A region of tissue is scanned as a series of 2-D frames. The correlation of substantially homogeneous speckle regions in the frames is determined and is compared with a precalibrated speckle correlation-versus-distance function to give an estimate of the distance between the frames. In some applications, the frames are partitioned into sub-frames, whose correlation values and distances are combined according to predetermined functions to give a distance estimate between the frames as a whole. The reliability of the distance estimates is improved and evaluated in various ways, for example, by comparing combinations of possible distances from end frames via intermediate frames, and by comparing computed frame or sub-frame velocities with a known or estimated transducer velocity. Once the relative distances of the 2-D frames are estimated, a 3-D image is compiled using registration techniques. Image frames need not be parallel.