Method for locating a small magnetic object in the human body

A piece of thin acupuncture needle lodged under the right scapula of a patient could not be found in surgical procedures accompanied by studies of 30 standard X-ray images. To locate it, the authors mapped the magnetic field component normal to a plane lying above the object, using a superconducting quantum interference device (SQUID). Assuming that the needle could be modeled as a magnetic dipole, it was possible to infer its lateral position, depth, orientation, and magnetic moment. With this information, directed CT scans, high resolution X-ray films, and the subsequent surgical removal of the needle proved that it could be located in the body with an accuracy of about 3 mm.