ESTRO 2020 Abstract Book

S293 ESTRO 2020

challenge of using MRI in RT. The aim of this work was to assess large FOV MRI distortions in a number of scanners used for RT throughout the UK. Material and Methods MRI scans of a GRADE phantom (Spectronic Medical AB, Helsingborg, Sweden) were acquired using a protocol recommended by the manufacturer (3D gradient echo, FoV 50x50x50 cm, voxel size 1x1x2 mm and Bandwidth 490 Hz/pixel) on the following 11 MRI scanners that are used for RT in the UK: Siemens Aera (Leeds, Guy’s and St Thomas’, Royal Marsden and Christie); Siemens Prisma (Leeds); Siemens Sola (Leeds); Siemens Skyra (Christie); Siemens Espree (Newcastle); Elekta Unity (Christie); Philips Ingenia 1.5T (Christie); GE Signa PET/MR (Newcastle). Scanners are referred to as 1-11 in a randomised order (for anonymity). Analysis was performed using MriPlanner (Spectronic Medical AB, Helsingborg, Sweden), the mean and max distortions and variance (δ 2 ) between scanners were reported at <100, 100-150, 150- 200, 200-250 and ≥250 mm from the isocentre. Results The range of the mean and max distortions for all scanners at 100-150 mm (representing the typical size of a brain) were 0.28-0.62 mm and 0.69-2.81 mm respectively. The range of the mean and max distortions for all scanners at 200-250 mm (representing the typical size of a pelvis) were 0.67-2.52 mm and 2.0-10.75 mm respectively. The mean and maximum distortions are shown in figure 1 and at <100, 100-150, 150-200, 200-250 and ≥250 mm from the isocentre, δ 2 was 0.004, 0.009, 0.045, 0.232 and 0.779 mm 2 respectively. The greatest mean geometric distortion (5.04 mm) observed was scanner 9 at ≥250 mm from the isocentre. For uses requiring a small FOV (less than 150 mm from the isocentre) such as brain imaging, both mean and maximum distortions would be < 2 mm except for the maximum distortion of one scanner. For uses requiring a large FOV (up to 250 mm from the isocentre) such as the head and neck and the entire pelvis, the mean distortions would be < 2 mm except for those from one scanner.

Conclusion The overall measurement error of the end-to-end phantom system was approximately 0.1 mm. Based on this system, the accuracy of ART dose delivery at a high-field MRL was found better than 1 mm. The precision of MRL ART dose delivery varied with direction and likely depended on voxel size. PH-0531 Assessment of geometric distortion of eleven clinical MRI scanners across the UK M. Alzahrani 1,2 , D. Broadbent 3 , R. Chuter 4 , B. Al-Qaisieh 3 , S. Jackson 4 , M. Hutton 4 , R. Johnstone 5 , S. Shah 5 , A. Wetscherek 6 , J. Chick 6 , J. Wyatt 7 , H. McCallum 7 , R. Speight 3 1 University of Leeds, Department of Biomedical Imaging Science, Leeds, United Kingdom ; 2 King Abdulaziz University, Department of Diagnostic Radiology, Jeddah, Saudi Arabia ; 3 Leeds Teaching Hospitals NHS Trust, Department of Medical Physics and Engineering, Leeds, United Kingdom ; 4 The Christie NHS Foundation Trust, Department of Medical Physics and Engineering, Manchester, United Kingdom ; 5 Guy’s and St Thomas’ NHS Foundation Trust, Department of Medical Physics and Engineering, London, United Kingdom ; 6 Joint Department at the Institute of Cancer Research and the Royal Marsden NHS Foundation Trust., Department of Medical Physics and Engineering, London, United Kingdom ; 7 Newcastle upon Tyne Hospitals NHS Foundation Trust and Newcastle University, Department of Medical Physics and Engineering, Newcastle, United Kingdom Purpose or Objective MRI is increasingly being used in radiotherapy (RT) in the UK for delineation, MRI-only planning and image guidance on an MR-linac. Geometric distortions are a known

Figure 1. A) The reported mean geometric distortion at different distances from the isocentre for all scanners. B) The reported maximum geometric distortion at different distances from the isocentre for all scanners