DW and perfusion-weighted MR imaging for assessment of stroke evolution

The combination of perfusion-weighted and DW MR imaging may provide more information than would either technique alone. Perfusion-weighted imaging involves the detection of a decrease in signal intensity as a result of the susceptibility or T2* effects of gadolinium during the passage of a bolus of a gadolinium-based contrast agent through the intracranial vasculature . A variety of hemodynamic images may be constructed from these data, including relative cerebral blood volume, relative cerebral blood flow, mean transit time, and time-to-peak maps.

In the context of arterial occlusion, brain regions with decreased diffusion and perfusion are thought to represent nonviable tissue or the core of an infarction. The majority of stroke lesions increase in volume on DW images, with the maximum volume achieved at 2–3 days.

When most patients with acute stroke are evaluated with both DW and perfusion-weighted MR imaging, their images usually demonstrate one of three patterns: A lesion is smaller on DW images than the same lesion is on perfusion-weighted images; a lesion on DW images is equal to or larger than that on perfusion-weighted images; or a lesion is depicted on DW images but is not demonstrable on perfusion-weighted images. In large-vessel stroke lesions (such as in the proximal portion of the middle cerebral artery), the abnormality as depicted on perfusion-weighted images is frequently larger than the lesion as depicted on DW images. The peripheral region, characterized by normal diffusion and decreased perfusion, usually progresses to infarction unless there is early reperfusion. Thus, in the acute setting, perfusion-weighted imaging in combination with DW imaging helps identify an operational “ischemic penumbra” or area at risk for infarction (Fig). 

Diffusion-perfusion mismatch after left middle cerebral artery stroke. The patient was imaged 3.8 hours after a witnessed sudden onset of a right hemiparesis. Transverse DW images demonstrate hyperintensity in the subcortical region, including in the lenticular nucleus and corona radiata (arrowheads, right-hand image in top row). Transverse cerebral blood volume (CBV) images demonstrate decreased dynamic cerebral blood volume in the region of hyperintensity on the DW images. However, there are areas of abnormal cerebral blood volume (arrows) that appear relatively normal on the DW study. Follow-up study performed 10 hours after the onset of symptoms demonstrates an increase in the size of the DW imaging abnormality (arrowheads, right-hand image in fifth row) as it extends into the region of brain that was previously normal on DW images but abnormal on cerebral blood volume images. 

 On the other hand, in small-vessel infarctions (perforator infarctions and distal middle cerebral artery infarctions), the initial lesion volumes on perfusion-weighted and DW images are usually similar, and the diffusion-weighted image lesion volume increases only slightly with time. A lesion larger on DW images than on perfusion-weighted images or a lesion visible on DW images but not on perfusion-weighted images usually occurs with early reperfusion. In this situation, the lesion on DW images usually does not change substantially over time.