Finite element method based simulation, desing, and resonant mode analysis of radio frequency birdcage coils used in magnetic resonance imaging
Author
Gürler, Necip
Advisor
İder, Y. Ziya
Date
2012Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
112
views
views
46
downloads
downloads
Abstract
Radio Frequency (RF) birdcage coils are widely used in Magnetic Resonance
Imaging (MRI) since they can generate very homogeneous RF magnetic field inside
the coil and have high signal-to-noise ratio (SNR). In practice, designing a
birdcage coil is a time-consuming and difficult task. Calculating the capacitance
value, which is necessary for the coil to resonate at the desired frequency, is the
starting point of the design process. Additionally, it is also important to know
the complete resonance frequency spectrum (or resonant modes) of the birdcage
coil that helps the coil designers to be sure that working mode is far away from
the other modes and so that tuning and matching procedures of the coil can be
done without interfering with the other modes. For this purpose, several studies
have been presented in the literature to calculate the capacitance value and the
resonant modes of the birdcage coil. Among these studies, lumped circuit element
model is the most used technique in capacitance and resonant modes calculations.
However, this method heavily depends on the inductance calculations which are
made under quasi-static assumptions. As a consequence of this assumption, error
in the calculations increases as the frequency increases to a point at which the
wavelengths are comparable with the coil dimensions. Additionally, modeling
the birdcage coil in a 3D simulation environment and making electromagnetic
analysis in the volume of interest is also important in terms of observing the electromagnetic
field distributions inside the coil. In this thesis, we have proposed
three different Finite Element Method (FEM) based simulation methods which
are performed using the developed low-pass and high-pass birdcage coil models
in COMSOL Multiphysics. One of these methods is the FEM based optimization
method in which magnitude of the port impedance or variance of H+ is
used as the objective function and the capacitance value is used as the control
variable. This is a new method proposed for calculating the capacitance value
of the birdcage coils. The other method is the eigenfrequency analysis which
is used to determine not only the resonant modes of the birdcage coil but also
the electromagnetic fields distributions inside the coil at these resonant modes.
To the best of our knowledge, FEM based eigenfrequency analysis of a birdcage
coil is also a new study in the field of MRI. The last method is the frequency
domain analysis which is used solve for the electromagnetic fields of a birdcage
coil for the specified frequency (or frequencies). One can also use this method to
estimate Specific Absorption Rate (SAR) at any object inside the coil. To make
these three simulation methods easily and according to the user-specified parameters,
we have developed two software tools using MATLAB which have also
graphical user interface (GUI). In order to compare the results of the proposed
methods and the results of the methods that use lumped circuit element model
with the experimental results, we have constructed two handmade birdcage coils
and made measurements for different capacitance values. Then, we have compared
the measured resonant modes with the calculated resonant modes; used
capacitance values with the calculated capacitance values. For the worst case (in
which the frequency is the highest), proposed FEM based eigenfrequency analysis
method calculates the resonant modes with a maximum of 10% error; proposed
FEM based optimization method calculates the necessary capacitance values with
20-25% error. Methods which use lumped circuit element model, on the other
hand, calculate the resonant modes and capacitance values with 50-55% error for
the worst case.
Keywords
RF Birdcage CoilsFinite Element Method
Lumped Circuit Element Model
Capacitance Calculation
Frequency Domain Analysis
Eigenfrequency Analysis