Fiducial-Based Translational Localization Accuracy of Electromagnetic Tracking System and On-Board Kilovoltage Imaging System

Purpose: The Calypso medical four-dimensional localization system uses AC electromagnetics, which do not require ionizing radiation, for accurate, real-time tumor tracking. This investigation compared the static and dynamic tracking accuracy of this system to that of an on-board imaging kilovoltage X-ray system for concurrent use of the two systems. Methods and Materials: The localization accuracies of a kilovoltage imaging system and a continuous electromagnetic tracking system were compared. Using an in-house developed four-dimensional stage, quality-assurance fixture containing three radiofrequency transponders was positioned at a series of static locations and then moved through the ellipsoidal and nonuniform continuous paths. The transponder positions were tracked concurrently by the Calypso system. For static localization, the transponders were localized using portal images and digitally reconstructed radiographs by commercial matching software. For dynamic localization, the transponders were fluoroscopically imaged, and their positions were determined retrospectively using custom-written image processing programs. The localization data sets were synchronized with and compared to the known quality assurance fixture positions. The experiment was repeated to retrospectively track three transponders implanted in a canine lung. Results: The root mean square error of the on-board imaging and Calypso systems was 0.1 cm and 0.0 cm, respectively, for static localization, 0.22 mm and 0.33 mm for dynamic phantom positioning, and 0.42 mm for the canine study. Conclusion: The results showed that both localization systems provide submillimeter accuracy. The Calypso and on-board imaging tracking systems offer distinct sets of advantages and, given their compatibility, patients could benefit from the complementary nature of the two systems when used concurrently.