Answer
In this phase, edema increases, maximizing at 48-72 hours, and MRI signals become more prominent and well demarcated. The ischemic area continues to appear as an area of hypointensity on T1-WI and as a hyperintense area on T2-WI. In addition, the mass effect can be appreciated in this phase.
In contrast-enhanced images, the arterial enhancement usually persists throughout the acute phase, while the parenchymal enhancement is usually appreciated at the end of this phase in complete infarction. In incomplete infarction, the parenchymal enhancement is usually earlier.
During this period, reperfusion occurs and petechial and frank hemorrhages can be observed, typically 24-48 hours after the onset of the stroke. Usually, petechial hemorrhages cause the "fogging" phenomenon, due to hemoglobin degradation products, that masks the infarction on T1-WI and T2-WI.
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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.
