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

Abstract

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.P art 1 of this review establishes the clinical context of CT perfusion (CTP). Next, a discussion follows on CTP map construction by using the maximal slope method 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 discusses the pearls and pitfalls of CTP map acquisition, postprocessing, and image interpretation. Issues including radiation doseϪreduction strategies, methods of correcting arterial input 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 parameters 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 provide evidence for the importance of the core/penumbra "mismatch" in patient triage for recanalization therapies beyond the current 3-hour therapeutic window for intravenous thrombolysis.

A Fast Nonlinear Regression Method for Estimating Permeability in CT Perfusion Imaging

“…[14][15][16] Subsequently, the CBV can be estimated by dividing the AUC of the tissue curve by the AUC of the AIF. 10 The bolus arrival time (BAT), which is in this study used to define the start of contrast enhancement, is defined as the 0.05% quantile of the gamma variate fit ( Figure 1B). …”

“…For example, the CBV, which should measure the intravascular volume only, may be overestimated by methods that do not take into account the additional extravascular distribution volume due to increased permeability of the blood-brain barrier. 9,10 As an alternative to the Patlak method, the use of a tissue perfusion model applied with nonlinear regression (NLR) uses the full length of the attenuation-time curves, does not transform the measurement errors, and allows for a simultaneous measurement of all perfusion parameters. For these reasons, NLR methods may provide a superior alternative to the use of Patlak plots in stroke imaging.…”

“…10 The values used for H LV and H SV are 0.45 and 0.25, respectively. 33 This correction affects all perfusion parameters that scale with the plasma concentration, i.e., the CBV, CBF, K trans , and V e .…”

Advanced neuroimaging in acute ischemic stroke: extending the time window for treatment

“…Cerebral blood volume is defined as the volume of blood passing through a volume of brain tissue per unit time, and MTT is defined as the average transit time of blood through a given brain region and is measured in seconds. 32 Neuronal dysfunction results after reduction in the CBF below 20 ml/100 g/min. 4,26 Further reductions in CBF result in failure of the sodium/potassium adenosine triphosphate pump and the ability to maintain the normal cellular osmotic gradient resulting in cellular death.…”

FDA Investigates the Safety of Brain Perfusion CT

“…When using such parameters, the effective radiation dose associated with a single slab PCT study is approximately equal to that of an unenhanced head CT, roughly 2-3 mSv. 10,11 A comprehensive stroke CT protocol that includes an unenhanced and postcontrast head CT, PCT, and CTA of the cervical and intracranial arteries may deliver a mean effective dose up to 6 times that of a standard, unenhanced head CT. 12 Not every scan sequence, however, need be performed for every patient. Dedicated stroke protocols should be tailored to specific clinical indications, and radiation reduction strategies such as adaptive dose modulation, not to mention MR imaging scanning when feasible, should be implemented as appropriate.…”