Spectral Doppler US.

Doppler ultrasonography (US) is used to measure blood velocity by means of the Doppler frequency shift of the echoes received from red blood cells. The two most common types of equipment for measuring Doppler shifts are continuous-wave (CW) Doppler and pulsed, or range-gated, Doppler. A CW Doppler transducer has a separate receiver and transmitter because it transmits continuously. Because of its continuous transmission, there is no depth resolution. CW Doppler systems, however, have high sensitivity and are not susceptible to aliasing. Pulsed Doppler systems transmit a pulse and measure the Doppler shift of the received echo signals from an operator-selectable depth interval (range gate). The ultrasound Doppler shifts occur in the audible range and are displayed in spectral form, with Doppler frequency shifts on the vertical axis and time on the horizontal axis so that the time-history of the blood flow is presented. The amplitude of the Doppler signal (proportional to the number of red blood cells at each velocity) is represented by the monitor gray-scale. The gray-scale image serves as a road map for range-gate placement. Pulsed Doppler US is not as sensitive as CW Doppler US, and aliasing can occur when high velocities are being measured.