Cerebral ischemia and infarction.

Cerebral infarction is most commonly related to atherosclerotic disease in the carotid and vertebrobasilar circulations. TIAs are manifestations of this same disease process and may occur before a complete infarction. The transformation of pale to hemorrhagic infarction may result from reperfusion of an infarct or from migration of an embolus. CT is often the primary imaging study to exclude frank hemorrhage. The use of MR is increasing, in both the acute and the subacute phases of stroke. The combination of MR imaging and MR angiography is gradually replacing CT, particularly in patients with TIAs. In addition, the depiction of minute alterations in contrast and subtle mass effect is visualized with MR more often than with CT. The precise localization of brain stem and posterior fossa infarctions is improved by the ability of MR to view complex structures in two or more orthogonal planes. Vascular structures are readily identified both on the spin echo images and with MR angiography. Slow or obstructed flow in the venous channels may be recognized, which makes MR the ideal examination for the evaluation of cerebral ischemia and infarction.