The Effect of Transponder Motion on the Accuracy of the Calypso Electromagnetic Localization System

Abstract

The Calypso localization system remotely detects miniature electromagnetic transponders implanted in a patient. The system is capable of continuously measuring the position of a treatment site during radiotherapy. If the site is moving during respiration then the system can be used for dynamic tracking of the target. With this application in mind we have made measurements to see if there is any change in localization accuracy due to transponder position or motion. Three localization transponders were mounted on a remote-controlled turntable that could move the transponders along a circular trajectory at speeds up to 3 cm/sec. A stationary calibration established the coordinates of six points on each transponder's circular path. We then compared position measurements taken while the transponders were in motion at a constant speed to the stationary coordinates. To synchronize the position of the moving transponders to a known point on the circular path we injected an RF noise pulse into the Calypso readout antenna when the turntable passed 0°. This produced outliers on the arc of moving transponder positions that could be used to compare the transponder positions in the direction of motion to the calibrated stationary positions. The stationary localization uncertainty varied from 0.02 to 0.06 cm per axis in the (x,y) plane, which is consistent with the observations of other researchers. We detected a small systematic trend in the stationary uncertainty as a function of transponder position along an arc 11 cm long. Lung tumors do not move more than about 3 cm in the most extreme cases. Therefore the Calypso system will have less than 0.02 cm systematic change in localization accuracy over the full range of tumor position. There was no statistically significant change in the transponder positions in either the radial or tangential direction when the transponders were in motion. If the Calypso system is used for real-time respiratory motion tracking, then tracking accuracy will be unaffected by transponder motion, at least for the velocity and position range that will be encountered in clinical applications. If the transponder is moving, its reported location corresponds precisely to its position at the midpoint of the readout interval, regardless of speed.