CT Aorta — Acute Aortic Syndrome

Unenhanced series through the thoracic aorta, then an arterial acquisition from the thoracic inlet to the common femoral arteries so the full extent of any dissection and its branch involvement is defined.

Typical, not policy

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

Suspected acute aortic dissection, intramural haematoma or penetrating atherosclerotic ulcer.
Suspected traumatic aortic injury.
Suspected aortic rupture or symptomatic aneurysm.
Malperfusion syndromes attributed to aortic pathology.

Technique

The unenhanced series is not optional in this protocol: intramural haematoma is a hyperdense crescent that becomes invisible once contrast is on board.
ECG synchronisation of the thoracic portion substantially reduces aortic root pulsation artefact and is recommended where the root and ascending aorta matter, which in acute dissection is always.
Coverage to the femoral arteries defines the distal extent and informs access planning.
The arterial acquisition is bolus-tracked rather than fixed-delay: monitoring region of interest in the descending thoracic aorta at the level of the carina or diaphragm, trigger at roughly 100-150 HU above baseline, then a post-trigger diagnostic delay of about 4-8 s. Craniocaudal acquisition starting at the inlet means the delay is short by design.
Where tracking is unavailable, a fixed delay of about 20-25 s from the start of injection is the usual published fallback, and a test bolus of 15-20 mL is the better option in a patient whose haemodynamics are clearly abnormal.
Transit time is prolonged in exactly the patients this protocol is used on — tamponade, severe aortic regurgitation, poor cardiac output, a large false lumen. Any of these makes a fixed delay unsafe as a timing method, and is the reason bolus tracking is the default here rather than a refinement.
The unenhanced series precedes the injection; it needs no delay stated, but it must not be skipped to save time.
These values are typical and move with scanner generation — a high-pitch dual-source acquisition covers inlet to femorals in a few seconds and behaves quite differently from a 64-slice platform taking 20 s over the same range.

Where it goes wrong

Omitting the unenhanced series is the classic protocol error and makes intramural haematoma undiagnosable.
Without ECG gating, motion at the aortic root produces a double contour that mimics a dissection flap in the ascending aorta.
A study cropped at the diaphragm misses the distal extent and the visceral branch involvement that determines management.
Streak from dense contrast in the left brachiocephalic vein can obscure the arch branches; right arm injection helps.

Clinical questions that reach this study

Suspected acute aortic syndrome Aortic aneurysm surveillance and post-repair follow-up Valvular heart disease — assessment and intervention planning Peripheral arterial disease — imaging before revascularisation Suspected renovascular hypertension / renal artery stenosis

Contrast

Iodinated, intravenous

Typically 70-100 mL of non-ionic iodinated contrast at 4-5 mL/s with a saline chaser.

Acquisition

Breathing: Single breath-hold where the patient can comply; quiet respiration otherwise.
Reconstruction: Thin axial source images with multiplanar and curved reformats along the aortic centreline, plus maximum-intensity projections.
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.

Non-contrast (unenhanced)No injection. Acquired before any contrast is given.
Shows intrinsic tissue attenuation, and nothing else. The transferable principle is that contrast is anti-signal for anything that is already dense: calcification, acute haemorrhage, urinary and biliary calculi, iodine-containing or haemorrhagic fluid, and intrinsic fat all lose conspicuity, or become uninterpretable, once surrounding tissue enhances. It is also the only baseline against which enhancement can be measured, so any protocol that quantifies enhancement or washout (a lesion "enhances by X HU", adrenal absolute washout, renal mass characterisation) is arithmetically impossible without it. Conversely, an unenhanced series adds dose and no information whenever the question is purely about vascularity or perfusion.
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. Circulation 2022.
ACR-NASCI-SIR-SPR practice parameter for the performance and interpretation of body CT angiography — bolus tracking, monitoring region of interest placement, trigger threshold and post-trigger diagnostic delay.