Tracking a 3-D ultrasound probe with constantly visible fiducials

A new method is presented for tracking a 3-D ultrasound (US) probe in space. The key is to use small fiducials that create point features in the US volume near the probe face and which are connected rigidly to the tracker on the probe. In this way, fiducials appear in every US volume and are used to calculate the position and orientation of the volume with respect to a fixed base. The main novelty is the elimination of a separate calibration device because calibration can be considered to be performed on every volume. After acquisition, the user simply marks the locations of the fiducials in the volume and solves a single equation to convert volume coordinates to the base. The point accuracy of the new method is slightly lower than those of conventional methods mainly because of the suboptimal appearance of the fiducials near the top of the image and little data averaging, but ease of use is improved.