The waveform is the record of a signal that varies over time. A blood flow signal for example, usually varies periodically with the cardiac cycle.

See also Coded Excitation, and Pulse Volume Recording.

Pulse volume recording is a method in which air-filled cuffs are placed segmentally on a limb and a difference in the limb volume associated with arterial pulsation is translated into a waveform. Evaluation of waveform change in this plethysmographic technique indicates the probability of the presence of an obstruction.

See also Plethysmography.

(AI) This index is the ratio between the acceleration of the Doppler spectral waveform and the relative peak systolic velocity. The systolic acceleration is determined by the change in distance between the begin of systolic flow and the peak systolic velocity (cm/sec), divided by the acceleration time (AT - time interval from the onset of flow to the initial peak).
The acceleration index is reported in frequency units as KHz/sec or velocity units as cm/sec2.

The autocorrelation is a mathematical procedure used to quantify periodicity in a Doppler signal and forms the basis of most color Doppler velocity estimators. The autocorrelation multiplies waveforms by successively time-shifted sections of itself.

Increasing the frequency of the transmitted power improves the image quality of ultrasound, but the improvement in resolution results in a decreased signal to noise ratio (SNR). Higher acoustic power levels can prevent the loss in SNR, but among other reasons, ultrasound regulations limit this to avoid heating or cavitation.
Coded excitation increase the signal to noise ratio without the loss of resolution by using coded waveforms. Coded excitation allows transmitting a long wide-band pulse with more acoustic power and high penetration of the sound beam.