Pulsed wave (PW) Doppler has the same emitted and reflected pulse frequency because it emits the next ultrasound pulse after receiving the previously reflected one. But, the forward blood flow will interact with the emitted ultrasound pulse and shorten its time of flight (TOF), which creates a positive TOF shift between the calculated TOF and detected TOF. If the velocity of forward flow is too fast and causes the TOF shift more than half of the calculated TOF, the reflected ultrasound pulses are considered as from the previously emitted pulses with longer TOF, which will show negative TOF shift and be misinterpreted as aliasing. This aliasing TOF shift can be completely rectified to its correct registration no matter how fast the forward flow velocity will be. So, the advantages of TOF shift theory can better quantitatively explain the spectral characteristics of PW Doppler, and more accurately calculate the flow velocity.