Female, 66 years old, with altered mental status. Innumerable scattered foci of restricted diffusion are seen involving the watershed regions of the cerebral hemispheres, the basal ganglia, left thalamus, and the posterior fossa. The most conspicuous areas of restricted diffusion are in the bilateral parietal and occipital lobes and the cerebellum. A relatively large area of mild restricted diffusion without correlating T2 signal abnormality is seen involving the right parietotemporal junction.Most of the above-mentioned areas are associated with non-masslike gyriform enhancement. The right parietal and bilateral occipital lesions show evidence of gyriform T1 hyperintensity suggesting laminar necrosis. No evidence of basal cistern or cranial nerve enhancement is seen.Apart from those lesions which are diffusion restricting, there are numerous additional areas of periventricular and subcortical T2 hyperintensity which do not diffusion restrict. No evidence of significant generalized mass effect is seen. No brain herniation is demonstrated. The ventricles and sulci are proportionately prominent compatible with volume loss. No abnormal fluid collections are detected.
Innumerable areas of restricted diffusion are seen indicating cytotoxic edema or a mixture of cytotoxic and vasogenic edema both supratentorially and infratentorially. There is a marked predominance of lesions in the cerebral watershed zones, the parietal and occipital lobes, and in the cerebellum. One rather large and ill-defined area at the right parietotemporal junction demonstrates a curious pattern of mild restricted diffusion with no correlating T2 signal abnormality. Most of the lesions demonstrate non-masslike gyriform enhancement indicating blood brain barrier breakdown.The distribution of the lesions noted above, with a preference for the watershed zones and posterior circulation, is very suggestive of a PRES-like phenomenon. Lesions related to PRES do not typically diffusion restrict to this degree, but it has been reported in severe cases. The rather curious pattern of mild restricted diffusion without T2 signal abnormality at the right parietotemporal junction is also supportive of an atypical vascular process such as PRES.Other differential considerations, including embolic ischemic disease and metastatic/neoplastic disease, are considered much less likely based on the imaging characteristics. 