Real-time B<sub>0</sub> compensation during gantry rotation in a 0.35-T MRI-Linac.
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2022-10
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Abstract
Background
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.Purpose
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.Methods
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.Results
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.Conclusion
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.Type
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Curcuru, Austen N, Taeho Kim, Deshan Yang and H Michael Gach (2022). Real-time B0 compensation during gantry rotation in a 0.35-T MRI-Linac. Medical physics, 49(10). pp. 6451–6460. 10.1002/mp.15892 Retrieved from https://hdl.handle.net/10161/26985.
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Deshan Yang
Deshan Yang is the professor of Medical Physics in the Department of Radiation Oncology, Duke University. He received his bachelor's degree in electronics engineering from Tsinghua University in 1992, a master’s degree in computer science from the Illinois Institute of Technologies in 2002, and his master’s and Ph.D. degrees in Biomedical Engineering from the University of Wisconsin-Madison in 2005. He spent two years as a postdoctoral researcher before joining Washington University in St. Louis as a faculty member. He worked as an instructor to a professor at Washington University in St. Louis between 2006 and 2021 before joining Duke University in 2021. His main research areas are medical image processing and analysis for radiation oncology applications, adaptive radiotherapy, cardiac radiosurgery, health information technologies for radiation oncology and medical physics.
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