Which statement about Doppler shifts is correct?

Which statement about Doppler shifts is correct?

• A. Doppler shifts always occur when the source and receiver are in motion.
• B. Doppler shifts occur only when the angle between motion and beam is 90 degrees.
• C. Doppler shifts occur when the source and observer are in motion and the angle is not 90 degrees.
• D. Doppler shifts never occur when motion exists.

Answer: (C)

Doppler shifts depend on motion along the direction of the ultrasound beam. A frequency change is produced only when there is relative motion and that motion has a component toward or away from the transducer, which means the angle between the motion and the beam is not 90 degrees. If the motion is perpendicular to the beam (angle of 90 degrees), the cosine factor becomes zero and no Doppler shift occurs. So shifts occur when there is motion and the angle is not 90 degrees, with stronger shifts as the angle approaches 0 degrees (motion directly toward or away from the beam). That’s why the correct statement is that Doppler shifts occur when the source and observer are in motion and the angle is not 90 degrees. If there is motion but the angle is 90 degrees, no shift; if there is motion along the beam, a shift is observed.

Doppler shifts depend on motion along the direction of the ultrasound beam. A frequency change is produced only when there is relative motion and that motion has a component toward or away from the transducer, which means the angle between the motion and the beam is not 90 degrees. If the motion is perpendicular to the beam (angle of 90 degrees), the cosine factor becomes zero and no Doppler shift occurs. So shifts occur when there is motion and the angle is not 90 degrees, with stronger shifts as the angle approaches 0 degrees (motion directly toward or away from the beam).

That’s why the correct statement is that Doppler shifts occur when the source and observer are in motion and the angle is not 90 degrees. If there is motion but the angle is 90 degrees, no shift; if there is motion along the beam, a shift is observed.