1. A method for non-invasively tracking a bone of a subject in three-dimensional space, the method comprising: (a) attaching a 3D or 4D volumetric ultrasound imaging transducer to the subject and acquiring a first reference image volume I 0 of a bone surface with the 3D or 4D volumetric ultrasound imaging transducer at an initial time t 0 , wherein the 3D or 4D volumetric ultrasound imaging transducer acquires 3D image pixel volumes; (b) determining an initial position of the bone using the first reference image volume I 0 ; (c) acquiring a second image volume I 1 of the bone surface with the 3D or 4D volumetric ultrasound imaging transducer at a second time t 1 ; (d) measuring a three-dimensional position of the 3D or 4D volumetric ultrasound imaging transducer with a three-dimensional position measuring device at the times t 0 and t 1 and associating each position with the corresponding image volumes I 0 and I 1 ; (e) 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; (f) determining a 3D transformation between the first reference image volume and the second image volume that corresponds to the best-match position; and (g) repeating steps c to f.