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A magnetic tracking system for monitoring the performance and activity of mechanical heart valve prostheses has been developed. Physicians may find the system valuable in detecting thrombosis (blood clots), scar tissue, or other complications that may indicate life-threatening medical conditions, such as heart attacks, strokes, or aneurysms. In this paper, a disk-shaped NdFeB magnet comparable in size to the floating disk of a single-leaflet tilting disk valve was employed as a magnetic marker. The orientation of the marker was tracked using standard least-squares localization algorithms with data from two sets of three mutually orthogonal magnetic field sensors fixed in an imaging plane. The best fitting magnetic dipole moment of the magnetic marker was determined by 360 sensor measurements. The orientation of the magnet was analyzed by rotating the magnet about a single axis through a range of 90deg at three imaging planes (3-6 in. from the sensors). The results show that the estimation error between the physical measurements and field-based predictions of marker orientation is within 2deg over all the measured angles for distances up to 6 in. from the imaging plane. This finding suggests that with optimization, the motion of magnetized prosthetic heart valves can be determined with high accuracy by an array of magnetic field sensors placed on the surface of the skin.