Room: Karl Dean Ballroom C
Purpose: MRI-Linacs have rotating gantries with large masses of steel shielding. Therefore, the magnetic field homogeneity can be sensitive to gantry angle. Reshimming the magnetic field for each treatment angle takes time and can impact the clinical workflow. Magnetic Resonance-Gradient Offset (MR-GO) uses a lookup table of shim current values to dynamically optimize the field shim for each gantry angle.
Methods: The field homogeneity was measured at 0.35 T (14.7 MHz) in a 24 cm internal diameter water phantom doped with 4.76 mM nickel sulfate hexahydrate. Free induction decays (FID) were acquired with the body coil [TR: 3 s, FA: 90Â°, 4 Averages, BW: 1260 Hz, 128 complex points]. The full width at half maximum (FWHM) was calculated for each FID spectrum. Measurements were made at each 15Â° increment of the gantry angle before and after gradient shimming and then calculated for every 5Â° increment using cubic interpolation. The gradient shim currents were used to generate a lookup table. During treatment delivery, the 2D cine true fast imaging with steady state precession (TrueFISP) sequence received the gantry angle from the motion controller. The corresponding shim currents were uploaded in real-time to the gradients before the subsequent image acquisition.
Results: In the phantom, MR-GO settings significantly reduced the field inhomogeneities for each gantry angle from a range of 2-10 ppm to <2 ppm. Our homogeneity specification is <5 ppm FWHM at each gantry angle.
Conclusion: MR-GO helped to minimize the variation in field inhomogeneity between gantry positions in a phantom. Future work will determine if the technique produces a significant improvement in image fidelity in vivo. However, the benefit of MR-GO may rise for MR-IGRT system operating at higher field strengths (e.g., 1.5 T).
Funding Support, Disclosures, and Conflict of Interest: Drs. Nikolova, Nikolov, and Lotey are employees of ViewRay. Washington University in St. Louis receives research funding, and consulting and speaking fees from ViewRay although such research funding was not used in this study. Dr. Gach owns common stock shares in ViewRay.