Optimum scan spacing for three-dimensional ultrasound by speckle statistics

A technique is introduced to determine the spacing between acquired two-dimensional (2-D) planes necessary to maintain resolution in the scan direction while maximizing scan speed for 3-D ultrasound (US) imaging. We performed statistical analysis on a series of 3-D scans of agar blocks with different image spacings, quantifying speckle size (S(c)) as the full-width half-maximum (FWHM) of the autocovariance function. S(c) was approximately constant at small 2-D image-plane spacings, but increased after the scan spacing surpassed some optimum point. Optimum spacing ranged between 0.075 and 0.4 mm, increasing with the axial depth and the number of focal zones. This latter dependence is a result of successive sampling by the US machine and the digitizer at unequal rates, as demonstrated through an analysis of the noise power spectra. This analysis predicted that S(c) may be significantly reduced by using a transducer sampling rate that is twice the digitization rate. This technique permits the calculation of the most efficient scan spacings for 3-D US imaging.