Multi-center diffusion-weighted MRI validation for 0.35T MR-Linac

MR-linac (MRL)-based radiotherapy allows daily anatomical and physiological imaging for adaptive treatment. Apparent diffusion coefficient (ADC) derived from diffusion-weighted imaging (DWI) is a quantitative biomarker correlated with tumor cellularity and response. Verifying ADC repeatability and reproducibility across centers is key for multi-center physiological adaptive radiotherapy trials on low-field MRLs.

Echo-planar imaging (EPI)-based DWI was performed on a commercial phantom containing 13 0-50% polyvinylpyrrolidone (PVP) vials. Diffusion weighting was applied in three orthogonal directions using two b-value sets: [0,30,60,90,120,150,180,200,500,1000] and [0,500,1000] s/mm². Reproducibility was assessed across four institutions using brain and torso coils. Repeatability was evaluated with four scans at one site. To assess gantry angle effects, scans were repeated at the angle inducing highest distortion. Coefficient of variation (COV) and normalized deviation from reference ADC values were calculated.

Using three b-values, COVs on a single MRL ranged 0.339-2.914% (brain coil) and 0.466-6.902% (torso coil); and with ten b-values, ranged 0.196-3.679% and 0.408-6.699%, respectively. Multi-site COVs ranged 0.329-3.413% (three b-values) and 0.499-2.420% (ten b-values) using brain coil, and 0.267-4.442% and 0.381-5.357% with torso coil. At zero-degree gantry angle, normalized root mean squared error (nRMSE) ranged 2.857-3.538% (single MRL) and 3.015-3.952% (multi-center). Distortion increased nRMSE to 10.767% and 6.236% with three and ten b-values, respectively.

These findings demonstrate excellent repeatability and reproducibility of ADC values on low-field MRLs across coils and b-value sets, supporting its use as a reliable biomarker in multi-center adaptive radiotherapy trials.