Answer
Oxygen extraction fraction (OEF) measured by positron emission tomography (PET) imaging is considered the criterion standard for imaging the ischemic penumbra in acute ischemic stroke. Until now, MR diffusion-perfusion imaging has been the only MR technique that measures this reversibly damaged brain area.
Blood oxygen level–dependent (BOLD) MRI is a technique that can be used to detect deoxyhemoglobin in the cerebral capillaries and veins as an MRI indicator of brain OEF. [4] Evidence suggests that BOLD MRI might provide a better estimation of the ischemic penumbra in acute ischemic stroke compared with MR diffusion-perfusion mismatch.
Resting state functional connectivity (FC) magnetic resonance imaging (MRI) (R-fMRI) measures, within a subject, the temporal correlation of the blood oxygenation level dependent (BOLD) signal across regions without any imposed task, providing a measure of temporal coherence of activity between brain regions. In a pilot study, we found that patients who received intravenous thrombolysis showed changes in resting state networks and functional outcomes over time. These findings point to an intriguing possibility that the improvement of resting state networks may reflect improved efficiency of brain activity that is potentially related to functional outcomes in acute stroke patients. [5]
Further validation of these techniques is required to confirm their clinical value in imaging of acute ischemic stroke.
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Magnetic resonance imaging in acute stroke. Left: Diffusion-weighted MRI in acute ischemic stroke performed 35 minutes after symptom onset. Right: Apparent diffusion coefficient (ADC) map obtained from the same patient at the same time.
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Magnetic resonance imaging in acute stroke. Left: Perfusion-weighted MRI of a patient who presented 1 hour after onset of stroke symptoms. Right: Mean transfer time (MTT) map of the same patient.
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Magnetic resonance imaging in acute stroke. Diffusion-perfusion mismatch in acute ischemic stroke. The perfusion abnormality (right) is larger than the diffusion abnormality (left), indicating the ischemic penumbra, which is at risk of infarction.
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The diffusion-weighted MRI reveals a region of hypointensity in the distribution of the right middle cerebral artery. Flanking the anterior and posterior regions of this abnormality are regions of hyperintensities, which represent regions of new infarct. The contiguity of these regions suggests that they are extensions of the old infarct.
