Effect of Doppler Examination Angle. All 8 images were acquired from the same location in the same artery within the same examination period using the Doppler equation to correct for the geometric angle between the ultrasound beam and the artery axis. All duplex Doppler ultrasound instruments use this method to adjust the velocity measurement value reported. This systematic measurement bias (higher values for larger angles) appears in all peripheral arteries. The sole exception is measurements distal to long straight arteries such as the distal superficial femoral artery. For equal Doppler angles, steering the ultrasound beam toward the feet (upper row) appears to provide similar values to steering the ultrasound beam toward the head (lower row), if the same Doppler angle is used, but the effect of this variable should be quantified.