Comparison of magnetic tracking and optical tracking by simultaneous use of two independent frameless stereotactic systems.

OBJECTIVE: The goal of this study was to compare the calculated accuracy and the true surgical accuracy of a magnetic and an optical tracking system at surgical registration and during surgery.
METHODS: Two Food and Drug Administration-approved, commercially available frameless stereotactic systems were used simultaneously in 70 surgical cases. The Compass Cygnus-PFS system (Compass International, Rochester MN) uses magnetic field referencing and tracking. The StealthStation (Medtronic SNT, Louisville, CO) uses optical referencing and tracking. Registration was performed for each of the systems using adhesive fiducial markers, cranial-implanted markers, anatomic landmarks, or a combination thereof. Preoperative imaging consisted of volumetric computed tomography, magnetic resonance imaging, or both. Calculated accuracy was given by each of the systems as the root mean square after registration. Surgical accuracy was assessed by comparing the anatomic accuracy of each system with a number of recognizable intraoperative anatomic landmarks.
RESULTS: Calculated accuracy (root mean square) was 1.4 +/- 0.6 mm using the magnetic system and 1.4 +/- 0.8 mm using the optical tracking system. In the 42 patients with implanted cranial fiducials, the calculated accuracies were 1.0 +/- 0.5 mm (magnetic) and 0.9 +/- 0.4 mm (optical). True surgical accuracy was considered good (3 mm or less) in both systems in 60 of 70 patients. In two patients, neither system was accurate. In eight patients, one of the two systems was considered inaccurate. Of these, the magnetic system was considered inaccurate three times and the optical system five times.
CONCLUSION: Magnetic referencing and tracking was found to be comparable with optical tracking both with regard to calculated and true surgical accuracy. Interference from metal objects in the magnetic field was seen rarely.