Large field of view distortion assessment in a low‐field MR‐linac

PURPOSE: MR-guided radiation therapy (RT) offers unparalleled soft tissue contrast for localization and target tracking. However, MRI distortions may be detrimental to high precision RT. This work characterizes the gradient nonlinearity (GNL) and total distortions over the first year of clinical operation of a 0.35T MR-linac. METHODS: For GNL characterization, an in-house large field of view (FOV) phantom (60 × 42.5 × 55 cm3 , >6000 spherical landmarks) was configured and scanned at four timepoints with forward/reverse read polarities (Gradient Echo sequence, FA/TR/TE = 28°/30 ms/6 ms). GNL was measured in Anterior-Posterior (AP), Left-Right (LR), and Superior-Inferior (SI) frequency-encoding directions based on deviation of the auto-segmented landmark centroids between rigidly registered MR and CT images and assessed based on radial distance from magnet isocenter. Total distortion was assessed using a 30 × 30 cm2 grid phantom oriented along the cardinal axes over >1 year of operation. RESULTS: The scanner's spatial integrity within the first ~10 months was stable (maximum total distortion variation = 10/6/8%, maximum distortion = 1.41/0.99/1.56 mm in Axial/Coronal/Sagittal planes, respectively). GNL distortions measured during this time period 1 mm. CONCLUSIONS: GNL distortions were negligible within a 10 cm radius from isocenter. However, in the periphery, non-negligible distortions of up to ~7 mm were observed, which may necessitate GNL corrections for MR-IGRT for treatment sites distant from magnet isocenter.