Real-time radiation beam imaging on an MR linear accelerator using quantitative T1 mapping

Background: Direct three-dimensional imaging of radiation beams could enable more accurate radiation dosimetry. It has been previously reported that changes in T-1-weighted magnetic resonance imaging (MRI) intensity could be observed during radiation due to radiochemical oxygen depletion. Quantitative T-1 mapping could increase sensitivity for dosimetry applications. Purpose: We use an MRI linear accelerator (MR-Linac) to visualize radiation delivery through the real-time effects of dose on the spin-lattice magnetic relaxation time (T-1) of water. We quantify the relationships between dose, spin-lattice relaxation rates (R-1) and dissolved oxygen concentration to further investigate the mechanisms of T-1 change. Methods: An ultrapure water phantom and a 1% agarose gel phantom were irradiated and imaged on a 1.5 T Elekta Unity MR-Linac. Radiation plans were created using the Monaco treatment planning system. Images were acquired before, during and after radiation. A dual-echo Look-Locker inversion recovery pulse sequence was used for simultaneous dynamic T-1/B-0 mapping. The change in R-1 with respect to dose (triangle R-1/triangle Dose) and the radiochemical oxygen depletion (ROD = triangle O-2/triangle Dose) were measured. The relaxivity of oxygen (r(1,O2) = triangle R-1/triangle O-2) in water was also measured in a separate experiment with samples of various dissolved oxygen concentrations. The minimum measurable dose over a 20-min period was estimated using a single-tailed 99th quantile Student's t-distribution. Results: Changes to R-1 were found to be spatiotemporally correlated to the predicted delivered radiation dose and persisted for at least 1 h after radiation. A complex dose plan could be imaged in the 1% agarose gel phantom, as the gel limits diffusion and convective mixing. In water, the triangle R-1/triangle Dose was found to be -1.0 x 10(-4) s(-1)/Gy, the r(1,O2) was found to be 5.4 x 10(-3) s(-1)/(mg/L), and the ROD was found to be -0.010 (mg/L)/Gy. Both r(1,O2) and ROD agree with published values. However, combining these two values yields a predicted triangle R-1/triangle Dose of -5.4 x 10(-5) s(-1)/Gy, indicating that radiochemical oxygen depletion alone under-predicts the MRI effect. The detection limit of R-1 was 1.1 x 10(-3) s(-1) which corresponded to a single-voxel minimum detectable dose of 11.1 Gy for this specific sequence. Conclusion: Quantitative T-1 mapping was used to image radiation dose patterns in real-time in water and agarose gel. Radiochemical oxygen depletion only partially explains the T-1 changes measured. Agarose gel could be used as a simple system for three-dimensional patient-specific quality assurance. Future applications may include in vivo dosimetry for FLASH radiotherapy, though improvements in acquisition methods and hardware are likely needed.