QA Optimization of 0.35T MR-Linac

Abstract

The 0.35 T MR-Linac combines MRI and linear-accelerator technology to enable organ-motion gating, adaptive radiotherapy, and radiobiological optimization through diffusion-weighted imaging (DWI). Operating at 0.35 T, it delivers a single-energy 6 MV flattening-filter-free (FFF) photon beam. Comprehensive quality assurance (QA) therefore covers both imaging and accelerator components. Acceptance testing protocols were evaluated to establish baselines for routine QA and to identify system limitations. Novel analysis methods quantified the magnetic-field effect on 6 MV FFF beam- scanning profiles. Large, medium, and small fields were characterized with twelve MR-compatible detectors—plastic scintillators, diodes, diamond detectors, and ionization chambers—spanning diverse shapes and sensitive volumes. A renormalization approach defined penumbra-region profile shapes for large fields, while quantifiable parameters captured the pronounced shifts in small-field profiles caused by the 0.35 T field. To meet small-field dosimetry requirements, field-output factors were measured with sixteen detectors (including Gafchromic film) and detector-specific correction factors from the smallest to the reference field were derived. Cherenkov corrections for plastic scintillators were refined through a novel multi-loop method. Dose- and latency-gating performance was evaluated with a plastic scintillator in a motion-gating phantom; synchronized linac, phantom, and detector signals yielded gating latency and dose-delivery accuracy across three gantry angles, all tracking algorithms, and slow, medium, and fast motion patterns. MRI commissioning was streamlined by optimizing 2-D T1 and T2 scan protocols on a large-FOV phantom, reducing acquisition time while maintaining image quality. Finally, clinical comparisons of four commercial phantoms imaged with the TRUFI sequence assessed distortion versus gantry angle and image-quality parameters according to each phantom’s specifications.