PHYSICS REVIEW Theoretic Basis and Technical Implementations of CT Perfusion in Acute Ischemic Stroke, Part 1: Theoretic Basis

SUMMARY: CT perfusion (CTP) is a functional imaging technique that provides important information about capillary-level hemodynamics of the brain parenchyma and is a natural complement to the strengths of unenhanced CT and CT angiography in the evaluation of acute stroke, vasospasm, and other neurovascular disorders. CTP is critical in determining the extent of irreversibly infarcted brain tissue (infarct “core”) and the severely ischemic but potentially salvageable tissue (“penumbra”). This is achieved by generating parametric maps of cerebral blood flow, cerebral blood volume, and mean transit time. Part 1 of this review establishes the clinical context ofCT perfusion (CTP). Next, a discussion follows on CTP map construction by using themaximal slopemethod and the 2 main deconvolution techniques, Fourier transformation (FT) and singular value decomposition (the latter being the most commonly used numeric method in CTP). Part 2 dis-cusses the pearls and pitfalls of CTPmap acquisition, postpro-cessing, and image interpretation. Issues including radiation dosereduction strategies, methods of correcting arterial in-put function (AIF) delay, the effect of laterality of AIF choice, vascular pixel elimination, the importance of correct cerebral blood flow (CBF) and cerebral blood volume (CBV) threshold selection, and the selection of appropriate perfusion parame-ters for correct estimation of penumbra are addressed. The review highlights the need for validation and standardization of important CTP parameters to improve patient outcomes and to design future randomized clinical trials that will pro-vide evidence for the importance of the core/penumbra “mismatch ” in patient triage for recanalization therapies be-yond the current 3-hour therapeutic window for intravenous thrombolysis. Clinical Context of CTP The management of acute ischemic stroke remains challeng-ing because there is a limited time window in which diagnosis must be made and therapy administered. Intravenous tissue plasminogen activator (tPA), to be used within 3 hours of stroke onset based on the 1995 National Institute of Neuro