Wei Luo1, Rui Liu2, Thomas Neuberger3,4, and Michael T Lanagan1,2
1Material Research Institute, University Park, PA, United States, 2Department of Engineering Science and Mechanics, University Park, PA, United States, 3Huck Institute of Life Science, University Park, PA, United States, 4Department of Biomedical Engineering, University Park, PA, United States
Synopsis
To maximize the
energy transfer to the cylindrical dielectric resonator utilized in magnetic
resonant imaging probe head, a three-loop coupling method was investigated
using electromagnetic field simulations. The simulation results demonstrate the
supreme performance of this coupling method and verify the previous preliminary
experimental results.Targeted audience
Engineers who are interested in utilizing a dielectric
resonator as a MRI coil.
Introduction
Cylindrical dielectric resonators (CDR)
operating in TE
01δ mode and HEM
11δ mode have been used in
recent years as MRI probes in high-field magnetic resonance imaging (MRI) (B
0 ≥ 7T)
[1-5]. Electromagnetic field simulations were
used to optimize the power transfer to the CDRs and the results were compared to
experimental data
[6]. A three-loop inductive coupling method is
shown by both experimental
[6] and simulation results to have the
best coupling to the TE
01δ mode of the CDR.
Methods
An MRI probe made of
CDR with a three-loop inductive coupling method
[6]
was modeled (Figure 1$$$b$$$) and simulated using the FDTD method.
The CDR ($$$ε_r$$$ = 173, outer diameter = 46.1
mm, height = 33.7 mm) has a hollow bore (diameter: 5.32 mm) in its center for sample placement. A three-turn
copper solenoid coil (height: 27 mm, diameter: 48 mm, wire diameter: 0.69 mm) was
wrapped around the CDR for coupling. The second simulated CDR MRI probe was coupled
by a single loop coil (diameter: 14.8 mm wire diameter: 1 mm) as shown in Figure
1$$$a$$$. In this model, the CDR kept the
same geometry but its $$$ε_r$$$ is
lowered to 160 which would provide some tunability at 14T in this setup. The single loop coupling coil was located right
beneath the CDR. In both models, a cylindrical ‘muscle’ phantom (diameter: 4.32
mm, length: 35 mm, $$$ε_r$$$
= 56, $$$σ$$$ = 0.85 S/m) was placed at the center of the CDR. Both MRI probes with muscle
phantom were positioned at the center of a copper shield (diameter: 55 mm,
length: 150 mm) and tuned to 600 MHz using two copper pieces located on the top
and bottom faces of the CDR (not shown in Figure 1). The coil efficiency, $$$|B_1^+|\sqrt{P_{dissp}}$$$,
and SNR maps in the phantom were calculated and compared. $$$|B_1^+|$$$ is the magnitude of the larger
circularly‐polarized magnetic component
of the EM field generated by the CDR MRI probe and $$$\sqrt{P_{dissp}}$$$ is the
total power dissipated in the phantom, coupling coil, CDR, and copper shield.
Their center line profile were also extracted and analyzed. All EM field
simulations were performed in a commercial package (XFdtd, Remcom, USA).
Results and Discussion
The coil efficiency (
Effcoil) and normalized SNR
maps (FA = 90˚) from both CDR MRI probes in the cross section of the phantom
along the phantom axis is shown in Figure 2 and Figure 3. Both, the
Effcoil and the SNR from the MRI probe with the three-loop inductive coupling method (Figure 2$$$b$$$ and Figure 3$$$b$$$) are higher at every point across the phantom than the one using a single coupling loop (Figure
2$$$a$$$ and Figure 3$$$a$$$). The intensity distributions of the
Effcoil and
SNR from both MRI probes are similar; the maximum locates at the phantom center
and the value falls off towards to the ends of the phantom. This also
shows in their center line profiles
(Figure 4 and Figure 5). Furthermore, Figure 4 and Figure 5 show
that, with the three-loop inductive coupling method, the maximum
improvement of
Effcoil and SNR are more
than order of magnitudes at both ends of the phantom; the minimum improvements are about 11% in
Effcoil
and 10% in SNR at the phantom center, respectively. The optimal imaging length,
where its SNR < 0.95×max(SNR) within the phantom, increases more than 46%
from 10.5 mm for the MRI probe using a
single coupling loop to 15.4 mm for the one using the
three-loop inductive coupling method. In conclusion, we expect a better imaging
quality across the full-length of the phantom using the CDR MRI probe with the three-loop inductive coupling method. The maximum
improvement would be located at both ends of the MRI probe and a maximum of 10%
increase is expect at the probe center.
Acknowledgements
NIH grant R24 MH106049 and NSF
grant DBI-1353816References
1] Haines et al., JMR, 2002;200:349-53. [2] Neuberger et al., CONCEPT MAGN RESON B,
2002;33B:109-14. [3] Aussenhofer et al., MRM, 2004;68:1325-31. [4] Aussenhofer et al., NMR Biomed, 2011;26:1555-61. [5] Aussenhofer et al., JMR, 2014;243:122-9. [6] Liu et al., ISMRM 2015;23:3102.