Brain damage resulting from traumatic brain injury or subarachnoid hemorrhage is a major cause of death and severe disability. A significant contribution to the morbidity arises from secondary brain ischemic damage. The cornerstone of neurocritical care management is optimization of cerebral blood flow to prevent these secondary damages. The challenge is to detect early signs of ischemia in order to start therapeutic measures. An ideal monitoring technique would enable continuous measurement of cerebral blood flow across the whole brain. Unfortunately, no current method allows such monitoring. We propose to develop a small doppler probe implantable in the skull, using Ultrafast Doppler technology, which is an imaging technic that allows a continuous evaluation of cerebral blood flow. It could provide a continuous monitoring of the cerebral blood flow in several region of the brain, and would constitute an ideal monitoring technique for brain injured patient.