Perfusion Computed Tomography for the Evaluation of Acute Ischemic Stroke: Strengths and Pitfalls.

Ischemic stroke results from occlusion of a cerebral artery, and it is the leading cause of disability and the fifth leading cause of death in the United States. Cerebral artery occlusion results in irreversible death of a component of cerebral tissue, which is referred to as the core infarction. There is an additional component of brain tissue that is ischemic, but viable, which is commonly referred to as the penumbra. The penumbra is at risk of irreversible infarction if timely restoration of blood flow is not achieved, and the preservation of the penumbra by restoration of arterial blood flow is the target of reperfusion therapy in the treatment of ischemic stroke. Increasingly, perfusion computed tomography CT (PCT) is performed for physiological evaluation of the brain parenchyma in the setting of cerebral ischemia, and this technique may be used to determine volume of core infarction and of the penumbra. The physiological data derived from PCT is typically displayed in perfusion maps, including cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT; Figure 1). Regions of brain with severely reduced CBV or CBF correspond to the region of core infarction. Regions of brain with prolongation of the MTT or its derivatives, the time-to-peak and time-tomaximum (Tmax) of the residue function, have been shown to accurately measure the penumbra in patients with acute ischemic stroke. PCT increases the sensitivity and specificity of the acute ischemic stroke diagnosis, aids in excluding stroke mimics, and informs about prognosis and treatment decisions. These important benefits of PCT must be weighed against several disadvantages of the technique. PCT necessarily results in increased radiation exposure to the patient, increased contrast administration that may result in renal injury, increased imaging time that may slightly delay treatment, and increased cost. Here we discuss PCT in the evaluation of cerebral ischemia with an emphasis on acute ischemic stroke. The most common variations in PCT technique and processing and future directions in PCT research are also discussed. CT Perfusion Technique: One Size Does Not Fit All There is significant variability in PCT technique between different institutions, and the PCT parameters used are influenced by the generation of CT scanner, processing software, prior institutional optimization, and even institutional inertia. We now discuss the most common variations in PCT technique, image processing, and challenges that result from the variation in PCT technique.