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

Area of Science:

• Medical Physics
• Radiotherapy Technology
• Biomarker Development

Background:

• Magnetic Resonance-Linear Accelerator (MR-linac) enables daily imaging for adaptive radiotherapy.
• Apparent diffusion coefficient (ADC) from diffusion-weighted imaging (DWI) is a quantitative biomarker for tumor cellularity and treatment response.
• Verifying ADC repeatability and reproducibility is crucial for multi-center trials using low-field MR-linacs.

Purpose of the Study:

• To assess the repeatability and reproducibility of ADC measurements on low-field MR-linacs.
• To evaluate the impact of different b-value sets and coils on ADC variability.
• To determine the influence of gantry angle-induced distortions on ADC accuracy.

Main Methods:

• Echo-planar imaging (EPI)-based DWI was performed on a phantom across four institutions.
• Two b-value sets ([0,30,60,90,120,150,180,200,500,1000] and [0,500,1000] s/mm²) were used.
• Repeatability was assessed via four scans at one site; reproducibility across sites using brain and torso coils.

Main Results:

• Coefficients of variation (COVs) for ADC were low, ranging from 0.196% to 6.902% across different b-value sets and coils.
• Multi-site COVs were also low, ranging from 0.267% to 5.357%.
• Gantry angle distortion increased normalized root mean squared error (nRMSE) to 10.767% with three b-values and 6.236% with ten b-values.

Conclusions:

• ADC measurements demonstrate excellent repeatability and reproducibility on low-field MR-linacs.
• The findings support the use of ADC as a reliable biomarker in multi-center adaptive radiotherapy trials.
• Low variability across coils and b-value sets enhances confidence in ADC's clinical utility.