S2256
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
banding through the prostate, preventing MR-linac treatment. Fig. 2c,d: bilateral hip implants with varying materials; left implant caused significant field disturbance. In all cases, banding artefacts aligned with predicted off-resonances of ±150 Hz. Across 15 patients, the median off-resonance in the central ROI around the target was –17 Hz (range: –46 to 0 Hz). One patient (Fig. 2b) showed –103 Hz. Conclusion: bSSFP banding artefacts can reliably indicate geometric accuracy in MRIdian reference scans for patients with metallic implants. Spatial integrity is preserved outside the most distal band, eliminating the need for additional fieldmap scans.
Digital Poster 3739
A simple method to assess the geometric fidelity of daily reference scans on a 0.35T MR-linac using bSSFP banding artefacts Rob H.N. Tijssen 1,2 , Marjolein N Hilberts 1 , Mariska de Smet 1 1 Department of radiation oncology, Catharina hospital, Eindhoven, Netherlands. 2 Department of biomedical engineering, Technical university, Eindhoven, Netherlands
Purpose/Objective: Metallic implants introduce magnetic field
perturbations that can distort MR images. Although distortions are reduced at 0.35T compared to higher field strengths, they remain relevant—especially with ferromagnetic materials like cobalt-chromium. A recent phantom study [1] provided insight into distortion effects across material types, but patient- specific assessment remains desirable, particularly when implant composition is unknown. As B0 fieldmap functionality is not available on the MRIdian system, this study explores whether the default balanced steady-state free-precession (bSSFP) reference scan can be used to assess geometric fidelity in clinical mode, circumventing the need for additional scans. Material/Methods: MRIdian daily reference images were acquired using a bSSFP sequence (TR/TE = 3.37ms/1.4ms, resolution = 1.5mm ³ , bandwidth = 535 Hz), which has a signal profile sensitive to local resonance frequency. This profile repeats every 1/TR Hz, with transition bands (low signal) appearing at off-resonances of 150±35 Hz for the given TR (see Fig. 1). This corresponds to a geometric shift of 0.3–0.5 mm, meaning spatial integrity is guaranteed outside the most distal banding artefact.To validate this, custom B0 fieldmaps (dual acquisition, single-slice gradient echo, TR/TE1/TE2 = 50ms/4ms/7.4ms) were acquired in 16 patients with pelvic tumours (prostate, rectum, lymph nodes), alongside vendor bSSFP images. Implants included iodine-125 seeds (1), gamma nails (2), and total hip replacements (11 unilateral and 2 bilateral), composed of stainless steel, ceramic, titanium, and cobalt- chromium alloys. Fieldmaps were generated in MATLAB and overlayed onto bSSFP images to assess off-resonance in a central 10 cm ROI around the target and at banding artefact locations. Results: Fig. 1 shows the bSSFP signal profile and an example fieldmap indicating off-resonance near a metallic implant. Fig. 2 shows bSSFP images with overlayed fieldmaps masked to display pixels at 150±35 Hz off- resonance. Fig. 2a: banding caused by hip implant and clothing buttons. Fig. 2b: hip implant producing
References: [1] Pouymayou, B. et al., phiRO (2024);30:100576 Keywords: image quality, QA, MR physics
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