This test is usually performed with the patient upright either standing or during head up tilt. A doppler probe will be placed on the head near the temple. The probe will emit high frequency sound waves (too high of a frequency to be heard) and detect them after they bounce off structures in the head. When a specific frequency is used, the moving blood inside the head causes changes to the sound waves that are related to how fast the blood is moving. This allows the determination of the speed of the blood in the cerebral vessels.