Real-time B₀ compensation during gantry rotation in a 0.35-T MRI-Linac

Rotation of the ferromagnetic gantry of a low magnetic field MRI-Linac was previously demonstrated to cause large center frequency offsets of ±400 Hz. The B₀ off-resonances cause image artifacts and imaging isocenter shifts that would preclude MRI-guided arc therapy.

The purpose of this study was to measure and compensate for center frequency offsets in real time during gantry rotation on a 0.35-T MRI-Linac using a free induction decay (FID) navigator.

A nonselective FID navigator was added before each 2D balanced steady-state free precession cine image acquisition on a 0.35-T MRI-Linac. Images were acquired at 7.3 frames per second. Phase data from the initial FID navigator (while the gantry was stationary) was used as a reference. The phase data from each subsequent FID navigator was used to calculate the real-time B₀ off-resonance. The transmitter/receiver phase and the phase accrual over the adjacent image acquisition were adjusted to correct for the center frequency offset. Measurements were performed using an MRI-Linac dynamic phantom prior to and while the gantry rotated clockwise and counterclockwise. Image quality and signal-to-noise ratio (SNR) were compared between uncorrected and B₀ -corrected MRIs using a reference image acquired while the gantry was stationary. Four targets in the phantom were manually contoured on the first image frame, and an active contouring algorithm was used retrospectively on each subsequent frame to assess image variations and calculate Dice coefficients. Additionally, three healthy volunteers were imaged using the same pulse sequences with and without real-time B₀ compensation during gantry rotation. Normalized root mean square errors (nRMSEs) were calculated for the phantom and in vivo to assess the efficacy of the B₀ compensation on image quality. The measured center frequency offsets from the volunteer and MRI dynamic phantom navigator data were also compared. The sinusoidal behavior of the center frequency offsets was modeled based on the gantry layout and long-time constant eddy currents resulting from gantry rotation.

The duration of the FID navigator and processing was 4.5 ms. The FID navigator resulted in a ≤11% drop in SNR in the phantom and in vivo (liver). Dice coefficients from the MRI-guided radiation therapy (MR-IGRT) phantom contour measurements remained above 0.8 with B₀ compensation. Without B₀ compensation, the Dice coefficients dropped below 0.8 for up to 21% of the time depending on the contour. Real-time B₀ compensation resulted in mean reductions in nRMSE of 51% and 16% for the MR-IGRT phantom and in vivo, respectively. Peak-to-peak center frequency offsets ranged from 757 to 773 Hz in the phantom and 760 to 871 Hz in vivo.

Dynamic real-time B₀ compensation significantly improved image quality and reduced artifacts during gantry rotation in the phantom and in vivo. However, the FID navigator resulted in a small drop in the imaging duty cycle and SNR.