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

Direct three-dimensional imaging of radiation beams could enable more accurate radiation dosimetry. It has been previously reported that changes in T₁-weighted magnetic resonance imaging (MRI) intensity could be observed during radiation due to radiochemical oxygen depletion. Quantitative T₁ mapping could increase sensitivity for dosimetry applications.

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 (T₁) of water. We quantify the relationships between dose, spin-lattice relaxation rates (R₁) and dissolved oxygen concentration to further investigate the mechanisms of T₁ change.

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 T₁/B₀ mapping. The change in R₁ with respect to dose (∆R₁/∆Dose) and the radiochemical oxygen depletion (ROD = ∆O₂/∆Dose) were measured. The relaxivity of oxygen (r_1,O2 = ∆R₁/∆O₂) 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.

Changes to R₁ 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 ∆R₁/∆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 ∆R₁/∆Dose of -5.4 × 10^-5 s^-1/Gy, indicating that radiochemical oxygen depletion alone under-predicts the MRI effect. The detection limit of R₁ was 1.1 × 10^-3 s^-1 which corresponded to a single-voxel minimum detectable dose of 11.1 Gy for this specific sequence.

Quantitative T₁ mapping was used to image radiation dose patterns in real-time in water and agarose gel. Radiochemical oxygen depletion only partially explains the T₁ 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.