The so-called diffusion/perfusion mismatch is used in ischaemic strokes to differentiate between the irreversibly damaged part of the brain (infarct core) and the functionally impaired but still viable brain tissue (penumbra).

Diffusion MRI (DWI: diffusion-weighted imaging) measures the movement of water molecules in the brain tissue and is the best available method for early detection and visualisation of the infarct core.

The perfusion sequence (PWI: perfusion-weighted imaging) is used to visualise the blood flow (perfusion) of the brain and, in this case, serves to visualise the areas of reduced blood flow.

By comparing the perfusion sequence with the diffusion sequence, the penumbral tissue that still needs to be saved can be determined. There is a "mismatch" if the perfusion sequence is greater than the diffusion sequence, i.e. if the zones of hypoperfusion are greater than the infarct core.

The diffusion/perfusion mismatch (DWI/PWI mismatch) can provide information for assessing the prognosis and guiding further treatment. With timely reperfusion therapy, the tissue in the penumbra can still be saved. In the absence of a penumbra, the expected benefit may be less than the risk of haemorrhage.