According to the Doppler equation, which of the following is NOT true?

• A. Increasing Doppler frequency will increase the frequency shift
• B. Increasing Doppler interrogation angle will increase the frequency shift
• C. No frequency shift is detected when the Doppler beam strikes the reflector with perpendicular incidence
• D. Velocity information can be obtained from the frequency shift if the angle of incidence is known

Answer: (D)

The key idea is that the Doppler frequency shift depends on the component of the target’s velocity along the ultrasound beam, captured by the cos of the incidence angle. The Doppler shift follows fD = (2 f0 v cos theta)/c, so as the angle theta increases, cos theta decreases. That means the frequency shift gets smaller, not larger, when you tilt the beam away from the direction of motion. Perpendicular incidence (theta = 90°) gives cos theta = 0, so no shift at all, which matches the statement about there being no frequency shift in that case. If you know the angle, you can solve for velocity from the measured shift, since v = (fD c)/(2 f0 cos theta). Because of this, the claim that increasing the interrogation angle increases the frequency shift is the one that isn’t true. The other statements align with the equation: higher transmitted frequency increases the shift, perpendicular incidence yields no shift, and velocity can be derived if the angle is known.