Abstract
MR‐guided radiotherapy requires strong imaging spatial integrity to deliver high quality plans and provide accurate dose calculation. The MRI system, however, can be compromised by the integrated linear accelerator (Linac), resulting in inaccurate imaging isocenter position and geometric distortion. Dependence on gantry position further complicates the correction of distortions. This work presents a new clinical application of a commercial phantom and software system that quantifies isocenter alignment and geometric distortion, as well as providing a deformation vector field (DVF). A large distortion phantom and a smaller grid phantom were imaged at multiple gantry angles from 0 to 330° on a 0.35 T integrated MR‐Linac. The software package was used to assess geometric distortion and generate DVFs to correct distortions within the phantom volume. The DVFs were applied to the grid phantom with resampling software then evaluated using structural similarity index measure (SSIM). Scans were also performed with a ferromagnetic clip near the phantom to investigate the correction of more severe artifacts. The mean magnitude isocenter shift was 0.67 mm, ranging from 0.25 to 1.04 mm across all angles. The DVF had a mean component value of 0.27 ± 0.02, 0.24 ± 0.01, and 0.19 ± 0.01 mm in the right‐left (RL), anterior‐posterior (AP), and superior‐inferior (SI) directions. The ferromagnetic clip increased isocenter position error from 1.98 mm to 2.20 mm and increased mean DVF component values in the RL and AP directions. The resampled grid phantom had an increased SSIM for all gantry angles compared to original images, increasing from 0.26 ± 0.001 to 0.70 ± 0.004. Through this clinical assessment, we were able to correct geometric distortion and isocenter shift related to gantry position on a 0.35 T MR‐Linac using the distortion phantom and software package. This provides encouragement that it could be used for quality assurance and clinically to correct systematic distortion caused by imaging at different gantry angles.
Highlights
We previously reported the impact of gantry position on image quality for the ViewRay MRIdian 0.35 T
This study presents the first clinical experience with the Modus QA QUASARTM MRID3D geometric distortion phantom and software system to measure the spatial distortion and imaging isocenter shift related to gantry position on an MR-linear accelerator (Linac) system
This study used two phantoms that were both imaged on a 0.35 T ViewRay MRIdian MR-Linac system in magnetic resonance imaging (MRI) QA mode. 3D slicewise images were acquired with the gantry in 12 different positions from 0◦ to 330◦ in 30◦ increments
Summary
Every advantage provided by MRI guidance is dependent on the spatial and geometric fidelity of the resulting images.MR images have intrinsic distortions which must be addressed prior to using images for target tracking or adaptive planning to prevent inaccurate dose calculation or improper beam gating.[8,9,10] These intrinsic distortions are generated by gradient nonlinearity, main static magnetic field (B0) inhomogeneity, imperfect shimming, and eddy currents, and manifest as distorted patient surface, spatial shift of the imaging volume, and distortion of internal structures.[8,11,12,13,14] On an MR-Linac system additional sources of distortion are introduced by the radiotherapy system due to imperfect radiofrequency shielding for Linac components and the presence of ferromagnetic material around the MRI bore. The work by Tijssen et al showed a varying level of gantry angle dependency for B0 field homogeneity across four Elekta Unity systems and four gantry angles, with one system showing no dependency and another displaying increased inhomogeneity with active shimming in place
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