CT Angiogram — Aortoiliac and Lower Limb Runoff

Supracoeliac or infrarenal abdominal aorta through to the pedal arches.

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

Peripheral arterial disease with lifestyle-limiting claudication or critical limb ischaemia, before revascularisation.
Acute limb ischaemia where the level of occlusion needs defining.
Suspected popliteal entrapment, aneurysm or trauma to the limb vasculature.
Assessment of bypass graft or stent patency.

Technique

Table speed must be matched to bolus transit. A fixed total scan time with variable pitch, tied to a bolus-tracking trigger in the aorta, is one published approach.
Bolus tracking with the monitoring region of interest in the supracoeliac or infrarenal abdominal aorta, triggering at roughly 100-150 HU above baseline, then a post-trigger diagnostic delay of about 3-5 s. The delay is short because the acquisition starts at the monitoring level and then chases the bolus down the leg — the timing problem in this study is not when to start, it is how fast to travel.
The acquisition duration is set rather than derived: published run-off protocols fix a total scan time of roughly 35-40 s from the aorta to the pedal arches and let the table speed fall out of that (of the order of 30 mm/s over a 120-130 cm range), so that the table never overtakes the bolus.
A test bolus of about 15-20 mL, monitored at the popliteal artery rather than the aorta, is the more informative option in slow-flow disease: measured aortopopliteal transit times range from under 10 s to well over 40 s between individuals, and that spread is far wider than any trigger threshold can absorb.
The biphasic injection exists to keep the distal vessels opacified once the table has outrun the leading edge of the bolus.
Faster scanners make this harder, not easier — a modern platform can cover the legs in well under the bolus transit time, so the acquisition is often deliberately slowed. A protocol transplanted from an older scanner without re-setting the scan duration is the usual way a department starts outrunning boluses.
Feet strapped together and legs immobilised: a small movement between the proximal and distal parts of a long acquisition ruins the reformats.

Where it goes wrong

Outrunning the bolus is the signature failure of this protocol — the calf and pedal vessels appear occluded when they are simply not yet opacified. A slow-flow patient with a large aneurysm or poor cardiac output is at highest risk.
Scanning too slowly allows venous contamination in the calves, which obscures the run-off vessels.
Dense infrapopliteal calcification blooms and can render small vessels unassessable; this is a known limitation rather than a fixable timing issue.
Movement of the legs mid-acquisition destroys curved reformats along the whole vessel.

Clinical questions that reach this study

Peripheral arterial disease — imaging before revascularisation

Contrast

Iodinated, intravenous

A large volume delivered as a biphasic injection — a short fast leading bolus followed by a longer slower phase — so arterial opacification is sustained the length of the leg. Published protocols use total volumes in the region of 120-180 mL at rates of roughly 3-5 mL/s, weight-adjusted.

Acquisition

Breathing: Breath-hold for the abdominal portion only.
Reconstruction: Thin axial source images with curved planar reformats along each named vessel, maximum-intensity projections and bone-removal volume rendering.
Preparation: Feet-first positioning, legs immobilised and slightly internally rotated with the feet strapped together. Power-injectable antecubital cannula; a biphasic injection is common.

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