SU‐C‐17A‐04: Distortion Characterization and Correction of MR‐SIM Images

Abstract

Purpose:Distortions in MRI compromise spatial fidelity, potentially impacting delineation and dose calculation. We characterized 2D and 3D large field of view (FOV), system‐level distortion to implement correction maps for MR‐SIM treatment planning.Methods:For 2D in‐plane distortion characterization, a 36×43×2 cm3 phantom with 255 known landmarks (∼1 mm3) was scanned using 1.0T high‐field open MR‐SIM in the transverse, sagittal, and coronal axes (TE/TR/α = 30/500 ms/90°, voxel size =1.4×1.4×5mm3). A binary template (reference) was generated from phantom schematic. An automated program converted MR‐SIM images to binary via masking, thresholding, and testing for connectivity to identify landmarks. Distortion maps were generated by centroid mapping. Images were corrected via warping with inverse distortion maps. Temporal stability was assessed. 3D distortion was characterized using a 40×40×40 cm3 phantom (3D T1‐FFE, TE/TR/α= 3.83/9 ms/10°, voxel size=1 mm3) and performing B‐spline‐based deformable image registration to CT‐SIM (reference). Displacement vector fields describing vector distortion magnitude and distortion histograms were generated.Results:Over 9 months, B0‐field distortions (mm) were 0.51 ± 0.84 (maximum = 5.5), 0.18 ± 0.84 (maximum = 5.5), and 1.03 ± 0.50 (maximum = 3.6) in transverse, sagittal, and coronal planes, respectively. Maximum distortions occurred 18–24 cm from isocenter for all planes. After implementing correction maps, distortions (mm) were reduced to 0.14 ± 0.19 (maximum = 2.12), 0.01 ± 0.21 (maximum = 1.68), and 0.17 ± 0.21 (maximum = 3.00) in transverse, sagittal, and coronal planes, respectively. 3D vector distortion was 0.59 ± 0.32 mm (maximum = 1.65) within 5 cm of isocenter. Distortions were non‐negligible (mean = 1.57 ± 1.06 mm, maximum = 6.26), 10‐15 cm from isocenter.Conclusion:Assessment of 2D distortion was conducted, revealing the need to further correct the periphery of MR‐SIM images. Future work includes generating 3D corrections and quantifying objectinduced distortions to improve planning accuracy with MR‐SIM.Research supported in part by a grant from Philips HealthCare