Most of the recent studies for image uniformity assessment involve signal to noise ratio (SNR) analysis computed over various combinations of mean grey level and variance [1–2] to quantify the speckle contrast and hence the depth of penetration (DOP). Speckle is auto correlated in time but not in space. Electronic noise always present at the bottom of the image, instead, is uncorrelated both in time and space. The extension of the area in which only the speckle is visible, gives an estimate of the penetration depth. The correlation coefficient of two subsequently acquired frames can be computed to quantify image uniformity where depth at which the correlation coefficient falls below a fixed threshold is defined as penetration depth.

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