Laboratory assessment of a miniature electromagnetic tracker

With the introduction of miniaturized electromagnetic digitizers, there comes a variety of potential for clinical applications for computer aided interventions using flexible instruments. Endoscopes or catheters can easily be tracked within the body. To evaluate the reliability of a new commercial tracking system, we measured the systematic distortions induced by various objects such as closed metallic loops, catheters, and ultrasound scan heads. In order to obtain the measurements, the Aurora ([)(TM)(]) (Mednetix/CH, NDI/Can) electromagnetic tracking system was used and data was transferred via PC serial port while running SuSE Linux 7.1. (SuSE, Gmbh, Nurnberg). Wire loops of different diameters, wire guides, optical tracking tools, and an ultrasonic (US) scan head were all tested for the extent to which interference exists, as well as having determined the influence of a C-arm fluoroscopy unit. To quantify the reliability of the system, the miniaturized sensor was attached onto non-metallic measurement rack and the transmitter was set to measure at three different distances within the digitizer range. The tracker was found to be more sensitive to distortions caused by objects nearer the emitter (average value 13.6 mm +/- 16.6 mm for wire loops positioned at a distance between 100 mm and 200 mm from the emitter). Distortions caused by objects near the sensor (distances smaller than 100 mm) are small (typical error : 2.2 mm +/- 1.9 mm) in comparison to the errors of other electromagnetic systems published in an earlier study of our group where we found an average error of 3.4 mm. Considerable distortions (error: 18.6 mm +/- 24.9 mm) were found, though, with the C-arm fluoroscopy unit, thus limiting the reliability of the tracker. The US scan head was found to cause significant distortions only at a distance between the emitter and the scan head less than 100 mm from the emitter in contrast to the average error of 3.8 mm +/- 6.3 mm at distances greater than 100 mm. Taking into account that significant distortions only occur in the presence of metallic objects close to the emitter, these results indicate the opportunities which are now available in surgical applications where flexible instruments may need to be monitored within the patient.