Doppler techniques are dependent on the transducers used. The transducer operating in continuous wave mode utilizes one half of the elements and is continuously sending sound energy while the other half is continuously receiving the reflected signals. If the transducer is being used in a pulsed wave mode, the whole transducer is used to send and then receive the returning signals.
Pulsed wave techniques allow the accurate measurement of blood flow at a specific area in the heart and the detection of both velocity and direction. Measurement is performed by timing the reception of the returning signals giving a view of flows at specific depths. The region where flow velocities are measured is called the sample volume. Errors in the accuracy of the information arise if the velocities exceed a certain speed. The highest velocity accurately measured is called the Nyquist limit.

See also Doppler Velocity Signal and Doppler Effect.

(CWD) Continuous wave (CW) Doppler is an ultrasound imaging mode, which records blood flow velocities along the length of the beam. Continuous wave Doppler uses different crystals to send and receive the signal. The transducer operating in continuous wave mode utilizes one half of the elements and is continuously sending sound waves of a single frequency while the other half is continuously receiving the reflected signals.
The advantages of a continuous wave transducer are a high sensitivity and no Nyquist limit. CW Doppler does not alias but has no depth precision and large gate. The beat frequency is the Doppler shift. CW Doppler echocardiography employs this technique to record the flow of blood through the cardiovascular system.

See also Cross Talk, Periorbital Doppler, and Mirror Artifact.

(PWD) Pulsed wave (PW) Doppler is a Doppler ultrasound mode that evaluates blood flow velocities in a range specific area along the length of the sound beam. Measured are changes in received frequency due to relative motion (flow) between a sound source (transducer) and sound receiver (transducer).
PW Doppler produces an audible signal as well as a graphical representation of flow. The Doppler shift produced by moving blood flow is calculated by the ultrasound system.

See also Amplitude Indicator, Pulsed Ultrasound.

(PD) Power Doppler imaging (PDI) is a Doppler technique, sensitive to low blood flow, allowing a complete visualization of detailed vascular blood structure. This medical imaging method is useable for detecting microbubbles during myocardial contrast echocardiography.

See also Resistive Index.

Selective detection of the microbubble contrast medium can be enhanced by Doppler processing that removes signals with zero Doppler frequency shifts. This will remove tissue harmonics. By detecting overlong bursts of inverted pulses and using Doppler detection methods, very high sensitivity to microbubbles can be achieved. The bubbles can be detected at sufficiently low incident power levels to avoid destroying them. Pulse inversion Doppler has demonstrated the first real-time images of myocardial perfusion using perfluorocarbon gas agents.

See also Pulse Inversion Imaging, Myocardial Contrast Echocardiography, and Perfluorochemicals.