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Characterization of Radiation Induced Current in RF coils of Linac-MR Systems Open Access


Other title
Radiation Induced Current
RF Coils
Type of item
Degree grantor
University of Alberta
Author or creator
Burke, Benjamin
Supervisor and department
Fallone, Gino (Oncology)
Rathee, Satyapal (Oncology)
Examining committee member and department
Robinson, Don (Oncology)
Santyr, Giles (Western University Physics)
Wachowicz, Keith (Oncology)
Morsink, Sharon (Physics)
Marchand, Richard (Physics)
Department of Physics
Medical Physics
Date accepted
Graduation date
Doctor of Philosophy
Degree level
Real-time MR imaging of the cancer patients undergoing external beam radiation treatment represents the next generation in image guided radiotherapy. However, the radio frequency (RF) coil of the MRI is exposed to the pulsed radiation of the linear accelerator in the systems where a medical linear accelerator is integrated with the MRI. This thesis is primarily concerned with the instantaneous effect of pulsed radiation on the RF coils, in particular the Radiation Induced Current (RIC). The RIC results from the charge imbalance created by the ejection of Compton electrons from the thin conductors of the RF coils during the pulsed irradiation. This work spans the initial observations of the RIC in real coils, a detailed characterization of the RIC and finally its impact on the MR image. The first part presented the measurements of the instantaneous RIC in two different MRI RF coils. Some basic characterization of the RIC included the isolation of the RF coil component responsible for RIC, the dependence of RIC on linear accelerator dose rate, and the effect of placing wax buildup on the coil to reduce RIC. The copper windings of the RF coils were isolated as the main source of RIC. A linear dependence of the RIC amplitude on dose rate was observed. The RIC was decreased with wax buildup, suggesting an electronic disequilibrium as the cause of RIC. In the second part, a buildup method of RIC removal in planar conductors is tested, a Monte Carlo method of RIC calculation in metal conductors is presented and validated, and the Monte Carlo method is used to examine the effects of magnetic fields on both planar conductor and practical cylindrical coil geometries. The buildup method of RIC removal is effective in planar geometries and in cylindrical coil geometries when the coil conductor is in direct contact with the patient. The presence of air gap between the coil and patient makes this method of RIC removal less effective although placing buildup still reduces the RIC by up to 60%. The SNR is improved in the images obtained concurrently with radiation if buildup is applied to the coil. The final part examined the effect of RIC on MR image quality. Imaging experiments were performed on the prototype linac-MR system and the RIC-induced signal-to-noise ratio (SNR) degradation of MR images was quantified. An image post-processing algorithm was proposed which removes some of the RIC-corrupted data from the MR images to recover some of the lost image SNR. The presence of RIC in MR RF coils leads to a loss of SNR which is directly related to the linac dose rate. The RIC related loss in SNR is likely to increase for future commercial systems that are likely to provide larger dose rate than the prototype system (250 cGy/min).. Some of this SNR loss can be recovered through the use of a post-processing algorithm which removes the RIC artefact from the image k-space.
Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
Citation for previous publication
Burke, B., Wachowicz, K., Fallone, B. G., & Rathee, S. (2012). Effect of radiation induced current on the quality of MR images in an integrated linac-MR system. Medical Physics , 39 (10), 6139-6147. Burke, B., Ghila, A., Fallone, B. G., & Rathee, S. (2012). Radiation induced current in the RF coils of integrated linac-MR systems: The effect of buildup and magnetic field. Medical Physics , 39, 5004-5014. Burke, B., Fallone, B., & Rathee, S. (2010). Radiation induced currents in MRI RF coils: application to linac/MRI integration. Physics in Medicine and Biology , 55, 735-746.

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