Systems and methods are disclosed providing for the use of ultrasound energy to measure blood flow within blood vessels by Doppler velocity measurement. Directional high-frequency pulsed-wave Doppler measurements can be made with a suitable ultrasonic needle transducers for in vivo measuring of blood flow. A needle probe can include a ultrasonic material such as PMN-PT. Such blood flow measurements can be made in any part of the body, e.g., in the central retinal vein and branch retinal veins.