1. A system for three-dimensional imaging of an object volume, comprising: an ultrasound transducer array for transmitting ultrasound beams and detecting ultrasound echoes reflected from said object volume at a multiplicity of sample volumes in a scan plane; means coupled to said ultrasound transducer array for acquiring imaging data derived from ultrasound echoes reflected from each one of a multiplicity of scan planes through said object volume; means for storing data for each of a multiplicity of image frames corresponding to said multiplicity of scan planes; means for determining a degree of correlation between the data of respective kernels of respective pairs of successive image frames, said respective degree of correlation being a function of a normalized sum of the absolute differences between corresponding data of said respective kernels; means for computing, as a function of said degree of correlation, a spacing between successive ones of said multiplicity of scan planes; a display monitor; and means for displaying a projected image on said display monitor which is a function of said spacing.