A method, comprising: generating, using a plurality of magnetic transmitters, a magnetic field in a region; introducing a field perturbing element into the region; calculating a reaction magnetic field in the region generated by a plurality of point sources in the field perturbing element; positioning a probe in the region and measuring a perturbed magnetic field at the probe; and determining a location of the probe in response to the measured perturbed magnetic field and the calculated reaction magnetic field, wherein introducing the field perturbing element into the region comprises measuring a location and an orientation of the field perturbing element with respect to axes defined by the magnetic transmitters, wherein the field perturbing element comprises a fluoroscope including a detector and a collimator, and further comprising adaptively locating the fluoroscope by: providing two reference sensors in the region, each reference sensor configured to generate a signal indicative of the perturbed magnetic field at the reference sensor; performing a preliminary registration of the fluoroscope with the region; and introducing three patch sensors, each patch sensor configured to generate a signal indicative of the perturbed magnetic field at the patch sensor; determining the location of the fluoroscope from the signals generated by the reference sensors and the patch sensors, and wherein the determination of the location of the fluoroscope comprises calculating the following cost function: C = ∑ sensors ‖ Meas − M r → sensor, o → sensor, φ fluoro, θ fluoro, d det, d col ‖ ∑ sensors ‖ Meas ‖ where M is a sum of field generated by the plurality of magnetic transmitters and the reaction magnetic field in the region, r sensor is the displacement of a sensor, o sensor is the orientation of a sensor, φ fluoro is the elevation of the fluoroscope, θ fluoro is the azimuth of the fluoroscope, d det is the distance of the detector, and d col is the distance of the collimator, each variable calculated with respect to the axes of the transmitters. generating, using a plurality of magnetic transmitters, a magnetic field in a region; introducing a field perturbing element into the region; calculating a reaction magnetic field in the region generated by a plurality of point sources in the field perturbing element; positioning a probe in the region and measuring a perturbed magnetic field at the probe; and determining a location of the probe in response to the measured perturbed magnetic field and the calculated reaction magnetic field, providing two reference sensors in the region, each reference sensor configured to generate a signal indicative of the perturbed magnetic field at the reference sensor; performing a preliminary registration of the fluoroscope with the region; and introducing three patch sensors, each patch sensor configured to generate a signal indicative of the perturbed magnetic field at the patch sensor; determining the location of the fluoroscope from the signals generated by the reference sensors and the patch sensors, and wherein the determination of the location of the fluoroscope comprises calculating the following cost function: C = ∑ sensors ‖ Meas − M r → sensor, o → sensor, φ fluoro, θ fluoro, d det, d col ‖ ∑ sensors ‖ Meas ‖ where M is a sum of field generated by the plurality of magnetic transmitters and the reaction magnetic field in the region, r sensor is the displacement of a sensor, o sensor is the orientation of a sensor, φ fluoro is the elevation of the fluoroscope, θ fluoro is the azimuth of the fluoroscope, d det is the distance of the detector, and d col is the distance of the collimator, where M is a sum of field generated by the plurality of magnetic transmitters and the reaction magnetic field in the region, r sensor is the displacement of a sensor, o sensor is the orientation of a sensor, φ fluoro is the elevation of the fluoroscope, θ fluoro is the azimuth of the fluoroscope, d det is the distance of the detector, and d col is the distance of the collimator, each variable calculated with respect to the axes of the transmitters.