Diffusion-weighted MRI as an evolving standard of care in acute stroke

To the Editor: Hacke and Warach1 evaluated the role of MRI in acute stroke. I agree that the ideal neuroimaging protocol in acute stroke should be sensitive in excluding intracranial hemorrhage. CT has been the preferred method of detecting acute hemorrhage because of its depiction of clotted blood as high density against the darker brain tissue. Conventional MRI techniques are insensitive to certain forms of bleeding.2-3 With the advent of echoplanar MRI, it is possible to detect acute intraparenchymal hemorrhage (IPH) using susceptibility-weighted imaging (SWI).2 Echoplanar SWI relies on the detection of deoxyhemoglobin (hypointensity) that develops in IPH in the first few hours after vascular rupture.3 Hacke and Warach1 thus recommend in their editorial that an MRI examination that includes diffusion, perfusion, MR angiography, T2weighted MRI, and SWI is sufficient for the acute stroke setting and would obviate the need for CT. Although I agree with the authors that streamlining of acute stroke imaging protocols is desirable to shorten the time to thrombolysis, their suggested MRI protocol may not be sufficiently sensitive to all types of intracranial hemorrhage. For example, any MRI protocol replacing CT in these patients should be capable of excluding acute subarachnoid hemorrhage (SAH) and intraventricular hemorrhage (IVH).4 None of the sequences suggested by Hacke and Warach1 have been tested in SAH or IVH. Because the subarachnoid and intraventricular compartments are oxygen rich, acute hemorrhages in these regions contain oxyhemoglobin for many hours, and may not form deoxyhemoglobin for several hours or days.3 Thus, SWI may not be capable of sensitively detecting these acute hemorrhages. Fortunately, the fluid-attenuated inversion-recovery (FLAIR) MRI technique has been developed and can be performed rapidly with using fast spin-echo (“fast FLAIR”). We have described a high-resolution FLAIR protocol that is performed in 2 minutes.5 FLAIR MRI has improved the detection of a variety of brain disorders, as compared with conventional MRI. The utility of FLAIR MRI relates to its sensitive detection of lesions causing T2 prolongation (hyperintensity), with nulling of normal CSF/brain background. Because oxyhemoglobin results in T2 prolongation,3 it is not surprising that FLAIR MRI has shown high sensitivity in diagnosing acute SAH and IVH.4 FLAIR MRI is comparable with CT in diagnosing SAH and IVH, and may even surpass CT (figure).4 Thus, I suggest that fast FLAIR MRI be added to acute stroke imaging protocols, and when combined with the protocol suggested by Hacke and Warach,1 FLAIR MRI should lessen the need for CT.