CT Aorta — Aneurysm Assessment and Planning
Thoracic inlet or diaphragm (as clinically indicated) to the common femoral arteries.
Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.
When to use it
- Sizing and morphology of a known aortic aneurysm before open or endovascular repair.
- Surveillance of an aneurysm where ultrasound is inadequate or the aneurysm is thoracic or thoracoabdominal.
- Assessment of iliac and femoral access vessels before an endovascular procedure.
Technique
- Diameters should be measured perpendicular to the centreline; axial diameters over-measure a tortuous aorta and are not comparable between studies measured differently.
- An unenhanced series can usually be omitted for elective sizing, which is the main dose saving over the acute protocol.
- Calcification and thrombus burden at the proposed landing zones are part of the report the interventional team needs.
- Bolus tracking is the default: monitoring region of interest in the aorta proximal to the aneurysm — the descending thoracic aorta, or the suprarenal aorta for an infrarenal sac — trigger at roughly 100-150 HU above baseline, then a post-trigger diagnostic delay of about 5-10 s.
- The post-trigger delay sits at the longer end of the angiographic range here, and deliberately: flow through a large sac is slow, so contrast that has reached the monitoring level has not yet reached the iliac and femoral access vessels the surgeon needs opacified. A delay copied from a carotid or coronary protocol will under-fill the distal run-off.
- A test bolus of 15-20 mL is the more robust choice in a very large aneurysm, in poor cardiac output, or where a previous study outran the bolus — it measures the individual transit time instead of assuming it.
- Where no tracking is available, a fixed delay of about 25-30 s from the start of injection is the published fallback for an aortoiliac study, longer than the 20-25 s used for the acute thoracic protocol for the same slow-flow reason.
- Timings are typical and scanner-dependent; faster acquisitions need proportionally more attention to the distal end of the coverage, not less.
Where it goes wrong
- Mixing axial and centreline measurements across serial studies manufactures apparent growth.
- Cropping the coverage above the femoral bifurcation leaves the access vessels unassessed and forces a repeat study.
- Poor arterial timing in a patient with a large aneurysm sac — flow is slow and the standard delay may be too short.
Clinical questions that reach this study
Contrast
Typically 70-100 mL of non-ionic iodinated contrast at around 4 mL/s with a saline chaser.
Acquisition
- Breathing
- Single breath-hold.
- Reconstruction
- Thin axial source images with centreline reconstruction for orthogonal diameter measurement, plus volume-rendered overviews for the surgical or endovascular team.
- Preparation
- Power-injectable cannula of at least 18-20G. Blood pressure cuff and monitoring per the acute pathway — a suspected acute aortic syndrome patient should not be left unattended in the scanner.
Phases
Each phase is authored once and shared across every protocol that uses it, so the physiology below is the same wherever you meet it.
- Early arterial phaseTypically ~15–25 s after the start of injection (or ~4–8 s after bolus trigger). Confirm locally.
Contrast is in arteries only; parenchyma has barely begun to enhance and veins are unopacified. The transferable principle is that this phase is a road map, not a lesion-detection phase: it defines arterial anatomy, variants, stenoses, dissection flaps and active arterial extravasation, and it deliberately trades away parenchymal contrast to do so. Any solid-organ lesion that depends on parenchymal enhancement difference will be under-called here, so an early arterial series is an addition to a diagnostic phase, never a substitute for one.
- CT angiography, bolus-trackedNo fixed delay. A monitoring ROI is placed in the target vessel (commonly the aorta) and acquisition triggers at a set attenuation rise — a ~100 HU threshold is widely used — followed by a short diagnostic delay of a few seconds for table movement and breath-hold instruction. Confirm locally.
This is not a separate physiological phase so much as a technique for hitting one reliably. The transferable principle is that a fixed delay assumes an average circulation, and the patients who most need vascular imaging — the shocked, the failing, the arrhythmic, the aneurysmal — are precisely those whose circulation time is furthest from average. Tracking the bolus in the target vessel replaces that assumption with a measurement, so peak arterial opacification is achieved in the individual patient. The trade-off is that the technique commits the scan to whatever the monitoring ROI sees: a badly placed ROI, a mistimed breath-hold or a threshold reached by a contralateral vein produces a systematically mistimed study rather than a slightly degraded one.
Safety checks this protocol carries
Derived from the contrast agent and phases above, not authored here — which is why they cannot drift apart from what the protocol actually does.
- Prior contrast reaction and elective premedication· nurse pre scan
- Intravenous access adequate for the planned injection· radiographer at scan
- Metformin and iodinated contrast· radiographer at scan
- Child-sized technique and contrast dose· radiographer at scan
- Pregnancy status before an ionising exposure· radiographer at scan
- Kidney function and intravenous iodinated contrast· radiographer at scan
References
- 2022 ACC/AHA guideline for the diagnosis and management of aortic disease.
- ACR-NASCI-SIR-SPR practice parameter for the performance and interpretation of body CT angiography — bolus tracking and the longer post-trigger delay appropriate to slow flow through an aneurysm sac.