Diameters perpendicular to the centreline at fixed landmarks, compared with the prior study at the same landmarks — growth rate and absolute size set surveillance and repair.
Orient first
- Normal diameters vary with age, sex and body size; indexed values help at the extremes.
- Degenerative, genetic (Marfan, Loeys–Dietz, Turner), bicuspid valve and inflammatory aetiologies differ in thresholds.
- Consistency of the method matters more than the method (inner-to-inner vs outer-to-outer: state it).
Acquire the study
- ECG-gated CT angiography of the thoracic aorta (gating removes root motion artefact); centreline and double-oblique reformats.
The manoeuvre
- Double-oblique reformats perpendicular to the centreline: diameters in mm at the sinuses, sinotubular junction, mid-ascending, proximal arch, mid arch, isthmus, mid-descending and at the diaphragm.
- State the measurement convention (inner-to-inner or outer-to-outer).
- Compare with the prior study at the SAME landmarks: growth in mm per year.
- Wall: thrombus, calcification, penetrating ulcers, intramural haematoma, periaortic change.
- Branch vessels and the aortic valve; associated coarctation.
What confirms it
- Diameter above the normal range for the segment (for example ≥ 45 mm ascending — verify the guideline in use) on double-oblique measurement.
What licenses you to exclude it
- Diameters within normal limits for age and body size on perpendicular measurement.
The classic misread
- Measuring on axial slices where the aorta is oblique — overestimation and false growth.
- Comparing with a prior study measured at a different landmark.