ABSTRACT
Most people working in CT departments are familiar with image acquisition and contrast enhancement protocols that aim to delineate structural features within the brain – for example, CT angiography for the cerebral arteries. On the other hand, experience using CT to depict perfusion and other aspects of vascular physiology may be less widespread. Nevertheless, some awareness of vascular physiology is required for optimal performance of CT cerebral 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 cerebral 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 perfusion CT are similar to those initial timing sequences with the temporal changes in contrast enhancement providing the information necessary for determining cerebral perfusion. However, the focus is not simply upon contrast enhancement within the major vessels but, more importantly, enhancement within the small vessels inside the cerebral tissues themselves. In effect, perfusion CT can be considered ‘CT angiography for the cerebral microcirculation’.
