he treatment of skull base lesions such as tumors, aneurysms, and traumatic lesions may require internal carotid artery (ICA) sacrifice. Pretherapeutic knowledge of carotid artery dependence is essential in these patients, since vascular bypass or alternative surgical approaches may be necessary in those unable to tolerate ICA sacrifice. Balloon test occlusion (BTO) is an angiographic test to evaluate ischemic tolerance after permanent occlusion of an ICA. During BTO, the cerebral hemisphere ipsilateral to the tested ICA may be perfused by collateral blood flow, depending on the development of collateral pathways including the circle of Willis (COW).