Figure 4a. Transverse power Doppler US images show the effects of pulse repetition frequency changes on feeding vessel measurement with the high-frequency transducer. Baseline settings are as follows: transverse view; insonation angle, 40°; peak systolic velocity, 1 m/sec; gain, 30%; filter, low; and color versus echo priority, 100%. Pulse repetition frequency is increased from (a) 500 Hz to (b) 1,000 Hz, (c) 3,000 Hz, and (d) 6,000 Hz, which results in marked decrease in display of flow within the apparent lumen of the normal vessel.