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Radio frequency noise studies for a linac-MRI system Open Access


Other title
image guidance
Radio frequency
Type of item
Degree grantor
University of Alberta
Author or creator
Lamey, Michael
Supervisor and department
Dr. Nicola De Zanche, Oncology, University of Alberta
Dr. Gino Fallone, Physics\Oncology, University of Alberta
Dr. Satyapal Rathee, Oncology, University of Alberta
Examining committee member and department
Dr. James Balter, Radiation Oncology, University of Michigan
Dr. Sharon Morsink, Physics, University of Alberta
Dr. Richard Marchand, Physics, University of Alberta
Department of Physics

Date accepted
Graduation date
Doctor of Philosophy
Degree level
A prototype system which has integrated a linear accelerator (linac) with a magnetic resonance imager (MRI) has been constructed at the Cross Cancer Institute. The real time operation of a linac-MRI system will require proper radio frequency (RF) shielding such that the MRI images can be acquired without extraneous RF noise from the linac. This thesis reports on the steps taken to successfully RF-shield the linac from the MRI such that the two devices can operate independently of one another. The RF noise from functioning multileaf collimators (MLC) is measured using near field probes and MRI images are acquired with the MLC near the MRI. This included measuring the RF noise as a function of applied magnetic field strength. Several measurement and simulation scenarios are discussed to determine the major sources of RF noise generation from the modulator of a linac. Finally RF power density levels are reported internally and externally to the RF cage which houses the linac and the MRI. The shielding effectiveness of the RF cage has been measured in the frequency range 1 – 50 MHz and is presented. MRI images of two phantoms are presented during linac operation. The MLC studies illustrate that the small RF noise produced by functioning MLC motors can be effectively shielded to avoid signal-to-noise degradation in the MRI image. A functioning MLC can be incorporated into a linac-MRI unit. The RF noise source investigations of the modulator of a linac illustrate that the major source of RF noise involves the operation of a magnetron. These studies also eliminate the pulse forming network (PFN) coil and the grid voltage spikes on the thyratron as possible major sources of RF noise. The main result is that for linac-MRI systems the modulator of a linac should be housed in a separate RF cage from the MRI. Finally imaging work with the linac operating illustrates that the accelerating structure of a linac and an MRI can be housed within the same RF cage. The 6 MV linac can be operated to produce radiation with no experientially measurable degradation in image quality due to RF effects.
License granted by Michael Lamey ( on 2009-12-22T16:05:32Z (GMT): 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 the above terms. The author reserves all other publication and other rights in association with the copyright in the thesis, and except as herein 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.
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