A technique has been developed, based on magnetic field measurements, to localize, in three dimensions, hypodermic and sewing needles lost in the human body. A theoretical model for the magnetic field generated by needles has been elaborated and experimentally validated. Using this model, the localization technique gives information about needle's centre, orientation and depth. The clinical measurements have been made using a SQUID system, with patients being moved under the sensor with the aid of an X-Y bed. The magnetic field associated with the remanent magnetization of the needle is acquired on-line and mapped over a plane. In all six cases that occurred, the technique allowed surgical localization of the needles with ease and high precision. This procedure can decrease the surgery time for extraction of foreign bodies by a large factor, and also reduce the generally high odds of failure.

A technique was previously developed, based on magnetic field measurements, to localize hypodermic and sewing needles lost in the human body, with the purpose of surgical extraction. The measurements are performed using a SQUID magnetometer, which detects the magnetic field associated with the remanent magnetization of the needle. The technique allowed easy surgical localivrtion of the needles with good precision in all six clinical cases studied so far. The procedure greatly decreases the surgery time for foreign body extraction, and also reduces the generally high odds of failure. This paper presents an improvement of the original algorithm, which is now independent of any constant magnetic field component, thus overcoming the main experimental difficulty usually found, namely that a SQUID system does not measure absolute fields.

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When using multichannel SQUID magnetometers for biomagnetic applications, the correct measurement of the Tesla/volt calibration factor of each channel is of extreme importance, in order to avoid gross errors when analyzing the results. We propose a general calibration method valid for all kinds of gradiometer arrays. One of the main properties of this method is that the gradiometer position inside the cryogenic Dewar does not need to be known precisely. When tested, calibration factors with accuracies ranging from 0.3% to 0.8% were obtained. It is based on a spatial Fourier technique and on the fact that the gradiometer acts as a discrete spatial filter.