The Doppler angle (angle of insonation) is the angle between the ultrasound beam and the direction of blood flow. Because the Doppler equation calculates velocity using the cosine of this angle, accuracy degrades significantly as the angle approaches 90° (cosine approaches zero, making small angle errors produce large velocity errors). For accurate velocity measurements, the Doppler angle should be 60° or less, and ideally 45–60°. Most ultrasound machines display an angle-correction cursor that the sonographer steers to align with the vessel wall.