Summary of Diffusion Perfusion mismatch

Look to the lesion seen in the diffusion weighted images and compare its size to the lesion seen in the perfusion weighted image.

1-If the lesion in DWI is smaller than that seen in PWI, this means that the obstructed vessel is a large vessel and that the stroke happened is a large vessel stroke  as the proximal part of the middle cerebral artery.

2-If the lesion in DWI is equal to or larger than that seen in PWI, this means early reperfusion, here usually lesion seen in DWI does not change over time.

3-If the lesion in DWI is seen while it is not seen in PWI, this also  means early reperfusion, here also usually lesion seen in DWI does not change over time.

4-If no lesion seen in DWI while there is a lesion seen in the PWI, this means ischemic penumbra which means ischemic area at risk for infarction. This usually occurs due to obstruction of perforator arteries or distal portion of middle cerebral artery. 

5- Brain regions with decreased diffusion and perfusion are thought to represent nonviable tissue or the core of an infarction.

Conclusion:

-DWI lesion<PWI = Large vessel stroke.

-DWI lesion= or > PWI = Early reperfusion.

-DWI lesion seen and PWI not seen = Early reperfusion.

-DWI lesion not seen and PWI seen = Ischemic penumbra.

-Brain region with decreased diffusion and perfusion = Non viable tissue.