SU-E-J-159: Electromagnetic Tracking during 4DCT

Real-time, electromagnetic tumor tacking during 4DCT is an unsolved problem. The underlying problem is due to eddy current magnetic fields generated within the conducting surfaces in response to the source's alternating magnetic fields.To solve this problem, we developed a method to separate the source from the background fields, which can be measured with simple search coil sensors, such as those used by the Calypso (Seattle, WA) tracking system.

We modeled the environment using ANSYS Maxwell electromagnetic simulation software. The gantry was modeled as a 5 mm thick × 1.2 m diameter stainless-steel cylinder, with variable length. The transponder solenoid was modeled as a 10 mm × 1 mm conducting cylinder, with azimuth directed current, which was assumed to have a frequency range of 300-500 kHz. The search coil configuration was assumed to be a 5 × 5 array of 5-7 cm square current loops with 6.7 cmseparation. An algorithm based on free space calculations and measurements was developed to calculate the solenoid position within the cylinder, in the presences of relatively large eddy magnetic fields that were generated at the same frequency as the source.

Of the various methods and sensor configurations investigated, we found a method that localized the transponder solenoid within 1 mm over all solenoid locations and gantry lengths. We also found that gradient techniques did not significantly increase localization accuracy as expected. Complex solutions were found but not suitable for rapid clinical implementation.

This method can be used to localize a Calypso® Beacon transponder during 4DCT to accurately track tumor positions. Furthermore, the method was based on inexpensive search coils with comparable dimensions to preserve the source-sensor distance. The next task is to create a system and investigate clinical implementation.