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Simulation and Background Noise Investigation of an MRI-linac Treatment Unit
M.
,
Lamey ,
M.
,
Carlone ,
S.
,
Steciw
and G.
,,
Fallone
Department
Introduction
 Image Guided Adaptive Radiotherapy (IGAR)
involves the use of an imaging modality to minimize
planning treatment volumes required to deliver a
specified dose.
 A major problem in the radiotherapy process
involves patient movement during treatment and
day-to-day organ movement.
 We propose the use of real time imaging during
radiotherapy treatment combining an Magnetic
Resonance Imaging (MRI) system with a linac.
- However a linac is a source of Radio Frequency
(RF) power; these RF signals can interfere with the
operation of the MRI because the MRI uses RF
signals in its transmitting and receiving coils.
of Physics, University of Alberta, Edmonton, Alberta, Canada
Department of Medical Physics, Cross Cancer Institute, Edmonton, Alberta, Canada
Department of Oncology, Cross Cancer Institute, Edmonton, Alberta, Canada
 Simulations will allow us to compare RF
measurements (B. Burke et al.) with simulated
results.
Materials and Methods
 We will solve Maxwell’s equations for the magnetic field of the system and the
RF noise through the use of finite element analysis (see Figure 1).
 The system to be modeled is broken into multiple linked simplified regions of
the original object, i.e. finite elements. Physical conditions, such as boundary
values, along with equations of equilibrium are applied to each element and a
system of equations is constructed. The system of equations is then solved.
 This finite element analysis technique will be used to simulate the magnetic field
and RF interference between the linac and MR in the system.
Objectives
• Combine a magnetic resonance imaging system
with a linac to image during treatment.
• The short term goal of the project is to determine
the necessary amount of shielding needed such that
the operation of the two devices in close proximity
will not affect one another deleteriously.
 We will also determine the necessary shielding
requirements such that magnetic field
interference will not alter the operation of the
linac.
Future Work
 Determine the antenna radiation pattern and
input impedance as a function of signal
frequency.
 Compare measurements and finite element
analysis simulations of the RF signal produced
by the linac.
- Interference of the magnetic field from the MRI
with the electron-beam steering process in the
linac just before the target (location of radiation
field production for treatment) may also be a
concern.
Figure 3. Blowup of the spherical-conical antenna which
will be used to study the RF noise from the linac.
Conclusions
Figure 1. Antenna design using COMSOL
multiphysics and the meshing (finite
elements) used to solve the radiation field
pattern.
Figure 2. Magnetic flux density of the
proposed 0.2T permanent magnet..
Figure 4. Electric potential isosurface around the
antenna (initial studies).
Results
 Magnetic field simulations of the proposed magnet have been completed using
the finite element simulation package, results are shown in Figure 2.
 An antenna has been designed in order to investigate the effects of the RF noise
on the MR system, see Figures 1 and 3.
 Initial simulation work of the electric potential of the antenna is shown in Fig 4.
References
1. S. Lim et al. A Tunable Electrically Small Antenna
for Ground Wave Transmission IEEE Trans. Ant.
Prop. 54 (2006) 417-421.
2. C. Balanis Antenna Theory: Analysis and Design
John Wiley and Sons, New York, 2005.
3. COMSOL multiphysics www.comsol.com