SYSTEMATIC DISTORTIONS IN MAGNETIC POSITION DIGITIZERS

Medical devices equipped with position sensors enable applications lik e image guided surgical interventions, reconstruction of three-dimensi onal 3D ultrasound (US) images, and virtual or augmented reality syste ms. The acquisition of three-dimensional position data in real time is one of the key technologies in this field. The systematic distortions induced by various metals, surgical tools, and US scan probes in diff erent commercial electromagnetic tracking systems were assessed in the presented work. A precise nonmetallic six degree-of-freedom measureme nt rack was built that allowed a quantitative comparison of different electromagnetic trackers. Also, their performance in the presence of l arge metallic structures was quantified in a phantom study on an acryl ic skull model in an operating room (OR). The trackers used were alter nating current (ac) and direct current (dc) based systems. The ac trac kers were, on average, distorted by 0.7 mm and 0.5 degrees by metallic objects positioned at a distance greater than 120 mm between the geom etrical center of the sample and the sensor. In the OR environment, th e ac system exhibits mean errors of 3.2 +/- 2.4 mm and 2.9 degrees +/- 1.9 degrees. The de trackers are more sensitive to distortions caused by ferromagnetic materials (averaged value: 1.6 mm and 0.5 degrees be yond a distance of 120 mm). The de tracker shows no distortions from o ther conductive materials but was less accurate in the OR environment (typical error: 6.4 +/- 2.5 mm and 4.9 degrees +/- 2.0 degrees). At di stances smaller than approximately 100 mm between sample and sensor er ror increases quickly. It is also apparent from our measurements that the influence of US scan probes is governed by their shielding materia l. The results show that surgical instruments not containing conductiv e material are to be preferred when using an ac tracker. Nonferromagne tic instruments should be used with de trackers. Static distortions ca used by the OR environment have to be compensated by precise calibrati on methods. (C) 1998 American Association of Physicists in Medicine [S 0094-2405(98)01611-3].