ESTRO 2021 Abstract Book

S622

ESTRO 2021

performance for MR image quality and quantitative MRI on a national level, centers that deviate can be identified and action can be taken to improve quality. In this study, the geometric accuracy of different MR scanners in eight centers was evaluated. Materials and Methods The steering committee of NIMBUS reached out to all RT centers in Denmark through an MRI QA workshop to launch the network. At the participating centers, a general MRI sequence (3D T1W GRE) was used to test geometric accuracy with adjustments in repetition time and echo time to accommodate differences in field strength. A large field of view phantom (Magphan RT 820) was used in a traveling-phantom setup for all acquisitions. The acquisitions were made in the period from March 2019 to December 2020. Distortion analysis of the MRI scans was performed in the web-based software Smári Image Analysis Service (The Phantom Laboratory, NY, USA). The analysis output was distortions as a function of distance to iso-center as well as maximum distortion and mean of the 10% maximum distortions within a 200 and 350 mm diameter spherical volume (DSV). The mean of the 10% maximum distortions was reported as a robustness check of the maximum distortion. Results Seven of eight Danish RT centers and one radiological department have participated in this study so far. In total, the centers had eight MRI scanners and two MR-linacs (Table 1) which were evaluated.

All maximum distortions were below 0.43 mm within 200 mm DSV and below 1.2 mm within 350 mm DSV, except for the Siemens Aera 1.5 T (Figure 1). The mean of the 10% maximum distortions was in general about 20% lower than the maximum reported values; for all scanners, except for the Siemens Aera 1.5 T, the mean of the 10% maximum distortions was less than 1 mm within the 350 mm DSV. No dependency on field strength was seen for the reported distortions.

Conclusion