1. A methods executed by a machine, of displaying a 3D image of a human tissue volume, said method comprising: (a) receiving a plurality of data sets, referred to as data slices, each said data slice representing a respective 2D slice of said tissue volume, said plurality of data slices having been generated by ultrasound scanning of said tissue volume; (b) defining (1) one or more regions within one of said data slices, referred to as regions 1a, 1b, and so on, within data slice 1, and (2) one or more regions within another of said data slices, referred to as regions 2a, 2b, and so on, within data slice 2; (c) performing a process of speckle decorrelation measurement to determine respective amounts of correlation between regions 1a and 2a, regions 1b and 2b, and so on; (d) utilizing said respective amounts of correlation to compute a relative positional difference vector representative of a relative positional difference between data slice 1 and data slice 2; (e) storing in a memory device (1) a representation of at least a portion of said data slice 1, (2) a representation of at least a portion of said data slice 2, and (3) said relative positional difference vector; (f) computing, from said relative positional difference vector, a relative position of each of said data slice 1 and said data slice 2 within a 3D image of the material; and (g) displaying said 3D image on a visual display.