How is the image acquisition timed following contrast administration in a CTA?
The image acquisition can be timed in one of three ways:
- • Empirical delay.
- • Bolus tracking.
- • Test bolus.
When using an empirical delay, the images are acquired after a preset delay from the time of contrast administration. This is the method generally used for imaging of the veins. The time delay depends on the vein of interest. To assess the infrarenal inferior vena cava (IVC) and the iliac veins, a delay of 140 seconds from the time of contrast injection is used. To assess the portal veins and the renal veins, the delay is typically 70 to 90 seconds. For assessment of the central veins of the chest and upper extremities, a delay of 90 seconds is used.
Imaging of the arteries requires more nuance because the window for optimal enhancement of the arteries is short. This is most often achieved by tracking the bolus of injected contrast material. A cursor is placed in a region of interest (ROI), which is the aortic segment most proximal to the artery of interest. For example, if the artery of interest is the celiac artery, then the ROI can be the descending thoracic aorta or abdominal aorta. After a specified time delay, known as the monitoring delay, low dose single slice images are obtained through the ROI every 1 to 2 seconds, while simultaneously tracking the attenuation of the aorta. When its attenuation exceeds a predetermined threshold, typically 130 HU, the scan commences after another time delay known as the scan delay.
When using a test bolus, a small volume of contrast material, typically 20 ml, is injected at the same rate as the main injection. An ROI is placed in the aorta and low dose single slice images are obtained. The attenuation of the artery is then measured on each image at different points in time, and the time to peak maximal enhancement (PME) is calculated. The image acquisition for the diagnostic scan is then timed by using the time to PME plus a few extra seconds because a larger contrast volume is administered.
Note that these techniques of image acquisition timing can also be utilized with MRA.
