TH-E-BRA-10: The 1.5 T MRI Accelerator for MRI during Radiation Delivery: Status Report

Abstract

Purpose: We have developed and built a prototype ring based accelerator around a 1.5 T MRI system for MRI guided radiation therapy. Methods: The design is a closed bore cylindrical 1.5 T MRI with a ring mounted 6 MV accelerator in the transversal mid-plane with a 7 mm non-rotating MLC without back-up collimators and flatness filter. The ring allows continuous rotation in both directions. The magnetic and RF interference between the MRI and the accelerator is overcome by active shielding and redesign of the Faraday cage respectively. The beam passage through the magnet and gradient coils is through a circumferential beam portal which is homogeneous and contains the equivalent of approximately 10 cm of aluminium. Results: After installation of the ring gantry around the MRI at zero field, radiation from the rotating accelerator was shown and arbitrary segments can be delivered. Recently the MRI is brought up to 1.5T and performance testing at 1.5 T is in progress. Radiation delivery was shown. For static gantry positions MRI can be performed, without striking image deterioration, substantiation is work in progress. Gantry rotation in both directions can be performed, but this does affect the MRI B0 field. These distortions are mapped using a standard field camera and a combination of active and passive corrections as presented by Amthor et al. (2011) seems feasible and is ongoing. Conclusion: The MRI accelerator is close to full functionality, that is, diagnostic 1.5T MRI during IMRT for delivering stereotactic precision radiation under MRI guidance. Amthor T. and Overweg J. Compensation of magnetic field perturbations for combined MRI/radiotherapy system. Eposter 526 in proc ESMRMB 2011Leipzig, Germany. Work is supported by Elekta, U.K. and Philips, Best, The Netherlands