BACKGROUND: Hippocampus is selectively susceptible to ischemic damage and it could be, in some conditions, considered as a border zone of brain ischemia. RESULTS: We illustrate this concept with three cases of hippocampal diffusion-weighted imaging (DWI) restrictions secondary to a carotid artery dissection with an ipsilateral fetal posterior cerebral artery (PCA) in two cases and to global brain hypoperfusion consequent to a cardiac arrest in the last case. CONCLUSION: The hypoperfusion induced by a cardiac arrest or an internal carotid dissection with an incomplete circle of Willis promotes hippocampal ischemia in the territories of the anterior choroidal artery and the longitudinal terminal segments of the hippocampal arteries.