Real‐time radiation beam imaging on an MR linear accelerator using quantitative <i>T</i> ₁ mapping

Abstract 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 (∆ R 1 /∆Dose) and the radiochemical oxygen depletion (ROD = ∆O 2 /∆Dose) were measured. The relaxivity of oxygen ( r 1,O2 = ∆ R 1 /∆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 ∆ R 1 /∆Dose was found to be −1.0 × 10 −4 s −1 /Gy, the r 1,O2 was found to be 5.4 × 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 ∆ R 1 /∆Dose of −5.4 × 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 × 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.

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