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SU‐F‐303‐04: Characterization and Correction of Gradient Non‐Linearity Induced Distortion On a 1.0 Tesla Open Bore MR‐SIM
Author(s) -
Price R.G.,
Kadbi M,
Kim J,
Chetty I.J.,
GlideHurst C
Publication year - 2015
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4925231
Subject(s) - imaging phantom , isocenter , transverse plane , physics , sagittal plane , distortion (music) , optics , nuclear medicine , mathematics , medicine , amplifier , optoelectronics , radiology , cmos , anatomy
Purpose: Implementation of MRI for single‐modality simulation is limited by system‐level distortions that may degrade treatment planning accuracy. We performed technical characterization of gradient non‐linearity (GNL) for large field‐of‐views (FOVs), developed a correction scheme, and quantified temporal stability. Methods: A 36×43×2cm 3 phantom with 255 known landmarks was scanned at isocenter in transverse, sagittal, and coronal planes (T1W‐FFE, TE/TR/α=5.54ms/30ms/28°, voxel size=1×1×2mm 3 , BW=191Hz/Pix, FOV=450×450×26mm 3 )using 1.0T open MR‐SIM with vertical magnetic field. Two scans were obtained in each orientation (positive/negative read gradient polarity of ±4.48mT/m) to isolate GNL using reverse gradient techniques. Four off‐axis positions (‐125 to 125mm) in superior‐inferior direction were evaluated. An automated MATLAB program was developed to identify control‐point locations relative to phantom schematics using masking, thresholding, and connectivity analysis. Distortion maps were interpolated between control points using singular value decomposition to fit to a sixth‐degree polynomial. In addition to vendor corrections, images were corrected via warping with inverse distortion maps. Temporal stability was assessed over 6 months. Results: Over the phantom FOV, mean±stdev (P5/P95) in mm were −0.07±1.10 (−1.83/1.92) in x‐direction and 0.10±1.10 (−1.5/2.15) in y‐direction for the transverse plane. Similar results were obtained in the sagittal plane. For coronal images, distortions were 0.4±1.16 (−1.32/2.50) and 0.04±0.40 (−0.52/0.77) in x‐ and y‐directions, respectively. Although negligible near isocenter (<1mm pixel width), 35% of pixels had distortions >1mm and 7% >2mm. Distortions were more pronounced 12.5cm off‐axis, with 44% of voxels >1mm, 17% >2mm, and 5% >3mm with largest distortions (∼8mm) occurring 22cm radial distance from isocenter. In‐plane distortions were reduced to <1mm up to ∼22.5cm from isocenter post‐correction. Distortion maps remained stable over 6 months with distributions varying <0.5mm between measurements. Conclusion: GNL distortion was non‐negligible. Further corrections were necessary to maintain <1mm accuracy for large FOVs. Statistical analysis of temporal stability revealed GNL distortion was consistent over 6 months. Research supported in part by a grant from Philips HealthCare (Best, Netherlands).