The use of the Doppler principle to measure blood velocity is efficiently exploited in clinical practice by precisely aiming a beam of ultrasound through a superficial “ultrasonic window” at the desired target. This allows insonation of otherwise obscure vessels, but it also results in a crucial limitation: The dependence of the velocity measurements on the angle between the ultrasound beam and the direction of blood flow. When a long segment of the vessel can be imaged in one plane, “correcting” the velocity by the appropriate factor (cosine of the angle) is now standard. The result is a more accurate measurement of true velocity, and normal and pathological ranges have been established using this straightforward adjustment.