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Real estate limitations placed a Proton Therapy Center (PTC) near a Magnetic Resonant Imaging (MRI) facility creating a high risk of interference.  The PTC uses a 9T magnet that moves and an 11 kW RF transmitter operating at frequencies sensitive to the MRI operation.  The unique approach to providing isolation, developed by Washington Laboratories was documented and published by the Journal of Proton Therapy under the title of:

Impact of the high magnetic field and RF power in a superconducting cyclotron on the operation of a nearby MRI facility

Chee-Wai Cheng, Steven G Ferguson, David Jordan, Frederick Jesseph, Mark Johnson, Lionel Bouchet, David Mansur, Barry Wessels


The purpose of this study is to investigate whether the 9T superconducting magnet (9TSM) and the 11kW RF source in the Mevion S250 proton therapy system (S250PTS) produce interference in a nearby MRI facility. The RF shielding of the University Hospitals Proton Therapy Center (UHPTC) was designed by one of the authors (S. Ferguson) of Washington Laboratories Ltd., Gaithersburg, MD. A preliminary test was conducted to assess the effect of an external magnetic field on MR imaging. A time-varying magnetic field of 60G (more than 100 times the expected variance from the Mevion S250 9TSM) was generated (to simulate a rotating magnetic field) in the walkway between the nearest MRI suite (MR5) and the UHPTC (wall-to-wall distance 2.8 m) while scanning of a phantom was performed in MR5. The 9TSM was ramped up slowly and magnetic field testing was conducted at the 2T point and finally at the 9T level (full current at 1990 A). The 9TSM was rotated to different angles (0o, 45o, 90o, 135o, 180o) while the magnetic field flux density was measured at different points inside the vault, in the walkway between the MR and UHPTC, and along the hallway inside the MR facility at each angle. The MR facility performed routine patient scanning on five clinical scanners throughout the 9TSM testing. With RF full power at 11 kW and the 9TSM rotated at different angles, the RF field strength was measured inside the vault and mid-point in the walkway (to evaluate the effectiveness of the RF shielding) and also between the two 3T MRI suites operating at 128 MHz center frequency where interference from the proton system RF emission would be a concern. Patient scanning was performed throughout the RF testing in all MRI suites. Finally, with the 9TSM at full current and RF full power, several phantom scans were performed in MR5 using MR scan sequences expected to be sensitive to time-varying magnetic fields and RF interference. No interference with MR imaging throughout the various RF and magnetic field testing was detected. This is the first report on interference of the strong time-varying magnetic field and RF power of the S250PTS in a nearby MRI facility. With proper RF shielding and site planning, the S250PTS may provide a viable solution to proton therapy in an urban setting where co-location is necessary or limited real estate restricts other installation options.

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Keywords: Superconducting, Synchrocyclotron, Magnetic interference, RF interference, RF shielding

Original Article:

Copyright (c) 2016 Chee-Wai Cheng, Steven G Ferguson, David Jordan, Frederick Jesseph, Mark Johnson, Lionel Bouchet, David Mansur, Barry Wessels

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This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.