The sample volume (or Doppler gate) is an adjustable cursor placed on the 2D image to define the specific location and depth from which pulsed wave Doppler velocity information is collected. Its size can be adjusted — smaller sample volumes improve specificity by sampling a narrower region; larger volumes capture flow across the full vessel diameter. Correct sample volume placement within the vessel lumen, combined with proper angle correction, is essential for accurate spectral Doppler velocity measurements.