 Rupture of a cerebral aneurysm causes subarachnoid hemorrhage SAH. Blood in the subarachnoid space of the brain can cause irritation of the cerebral blood vessels, leading to constriction of these vessels, a phenomenon known.as vasospasm. Cerebral vasospasm can cause stroke and possibly death. Of all the patients with SAH, approximately 20-40 will suffer from clinical vasospasm and more than 60 of those patients will never get back to their previous functional status. Tools to identify early vasospasm and thus early treatment could greatly decrease the morbidity and mortality following SAH. 