Diffusion and Perfusion MRI Imaging in Acute stroke

During the last 2 decades, MRI techniques have gained an increasing role in acute stroke management. Animal and human studies have demonstrated that diffusion-weighted imaging (DWI) combined with perfusion-weighted imaging (PWI) sequences including regional cerebral blood volume (rCBV), relative CBF (rCBF), and mean transit time (MTT) maps can accurately represent the ischemic penumbra and predict the final size of the infarct. DWI is an advanced MRI technique that is able to detect water diffusion in brain tissue. Compared to normal brain tissue, ischemic brain tissue water diffusion coefficients change and can be represented as signal intensity changes of MRI. DWI MRI correlates well with the cellular metabolism and cytotoxic edema and predicts an rCBF below 15 to 20 mL/100 g/min, allowing ultra-early diagnosis of stroke. The protons on water molecules in ischemic areas have restricted diffusion and will generate hyperintense signals and display a correspondent signal drop on the apparent diffusion coefficient maps. Hyperintense lesions on DWI are considered the MR equivalent of the infarction core, whereas the mismatch between the DWI abnormal signal and perfusion deficit on PWI represents the ischemic penumbra . DWI and PWI enable the diagnosis of small-vessel stroke (lacunes). Patients with lacunar strokes can have severe deficits that mimic large-vessel occlusions but are caused by small-vessel occlusions. MRI techniques can help guide the use of intravenous thrombolytic and mechanical revascularization techniques.

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Initially considered a reflection of irreversible ischemic damage, DWI lesions are reversible to some degree. In cases of early reperfusion, these images can overestimate the ischemic penumbra. These hyperreperfused areas depict regions of benign oligemia rather than ischemia. Therefore, an ischemic lesion can be divided into four regions: ischemic core, reversible DWI lesions, penumbra, and benign oligemia; both penumbra and reversible DWI lesions are targets for early thrombolysis.