CT Angiogram — Intracranial Arteries
Skull base (or arch, if the great vessels are also in question) to the vertex, covering the circle of Willis and its major branches.
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 large vessel occlusion in acute ischaemic stroke, to select for thrombectomy.
- Suspected intracranial aneurysm, including after a positive non-contrast CT for subarachnoid haemorrhage.
- Suspected intracranial arterial dissection, vasculitis or reversible vasoconstriction.
- Pre-operative or pre-procedural vascular mapping, and follow-up of treated aneurysms where metal artefact permits.
Technique
- Bolus tracking or a test bolus is preferred to a fixed delay; published cerebral protocols commonly place the monitoring region of interest in the aortic arch or descending aorta and trigger at an enhancement threshold in the region of 100-150 HU.
- "No fixed delay" is not the whole answer: once the threshold is crossed, the diagnostic acquisition follows after a short post-trigger delay, typically of the order of 4-6 s for an arch or descending aortic monitoring position. That interval is the time the intracranial arteries need to fill, and it is scanner-dependent — the table movement and any breathing instruction built into the platform are part of it.
- Where bolus tracking is unavailable, a test bolus of about 15-20 mL injected at the diagnostic rate with low-dose monitoring images every 1-2 s gives the individual peak arrival time, and the diagnostic scan is started at that peak plus a few seconds. A blind fixed delay — around 15-20 s from the start of injection is the usual published fallback — is the least reliable of the three and is a poor choice in low cardiac output.
- Acquisition is often run caudocranially to limit venous contamination in the head.
- Reported bolus-tracking thresholds and post-threshold delays differ appreciably between published protocols and scanner platforms; the local number is the one that applies. Faster wide-detector and high-pitch systems shorten the acquisition and therefore change the delay that gives a purely arterial study.
Where it goes wrong
- A small or peripheral cannula caps the achievable injection rate and is the commonest reason for poor arterial opacification.
- Triggering late produces venous contamination that obscures small aneurysms at the skull base.
- Aneurysm clips, coils and dental amalgam create streak that can simulate or conceal a vascular abnormality; thin reformats through the artefact are more informative than a repeat scan.
- A CTA acquired without a preceding unenhanced head CT cannot answer the haemorrhage question that usually precedes it.
Clinical questions that reach this study
Contrast
Typically 40-70 mL of non-ionic iodinated contrast at 4-5 mL/s, followed by a saline chaser of around 20-30 mL.
- A saline flush is part of the protocol, not an optional extra: it tightens the bolus and clears dense contrast from the subclavian vein.
Acquisition
- Breathing
- Quiet respiration; no swallowing.
- Reconstruction
- Sub-millimetre axial source images with coronal and sagittal reformats, plus maximum-intensity projections and volume rendering reviewed on a workstation.
- Preparation
- A cannula that will tolerate a power injection at 4-5 mL/s; a small hand cannula is the single commonest cause of a non-diagnostic study. Right antecubital access is preferred where a combined arch acquisition is planned, to keep dense contrast out of the left brachiocephalic vein.
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
- Kidney function and intravenous iodinated contrast· radiographer at scan