ABSTRACT
Most people working in computed tomography (CT) departments are familiar with image acquisition and contrast enhancement protocols that aim to delineate the blood vessels supplying a particular organ, for example CT angiography for the renal or pulmonary arteries. On the other hand, experience using CT to depict perfusion or other aspects of vascular physiology within those organs may be less widespread. Nevertheless, some awareness of vascular physiology is required for the optimal performance of CT angiography in that the timing of the image acquisition must be matched to the temporal changes in vascular enhancement as the contrast medium passes through the circulation. Many modern CT systems now incorporate an automated method for timing the image acquisition in accordance with vascular enhancement. However, prior to this development, it was commonplace to acquire a rapid sequence of images without table movement during a small initial test injection of contrast medium. This sequence of images was used to identify the time of optimal vascular enhancement and so guide the timing of the main CT angiography study. Conceptually, image acquisition and contrast protocols for CT perfusion in oncology are similar to those initial timing sequences, with the temporal changes in contrast enhancement providing the information necessary for determining tumor perfusion. However, the focus is not simply upon contrast enhancement within the major vessels but, more importantly, enhancement within the small vessels inside the tumor itself. In effect, CT perfusion can be considered ‘CT angiography for the tumor microcirculation’.
