Pulse Repetition Frequency (PRF) is the number of ultrasound pulses the transducer transmits per second. Higher PRF allows faster Doppler sampling (raising the Nyquist limit and reducing aliasing) but limits maximum imaging depth, because the system must wait for echoes from deep structures before firing the next pulse. In color and pulsed wave Doppler, adjusting the PRF (velocity scale) is the primary tool for optimizing the display of fast or slow flow.