Methods and apparatuses for displaying and using a shaped field of a repositionable magnet to move, guide, and/or steer a magnetic seed or catheter in living tissue for medicinal purposes. A moveable magnet assembly and a portion of a patient's body undergoing magnetically-aided surgery are both provided with fiducial markers. The portion of the patient's body is fixed in a location in which the fiducial markers are sensed and located by a set of localizers. The positions of the fiducial markers are determined by a processor, which operates on a stored representation of the magnetic field of a magnet in the magnet assembly to provide a display of the present magnetic field of the magnet. This display may be superimposed over an MRI, X-ray or CAT image during surgery. The repositionable magnet can be an electromagnet. In some embodiments, a computer calculates orientations and currents for an external electromagnet to move the implanted magnetic object in the patient's body through a desired path. The display can provide real-time imaging of the implanted magnetic object in comparison to the desired path as the object is moved.