Contrast timing and bolus tracking

Contrast is only useful where it is when the scanner passes. Arterial, portal venous and delayed phases are moments in one moving bolus, and bolus tracking is how you catch the right one.

By the ScanFlo Editorial Team · Updated 28 September 2026

The same injection shows the arteries at twenty seconds, the liver at seventy and the collecting systems several minutes later. Modern scanners cover the chest and abdomen in a few seconds, so the scan is a snapshot of a moving bolus. Getting the phase wrong costs more than any other parameter, because it cannot be fixed afterwards without injecting again.

Phases

Those times are typical, not fixed. The contrast arrives when the patient's circulation delivers it, and cardiac output, injection rate, cannula position and a proximal stenosis all move it. A fixed delay works reasonably for the portal venous phase, which is broad. Arterial phases are narrow and patient-dependent, and that is where a fixed delay most often misses.

Bolus tracking

Bolus tracking measures the arrival instead of guessing it. A single low-dose monitoring slice is planned at a level the bolus will cross, most often the descending or abdominal aorta. A region of interest is drawn inside the vessel. As the injection runs, the scanner repeats that slice every second or so and plots the HU in the ROI. When enhancement crosses the trigger threshold, the diagnostic scan starts after a short delay that covers table travel and the breath-hold instruction.

Thresholds vary by department and indication; many trigger at around 100 to 150 HU. The things that go wrong are practical. The ROI is placed in the wrong vessel or drifts out of it when the patient breathes. The monitoring level misses the bolus because of anatomy. The injection extravasates and the curve never climbs. If the curve is flat when it should be rising, check the cannula and the ROI before waiting longer. A manual start is always available.

The monitoring scans carry their own dose. It is small per image, but it counts toward the exam total you read in dose metrics, so keep the monitoring interval and the number of repeats sensible.

Test bolus

A test bolus injects a small amount of contrast first and images one level repeatedly to find the time to peak. The main injection is then scanned at that time plus a margin. It costs extra contrast and a few minutes. In return you see the whole curve, which helps when timing is critical, as in coronary CTA or when the circulation is unusual.

The injection itself

Timing is only half of enhancement; the other half is how much iodine arrives per second. Faster injection rates and higher iodine concentration give a tighter, brighter bolus for angiography. A saline chaser pushes the contrast out of the arm veins and sharpens the bolus. A large-bore cannula in the antecubital fossa makes high rates possible. Portal venous imaging depends more on total iodine dose, often weight-based, than on rate.