Program Information
Design of Passive Magnetic Shielding for MRI-Linac Systems
B Whelan1*, B Oborn2 , S Kolling1 , D Constantin3 , P Keall1 , (1) University of Sydney, Sydney, NSW,(2) Illawarra Cancer Care Centre, Wollongong, NSW,(3) Varian Medical Systems, Palo Alto, CA,
Presentations
TU-FG-FS2-5 (Tuesday, August 1, 2017) 1:45 PM - 3:45 PM Room: Four Seasons 2
Purpose: Passive magnetic shielding in MRI-Linac systems is used to reduce the magnetic field around sensitive Linac components, but also causes distortion in the MR imaging field. This can cause image artifacts and reduce geometric fidelity. In this work, the fundamental principles of passive shielding are applied to design shielding for a side–coupled 6MV Linac in a 1T magnet. Both in-line and perpendicular MRI-Linac configurations are investigated.
Methods: Shields were designed for a Varian 600C Linac operating with a 1.0 T MRI at SID 1.5m using FEM modelling (Comsol). Sensitivity data for the Linac, electron gun and magnet were taken from the literature. Design goals were mean field <2G (perpendicular), <40G (in-line) and MRI homogeneity (30cm DSV<100PPM). The following shield parameterisations were explored: Shield thickness (1-20mm,1mm intervals), shield length (100-400mm,50mm), number of concentric shield layers (1-5,1), and material (carbon steel or mu-metal). Shield efficacy (average field without/with shield) and 30cm DSV distortion were recorded for all cases.
Results: The mean magnetic field was reduced from 985G to 0.5G (perpendicular) and 413 to 4.3G (in-line) with magnet distortion <60PPM, exceeding design objectives. The same shield geometry is around twice as effective and induces less distortion in the perpendicular configuration than the in-line orientation. However, Linacs are around 25 times more sensitive to magnetic fields in this configuration. The use of concentric shield layers was a highly effective strategy in both orientations. Mu-metal is an extremely effective shielding material, but saturates at low field. Therefore, shielding configurations of outer layers of steel with an inner layer of mu-metal was adopted.
Conclusion: We have investigated key factors impacting passive magnetic shield design for MRI-Linac systems, and applied the results to design passive shields in both in-line and perpendicular configurations of a 1.0 T magnet which will allow mutual interoperation of linac and MRI.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by an Australian NHMRC program grant (APP1036075)
Contact Email: