CT Perfusion Imaging in Acute Stroke

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Acute Stroke CT Imaging Protocol

At a recent meeting of stroke radiologists, emergency physicians, neurologists, and National Institutes of Health (NIH) administrators in Washington, DC, sponsored by the NIH and the American Society of Neuroradiology, both the technical and clinical issues regarding advanced acute stroke imaging were discussed. A position paper of the expert consensus achieved was published simultaneously in the American Journal of Neuroradiology and Stroke.40, 41 In these articles, expert recommendations for

Quantitation and resolution

While CTP and MR-PWI both attempt to evaluate capillary-level hemodynamics, their differences should be considered. Dynamic susceptibility contrast (DSC) MR-PWI techniques rely on the T2* effect induced in adjacent tissues by high concentrations of intravenous gadolinium; CTP relies on direct visualization of intravascular contrast material. The linear relationship between contrast concentration and CT attenuation more readily lends itself to quantitation, which is not easily achieved with

Radiation dose in CTP: facts, publicity, and response

On October 8, 2009, the US FDA issued an initial notification regarding a safety investigation of facilities performing CTP scans. Because of incorrect settings on the CT scanner console of a single facility, more than 200 patients over a period of 18 months received radiation doses that were approximately 8 times the expected level. Approximately 40% of the patients lost patches of hair as a result of the overdoses.75 These incidents followed another unrelated incident in a community hospital

CTP methodology: general principles

Perfusion-weighted CT and MR, in distinction to those of MR and CTA which detect bulk vessel flow, are sensitive to capillary, tissue-level blood flow.80 The generic term “cerebral perfusion” refers to tissue level blood flow in the brain. This flow can be described using a variety of parameters, which primarily include cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT). Definitions of these parameters are as follows.

CBV is defined as the total volume of blood

Mathematical Models for CBF and MTT Calculation

The two major mathematical approaches involved in calculating CBF and MTT are the deconvolution and nondeconvolution-based methods (for a comprehensive review see Konstas and colleagues79). Deconvolution techniques are technically more demanding and involve more complicated and time-consuming processing, whereas nondeconvolution techniques are more straightforward, but depend on simplified assumptions regarding the underlying vascular architecture. As a result, the interpretation of studies

General Principles

In urgent clinical cases, perfusion changes can often be observed immediately following scanning by direct visual inspection of the axial source images at the CT scanner console. Soft copy review at a workstation using “movie” or “cine” mode can reveal relative perfusion changes over time. It has been suggested that arterial-phase CTP source images (CTP-SI) closely correlate with CTP-CBF and venous-phase CTP-SI with CBV; hence a visual inspection of the CTP-SI can determine penumbra, core, and

Correction for delay and dispersion of the AIF: why it matters

Calculation of CBF requires knowledge of the AIF, which in practice is estimated from a major artery, assuming that it represents the exact and only input to the tissue voxel of interest, with neither “delay” nor “dispersion.” There are several clinical situations, however, where the AIF TDC will lag, and the tissue TDC will lag behind the AIF curve (“delay”). AIF delay can occur due to extracranial causes (atrial fibrillation, severe carotid stenosis, poor left ventricular ejection fraction)

Clinical applications of CTP

Current indications for performing CTP imaging of acute stroke include: (1) when DWI is unavailable, (2) when the (a) diagnosis, (b) type of stroke, or (c) extent of ischemia is uncertain based on the clinical examination and other imaging, or (3) for postaneurysmal subarachnoid hemorrhage (SAH) vasospasm risk assessment (given cumulative radiation dose issues, performing more than 2 or 3 serial CTPs per admission is ill advised). Although to date there is no high level of evidence to suggest

Image interpretation: infarct detection with CTA-SI

Several groups have suggested that CTA source images, like DWI, may sensitively delineate tissue destined to infarct despite successful recanalization.26, 32, 117 The superior accuracy of CTA-SI in identifying infarct “core” compared with unenhanced CT has been unequivocally established in multiple studies.45, 117, 118, 119, 120 Theoretical modeling indicates that early generation CTA-SI—assuming an approximately steady state of contrast in the brain arteries and parenchyma during image

CTP interpretation: ischemic penumbra and infarct core

CTP maps cannot and should not be interpreted in a vacuum. Accurate CTP image interpretation requires correlation with the clinical presentation, noncontrast CT (NCCT), CTA, CTA-SI, and CT-MTT/CBV/CBF data. A qualitative estimate of the presence and degree of ischemia is typically what is required for guiding clinical management, and this can be accomplished with a variety of vendor platforms, despite the current lack of complete standardization with regard to acquisition, postprocessing, and

Imaging predictors of clinical outcome

Predicting outcome is perilous. The penumbra is dynamic, and several factors influence its fate, including time-post ictus, residual and collateral blood flow, admission glucose, temperature, hematocrit, systolic blood pressure, and treatment, including hyperoxia.147 As already mentioned, CTA/CTP has the potential to serve as a surrogate marker of stroke severity, likely exceeding the NIHSS score or ASPECTS as a predictor of outcome.27, 28, 29, 30, 31, 32, 33, 34, 35

Summary

As new treatments are developed for stroke, the potential clinical applications of CTP imaging in the diagnosis, triage, and therapeutic monitoring of these diseases are certain to increase.

Validation and standardization of CTP methodology will be crucial for the widespread acceptance of advanced imaging in patient selection for novel stroke therapies. Expert consensus regarding standardization of CTP image acquisition has already been reached at the “Advanced Neuroimaging for Acute Stroke

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