry; 30 mm or more is the conventional AAA definition. The AAA-sac and growth entries are the surveillance conversation. Name outer-to-outer versus inner-to-inner
Outer-to-outer versus inner-to-inner is a real disagreement — name the convention. Do not reuse a thoracic-aorta number here.
USG · CT
Diagnostic criteria
Abdominal aorta · maximum outer-wall AP diameterunder 30 mm
Measure outer wall to outer wall, perpendicular to the vessel axis; an oblique cut over-reads.
≥5 mm in 12 months is the conventional acceleration flag; a shorter-interval jump that the local surveillance table treats as equivalent is the same finding — confirm that table
Measurement error on ultrasound is a few millimetres — do not call 2 mm of "growth" between noisy studies. Versioned criterion — verify against the current edition before clinical use.
USG · CT
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.