Which parameter represents the angle between the ultrasound beam and the direction of flow, essential for velocity calculations?

• A. Blood viscosity
• B. Angle of incidence
• C. Depth of tissue
• D. Doppler frequency

Answer: (B)

The angle between the ultrasound beam and the direction of flow is what lets us convert the Doppler shift into a velocity. In Doppler ultrasound, the measured frequency shift depends on the cosine of that angle, so the velocity is derived by dividing by cos(theta). If the beam runs parallel to the flow (theta near 0), cos(theta) is close to 1 and velocity is calculated most reliably. If the beam is perpendicular to the flow (theta around 90 degrees), cos(theta) is zero and the Doppler signal vanishes, making velocity impossible to determine. So this angle, known as the angle of incidence, is essential for accurate velocity calculations. Other factors like blood viscosity or tissue depth don’t set the velocity from the Doppler equation, and the Doppler frequency is the measured shift used to compute velocity after applying the cos(theta) correction.