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therapy have shown that the technique may visualize salvage of mismatch-defined penumbral tissue with smaller volumes of infarction among patients who have successful recanalization.134,136 Efforts are under way to develop multiparametric MRI criteria that could identify regions of irreversible infarction from potentially reversible ischemia or portend a high risk of hemorrhagic complications after thrombolytic therapy.137–139 A recent phase II trial of intravenous administration of the thrombolytic agent desmoteplase showed a signal of potential therapeutic benefit when MRI was used to select patients with diffusion–perfusion mismatch for treatment 3 to 9 hours from onset.140 However, insufficient evidence currently exists to recommend this approach for selecting patients for acute therapies in routine practice. Two prospective studies recently demonstrated that MRI is as accurate as CT in detecting hyperacute intraparenchymal hemorrhage in patients presenting with stroke symptoms within 6 hours of onset when gradient echo MRI sequences were used.88,141 These findings suggest that MRI may be used as the sole imaging modality to evaluate acute stroke patients, including candidates for thrombolytic treatment. However, in patients presenting with symptoms suggestive of subarachnoid hemorrhage, a CT scan should be performed. Gradient echo sequences also have the ability to detect clinically silent prior microbleeds not visualized on CT. Some data suggest that microbleeds represent markers of bleeding-prone angiopathy and increased risk of hemorrhagic transformation after antithrombotic and thrombolytic therapy.142–144 However, other studies have not found an increased risk in patients with small numbers of microbleeds.145 The importance of the presence of large numbers of microbleeds on MRI in thrombolytic decision making remains uncertain. MR angiography is increasingly used for noninvasive screening of the extracranial and intracranial circulation. When compared with digital subtraction angiography for detection of cervical and intracranial stenoses, sensitivity and specificity have ranged from 70% to 100%.146,147 In the intracranial vasculature, MR angiography is useful in identifying acute proximal large-vessel occlusions but cannot reliably identify distal or branch occlusions. A potential diagnostic advantage of MRI over CT in non-tPA situations in suspected stroke has been demonstrated. MRI is better at distinguishing acute, small cortical, small deep, and posterior fossa infarcts; at distinguishing acute from chronic ischemia; and at identifying subclinical satellite ischemic lesions that provide information on stroke mechanism.95,124,129,148–167 Limitations of MRI in the acute setting include cost, relatively limited availability of the test, and patient contraindications such as claustrophobia, cardiac pacemakers, or metal implants. Advantages include the avoidance of exposure to ionizing radiation and iodinated contrast and greater spatial resolution 4. Other Brain Imaging Techniques Oxygen-15 positron-emission tomography may quantify regional brain perfusion and oxygen consumption.168–172 However, logistical and pragmatic considerations limit the application of positron-emission tomography in the setting of acute stroke. Xenon-enhanced CT provides a quantitative measurement of cerebral blood flow by using inhaled xenon but is not currently widely available.173 Single-photon emission CT, which is minimally invasive and measures relative cerebral blood flow, might be able to identify thresholds for reversible ischemia and could be helpful in predicting outcomes or monitoring responses to treatment.174–176 Limitations include lack of availability, expense, and difficulty associated with tracer preparation. B. Other Vascular ImagingIn addition to the aforementioned CT and MR angiography, transcranial Doppler ultrasonography, carotid duplex sonography, and catheter angiography have been used to detect intracranial or extracranial vessel abnormalities. Transcranial Doppler ultrasonography and angiography have been used to monitor the effects of thrombolytic therapy over time and can help
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