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The physical principles of continuous-wave and pulsedwave Doppler as well as the processing methods to detect the direction of flow are reviewed. An overview of Doppler instrumentation, including duplex, zero crossing, range-gated, and color flow imaging detection systems is presented. The technique of spectral analysis is explained with multiple illustrative examples. Aliasing, which arises from the limited sampling rate of pulsed wave instruments, is discussed in detail.

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Thermal and mechanical indices are acoustic output descriptors that provide a means of assessing the potential for biologic effects from exposure to diagnostic ultrasound. Models and assumptions used in the estimation of acoustic output indices are discussed with illustrative diagrams. The defining equation for each index is presented. Safety guidelines based on the index values are reviewed.