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acute stroke imaging. The multimodal CT approach may include noncontrast CT, perfusion CT, and CT angiography studies. Two types of perfusion techniques are currently available. Whole-brain perfusion CT provides a map of cerebral blood volume, and it is postulated that regions of hypoattenuation on these cerebral blood volume maps represent the ischemic core.110 Although this technique has the advantage of providing whole-brain coverage, it is limited by its inability to provide measures of cerebral blood flow or mean transit time. Alternatively, the second technique, dynamic perfusion CT, has the potential to provide absolute measures of cerebral blood flow, mean transit time, and cerebral blood volume. Dynamic perfusion CT is currently limited to 2 to 4 brain slices and provides incomplete visualization of all pertinent vascular territories. Recent reports demonstrate a high degree of sensitivity and specificity for detecting cerebral ischemia with both of these perfusion CT techniques.111–113 In addition, several studies have suggested that perfusion CT may be able to differentiate thresholds of reversible and irreversible ischemia and thus identify the ischemic penumbra.114,115 Helical CT angiography provides a means to rapidly and noninvasively evaluate the vasculature, both intracranially and extracranially, in acute, subacute, and chronic stroke settings and thus to provide potentially important information about the presence of vessel occlusions or stenoses.116,117 The feasibility of this technique has been demonstrated in the acute stroke setting, with preliminary data suggesting high diagnostic accuracy for evaluation of large-vessel intracranial occlusions as compared with ultrasound and digital subtraction angiography.118–120 These techniques have the advantage of relatively rapid data acquisition and can be performed with conventional CT equipment. Disadvantages include iodine contrast and additional radiation exposure. The role of perfusion CT and CT angiography in making acute treatment decisions has not yet been established. 3. Multimodal MRI The multimodal MRI approach for acute stroke evaluation includes diffusion-weighted imaging (DWI), perfusion-weighted imaging (PWI), MR angiography, gradient echo, and often fluid-attenuated inversion recovery or T2-weighted sequences. Standard MRI sequences (T1 weighted, T2 weighted, and proton density) are relatively insensitive to the changes of acute ischemia.121 DWI allows visualization of ischemic regions within minutes of symptom onset122–131 and early identification of the lesion size, site, and age. It can detect relatively small cortical or subcortical lesions, including those in the brain stem or cerebellum, areas often poorly visualized with standard CT scan techniques. DWI also provides information about the involved vascular territory and has a high sensitivity (88% to 100%) and specificity (95% to 100%) for detecting ischemic lesions, even at very early time points. PWI, usually performed with the rapid administration of an intravenous paramagnetic contrast agent, provides relative measures of cerebral hemodynamic status. Investigations of the best PWI analytical method focus on identifying the highest correlation of ischemic volume with acute clinical deficits (symptomatic hypoperfusion) or with volume of chronic infarct (tissue at risk). Studies have demonstrated that the initial volumes of the lesions seen on DWI and PWI correlate well with the final size of the stroke found on follow-up brain imaging.129,132,133 In addition, these lesion volumes correlate well with severity of stroke as rated by both clinical scales and outcomes. These findings suggest that DWI might provide helpful early prognostic information.125,132 The ischemic penumbra is roughly approximated on MRI as regions of perfusion change without a corresponding diffusion abnormality (diffusion–perfusion mismatch). However, several studies indicate that, at least in some circumstances, the initial diffusion abnormality is reversible and the visually thresholded perfusion volumes overestimate the penumbra.134,135 Sequential MRI studies performed in patients being treated with thrombolytic
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