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

Motivation: Direct imaging of radiation beam effects could enable more accurate dosimetry and in vivo dose verification. Goal(s): Our goal was to detect immediate changes in T1 relaxation due to water radiolysis induced oxygen depletion. Approach: We used dynamic T1 mapping on an MR-Linac to simultaneously irradiate and image water phantoms. Results: We observed real-time changes in T1 with a T1/Dose slope of 0.71 ms/Gy. The current sensitivity limit was estimated to be 3 Gy for a 10 min scan. Three-dimensional spatial patterns in T1 were consistent with the predicted dose profile. Impact: Real-time visualization of radiation beam effects using quantitative T1 mapping may enable new radiation dosimetry methods. This study may lead to volumetric in vivo dose verification and imaging of transient oxygen depletion in high dose rate (“FLASH”) radiotherapy.