String test objects are used to evaluate which aspect of Doppler performance?

• A. Signal amplitude
• B. Doppler accuracy
• C. Color encoding fidelity
• D. Velocity resolution

Answer: (B)

String test objects provide a controlled, known velocity source inside the ultrasound beam. By moving a string at precise speeds, the Doppler system produces frequency shifts that should translate into those exact velocities. This lets you directly gauge how accurate the Doppler velocity readout is—whether the measured values match the true speeds across depths and angles. If the readings line up with the known motion, Doppler performance is validated. The other aspects aren’t the primary focus here: signal amplitude tests look at echo strength, color encoding fidelity concerns how color is displayed rather than velocity, and velocity resolution measures the ability to separate very close speeds rather than the overall accuracy of the velocity estimate.