Watershed infarctions typically occur at the junctional zones or border zone of the arterial territories due to reduced cerebral perfusion of variable etiologies.

1. external or Cortical watershed infarcts: these are the more common ( and widely published ) variety of watershed infarctions. These are seen at the junction of the major arterial territories, most commonly between the anterior cerebral artery (ACA) - middle cerebral artery (MCA) and middle cerebral artery (MCA) – posterior cerebral artery (PCA). These are seen as larger confluent white matter hyper-intensities on T2 weighted MR images and are usually wedge-shaped or oval in shape
2. internal watershed infarcts: these are relatively smaller foci of restricted diffusion seen symmetrically in both cerebral hemispheres, in the deep white matter, subcortical white matter, and in the basal ganglia region in particular in the zone of the lenticulostriate perforator circulation

Case courtesy Dr. Ahmed Ajamy Elsayed