The timing of imaging acquisition is usually stated from the commencement of the intravenous contrast medium injection. For some protocols a standard delay may be used, e.g. portal phase abdominal imaging at 70–90 s. The arrival of contrast medium in the aorta, however, varies depending on cardiac output and other factors. Individualised timing is possible using bolus triggering: a region of interest may be preset, the CT data acquisition being triggered to start when the arrival of contrast medium reaches a critical threshold. This latter technique is particularly useful in CT angiography or for arterial phase imaging of the liver and pancreas.

The advent of multidetector CT brought multiphase CT protocols and, given the speed of current equipment, it would be easy to obtain these on every patient. However, in order to reduce radiation dose, it is important to only obtain relevant phases. For example, unenhanced images are essential in characterising adrenal lesions but have no value in routine oncological staging studies, whereas arterial phase pancreatic studies are crucial in staging pancreatic carcinoma but are of no value in acute pancreatitis.

In some instances, e.g. CT urography, it has become standard practice in many centres to split the bolus of intravenous contrast medium into two or three aliquots that are given at different times, and then perform a single study effectively giving several different phases, i.e. nephrographic and excretory, rather than perform multiphase imaging.

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