CT Perfusion in Acute Stroke
Section snippets
Acute stroke protocol
A protocol for the imaging of acute stroke should address the central questions necessary to triage patients appropriately (see Box 2). The acute stroke protocol used at our institution has three components: unenhanced CT, arch-to-vertex CTA, and dynamic first-pass cine CTP. A similar CTA/CTP protocol, or its equivalent, could be applied using any commercially available multidetector-row helical CT scanner with only minor variations.
The role of unenhanced CT in stroke triage is principally to
Quantitation and resolution
Although CTP and MR-PWI both attempt to evaluate the intricacies of capillary level hemodynamics, the differences in technique create several important distinctions that should be considered (Fig. 1). Although dynamic susceptibility contrast (DSC) MR-PWI techniques rely on the indirect T2* effect induced in adjacent tissues by high concentrations of intravenous gadolinium, CTP relies on direct visualization of the contrast material. The linear relation between contrast concentration and
CT perfusion: general principles
Perfusion-weighted CT and MR imaging techniques—as opposed to those of MR angiography and CTA, which detect bulk vessel flow—are sensitive to capillary tissue level blood flow [70]. This evaluation of capillary level hemodynamics extends the traditional anatomic role of imaging to provide insight into the delivery of blood to brain parenchyma. The idea of contrast-enhanced CTP imaging emerged as early as 1976, when a computerized subtraction technique was used to measure relative cerebral blood
CT perfusion theory and modeling
Although easy to define in theory, the perfusion parameters of CBV, CBF, and MTT can be difficult to quantify in practice. The dynamic first-pass approach to CTP measurement involves the dynamic intravenous administration of an intravascular contrast agent, which is tracked with serial imaging during its first-pass circulation through the brain tissue capillary bed. Depending on the assumptions regarding the arterial inflow and the venous outflow of the tracer, the perfusion parameters of CBV,
CT perfusion postprocessing
In urgent clinical cases, perfusion changes can often be observed immediately after 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, although advanced postprocessing is required to appreciate subtle changes and to obtain quantification. Axial source images acquired from a cine CTP study are networked to a freestanding workstation for detailed
Clinical applications of CT perfusion
Indications (and potential indications) for advanced functional imaging of stroke in the first 12 hours include the following: (1) exclusion of patients most likely to hemorrhage and inclusion of patients most likely to benefit from thrombolysis; (2) extension of the time window beyond 3 hours for intravenous and 6 hours for anterior circulation intra-arterial thrombolysis; (3) triage to other available therapies, such as hypertension or hyperoxia administration; (4) disposition decisions
CT perfusion interpretation: infarct detection
A number of groups have suggested that CTA source images, such as DWI, can sensitively detect tissue destined to infarct despite successful recanalization (Fig. 3) [29], [93], [94]. Theoretic modeling indicates that CTA-SI, assuming an approximately steady state of contrast in the brain arteries and parenchyma during image acquisition, are predominantly blood volume weighted rather than blood flow weighted, although this has yet to be validated empirically in a large series [20], [43], [44],
CT perfusion interpretation: ischemic penumbra and infarct core
An important goal of advanced stroke imaging is to provide an assessment of ischemic tissue viability that transcends an arbitrary clock-time [103], [104], [105]. The original theory of penumbra stems from experimental studies in which two thresholds were characterized [106]. One threshold identified a CBF value below which there was cessation of cortical function, without increase in extracellular potassium or reduction in pH. A second lower threshold identified a CBF value below which there
Imaging predictors of clinical outcome
Predicting outcome is perilous. The penumbra is dynamic, and several factors influence its fate, including time after ictus, residual and collateral blood flow, admission glucose, temperature, hematocrit, systolic blood pressure, and treatment (including hyperoxia) [121]. 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 [24], [25], [26], [27], [28], [29], [30], [31], [32].
Experimental applications of CT perfusion in stroke
The additional information about capillary level hemodynamics afforded by CTP could be particularly important in future clinical trials of acute stroke therapy, in which CTP could refine the selection of subjects to include only those patients most likely to benefit from treatment; this imaging-guided patient selection may help to demonstrate beneficial effects that would be obscured if patients without salvageable tissue were included. CTA combined with CTP could be used to identify patients
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.
Technical advances in scanner hardware and software should no doubt continue to increase the speed, coverage, and resolution of CTP imaging. CTP offers the promise of efficient use of imaging resources and, potentially, of decreased morbidity. Most importantly, current CT technology already permits the
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