The effect of transponder motion on the accuracy of the Calypso Electromagnetic localization system.

PURPOSE: To determine position and velocity-dependent effects in the overall accuracy of the Calypso Electromagnetic localization system, under conditions that emulate transponder motion during normal free breathing. METHODS AND MATERIALS: 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/s. A stationary calibration established the coordinates of multiple points on each transponder's circular path. Position measurements taken while the transponders were in motion at a constant speed were then compared with the stationary coordinates. RESULTS: No statistically significant changes in the transponder positions in (x,y,z) were detected when the transponders were in motion. CONCLUSIONS: The accuracy of the localization system is unaffected by transponder motion.

Accuracy of a wireless localization system for radiotherapy.

PURPOSE: A system has been developed for patient positioning based on real-time localization of implanted electromagnetic transponders (beacons). This study demonstrated the accuracy of the system before clinical trials. METHODS AND MATERIALS: We describe the overall system. The localization component consists of beacons and a source array. A rigid phantom was constructed to place the beacons at known offsets from a localization array. Tests were performed at distances of 80 and 270 mm from the array and at positions in the array plane of up to 8 cm offset. Tests were performed in air and saline to assess the effect of tissue conductivity and with multiple transponders to evaluate crosstalk. Tracking was tested using a dynamic phantom creating a circular path at varying speeds. RESULTS: Submillimeter accuracy was maintained throughout all experiments. Precision was greater proximal to the source plane (sigmax = 0.006 mm, sigmay = 0.01 mm, sigmaz = 0.006 mm), but continued to be submillimeter at the end of the designed tracking range at 270 mm from the array (sigmax = 0.27 mm, sigmay = 0.36 mm, sigmaz = 0.48 mm). The introduction of saline and the use of multiple beacons did not affect accuracy. Submillimeter accuracy was maintained using the dynamic phantom at speeds of up to 3 cm/s. CONCLUSION: This system has demonstrated the accuracy needed for localization and monitoring of position during treatment.