The beam vessel angle is between the axis of the ultrasound beam and the axis of a vessel lumen. This angle is equal to the Doppler angle when flow is parallel to the vessel axis.
Also called angle of attack.

The Doppler angle (theta) is the angle of incidence between the ultrasound beam and the estimated flow direction (parallel to the long axis of the vessel). If the beam is parallel to the flowing blood, the Doppler angle is zero, and the determination of flow is most accurate.

See also Beam Vessel Angle, Doppler Effect and Doppler Ultrasound.

(short for ultrasound beam) The sound beam is the confined, directional beam of ultrasound traveling as a longitudinal wave from the transducer face into the propagation medium. The near field and the far field are two separate regions along the beam. Sound beams are either steered mechanically or electrically. Both rapidly sweep sound waves through tissues.

See also Sheer Wave, Beam Vessel Angle, Beam Steering, and Huygens Principle.