The Design And Implementation Of A Novel Compact Linear Accelerator-Based Magnetic Resonance Imaging-Guided Radiation Therapy (Mr-Igrt) System

To report on the novel design of a linear accelerator–based MRI-guided radiation therapy system that completely addresses RF and magnetic field interference, as well as radiation attenuation or perturbations in a compact system that can be housed in existing/conventional radiation therapy vaults. The MR-IGRT system was designed to provide simultaneous MR imaging combined with a spectrum of external beam radiation therapy techniques (3D, IMRT, SBRT, and SRS) at the same isocenter. The device consists of 3 main components: (1) a vertically split (double-donut) solenoidal superconducting 0.35 T MRI; (2) a ring-gantry mounted 6-MV flattening filter–free (FFF) linear accelerator coupled with a truly double-focused 4-mm multileaf collimator; and (3) an integrated high-performance computing adaptive planning and therapy delivery software. A novel RF shielding and absorption technology was designed to completely and robustly eliminate linear accelerator–generated RF presence in the treatment room (MRI environment). Simultaneously, a novel magnetic shielding sleeve system was designed to create regions of low (sub-Gauss) level magnetic fields within the ring-gantry, allowing mounting of all major linear accelerator components (pulse transformer, gun driver, magnetron, port circulator, and accelerator body [gun, resonating cavity, and target]) in a 90-cm source-to-isocenter distance gantry. The design results in radiation beams that are not attenuated by any RF or magnetic field shielding components enabling delivery of virtually pure linear accelerator-based radiation to patients in an MRI environment. Prototype testing demonstrated complete elimination of linear accelerator RF inside the treatment room by measurement with a spectrum analyzer and tuned microwave antenna. The levels of RF in the room with and without operation of the linear accelerator were nearly identical. The magnetic field inside of the magnetic shielding sleeves was measured with a Hall Effect magnetometer demonstrating less than one Gauss level impact of the MRI on all major linear accelerator components. A novel on-gantry shimming system allows for operation of the linear accelerator and collimator system while maintaining <25 ppm distortions over a 45-cm spherical field of view. The linear accelerator delivers unattenuated and unperturbed FFF radiation. The device design demonstrates feasibility of practically ideal harmonization of MRI and linear accelerator in a single system. As previously reported, the 0.35T MRI provides high fidelity imaging devoid of significant chemical shift, magnetic susceptibility distortions, and SAR enabling high fidelity planar frame rates of 8 FPS with 2-mm resolution. The design also enables in-field retrofitting of Co-60 based versions of this system.