1. A method for non-invasively tracking a bone of a subject in three-dimensional space, the method comprising: (a) acquiring a first reference image volume I 0 of a bone surface with a volumetric ultrasound imaging transducer at an initial time to; (b) acquiring a second image volume I 1 of a bone surface with the volumetric ultrasound imaging transducer at a second time t 1 ; (c) measuring the three-dimensional position of the volumetric imaging transducer with a three-dimensional position measuring device at times t 0 and t 1 and associating each position with the corresponding image volumes I 0 and I 1 ; (d) searching for a relative position of I 0 and I 1 for which overlapping portions of the image volumes of I 0 and I 1 are closest aligned to one another and defining this position as a best-match position; (e) determining the 3D transformation between the first reference volume and the second image volume that corresponds to the best-match position; and (f) repeating steps b to e.