Towards an understanding of radiation-induced R1 changes in irradiated water on an MR linear accelerator

Motivation: Absorbed radiation dose can be directly imaged on an MR-Linac using T1 mapping, hypothesized to result from depletion of paramagnetic dissolved oxygen. However, it is unclear whether T1 changes are solely caused by this effect. Goal(s): To determine whether radiochemical oxygen depletion is solely responsible for radiation-induced R1 changes. Approach: Water phantoms with different dissolved gas compositions were irradiated and T1 mapping was acquired on an MR-Linac. Dissolved oxygen was measured using a polarographic probe. Results: Oxygen depletion explains 54% of radiation-induced R1 change in the atmospheric gas phantom. R1 change in the nitrogen-bubbled phantom was caused only by oxygen depletion. Impact: An understanding of the mechanism behind radiation-induced R1 changes could determine if radiation beam effects can be imaged in other systems and in vivo.