A Doppler sample volume defines the depth interval from which signals are sampled. How does increasing the sample volume length affect velocity measurement accuracy?

• A. Longer volumes improve spatial precision by averaging more velocities
• B. Longer volumes reduce spatial precision by averaging more velocities
• C. Longer volumes have no effect on accuracy
• D. Longer volumes increase temporal resolution

Answer: (B)

The key idea is that the Doppler sample volume sets the depth range from which velocities are measured. When you increase its length, you’re averaging velocities over a larger slice of tissue along the beam. The velocity you report is effectively the average over that depth interval, so any real velocity differences within that span get smeared out. This reduces spatial precision, because you can’t localize the velocity to a narrow depth. The longer gate can improve signal stability by averaging more echoes, but it does not enhance depth-specific (spatial) accuracy and it does not increase how fast you can update the velocity reading.