Watershed infarctions occur in areas between vascular supplies. Most often, watershed infarctions are seen between the MCA and ACA territories, or between the MCA and PCA territories. However, deep watershed infarcts may occur between deep and superior penetrating arteries. In the pathologic examples above (axial on the left, coronal on the right), the arteries have been filled with dye.  The red area is the deep watershed between superficial artery (insular artery (IA - branch of the MCA) and cortical medullary penetrating arteries (CMPA)) and the deep arteries (lenticulostriates (LCA) and thalamic performating arteries (TA)).  These infarcts are usually the result of large vessel occlusive disease (e.g., carotid artery stenosis) or cardiac embolus. Adapted from Emre Kumral, Tolga Özdemirkiran, and Yaprak Alper. Strokes in the subinsular territory: Clinical, topographical, and etiological patterns. Neurology 2004 63: 2429-2432.