(FFT) The fast Fourier transformation is a particularly fast and efficient computational method of performing a Fourier transformation, which is the mathematical process by which raw data is processed into a usable image.
The fast Fourier transform analyzer is a common device that performs spectral analysis in ultrasound instruments. In this case, it displays different quadrature Doppler frequencies or reflector velocities when a sample volume cursor is used along time. The Doppler frequency is proportional to the spectral reflector velocity.

See also Proportionality Constant, and Sampling Rate.

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(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.