Documentation and measurement of intraoperative cerebral blood flow during various neurosurgical procedures is not only valuable but also very informative. There are various methods by which qualitative and quantitative measurement of blood flow have been developed over the years. The use of perivascular ultrasonic flow more recently is fast gaining popularity. We describe the technique, principle and application of ultrasonic perivascular micro-flow probe quantitative measurement of selective vessel flow in neurosurgery.