If denotes the standard deviation of the Doppler spectrum, the Doppler variance () can be derived from autocorrelation technique.25,51,77 Usually, averaging is done to improve SNR. The averaging can be performed either in the lateral direction51 or in both lateral (temporal) and axial (depth) directions.24,25,52 Compared to lateral averaging, only Eq. (5), averaging along both the lateral and depth directions [Eq. (6)], can reduce the background noise and improve the image quality:25,36Display Formula
(6)Here, represents different data points along each axial scan and represents different axial scans; is the time interval between sequential -scans; is the number of -lines that are averaged; and is the number of depth points that are averaged. Values of and are application dependent. Generally, a larger and will increase SNR, but they also increase computation time and decrease resolution.