Zhiyue J Wang1,2, Alexander Ivanishev1, Keith M Hulsey1, Dah-Jyuu Wang3, and Robert E Lenkinski1
1UT Southwestern Medical Center, Dallas, TX, United States, 2Children's Medical Center Dallas, Dallas, TX, United States, 3Children's Hospital of Philadelphia, Philadelphia, PA, United States
Synopsis
Traveling
wave MRI uses a wave-guide for RF transmission. The metal bore of the scanner magnet serves as
a wave-guide and extensions using conductor sheets may be added. Although
dielectric materials are frequently introduced into the system, their intended function
has been to modify the behavior of the wave-guide. In this work, we show that a
dielectric material may be used as a wave-guide by itself, in a fashion similar
to optic fibers guiding light transmission. We conducted MRI experiments at 7T
using an insulator wave-guide constructed by filling a PVC tube with
deionized water.Introduction
Traveling wave MRI allows RF
excitation and signal detection in far field [1] using a wave-guide.
The metal bore of the scanner magnet serves as the wave-guide, though
extensions using conductor sheets may be added [2]. Although dielectric
materials are frequently introduced into the system, their intended function
has been to modify the behavior of the wave-guide [3,4]. In this work, we
investigate the proposition that a dielectric material may serve as a
wave-guide by itself, in a fashion similar to optic fibers guiding light
transmission. We conducted MRI experiments at 7T using an insulator wave-guide
constructed by filling a PVC tube with deionized water.
Materials and Methods
Rationale: Electromagnetic fields can propagate along
a dielectric cylindrical rod surrounded by a vacuum [5]. The lowest TE and TM
modes have a non-zero cutoff frequency f
given by 2πfa×(ε1-1)0.5 /c=2.405 (here a
and ε1 are the radius
and dielectric constant of the cylinder, and c
is the speed of light). For 7T proton MRI, the propagation of the lowest TE and
TM mode in a tube filled with water requires
a>4.3 cm. On the other hand, the HE11 mode does
not have a cutoff frequency and can propagate in a cylinder of any radius.
For the HE11 mode, both longitudinal and transverse components are
present for H and E fields, and the transverse component of both H and E
fields are approximately uniform near the center of a transverse cross section
[6]. For
a cylindrical dielectric wave-guide, the electromagnetic fields decrease
exponentially with the radial distance from the tube following a modified
Bessel function of the 2nd kind with a length constant comparable to the RF
wavelength inside the wave-guide. This property allows the wave to propogate
down the wave-guide with minimal interaction with the magnet bore. The field at
the ends of the wave-guide propogates relatively freely, allowing the
wave-guide to couple with both the coil and the sample.
Experiments: Three PVC tubes were capped, filled with
deionized water and tested as dielectric wave-guides. Tube 1 had an inner
diameter (i.d.) of 6.4 cm, a wall thickness of 0.6 cm, and a length of 120 cm
allowing only the HE11 mode to propagate. To demonstrate the wave guiding capability,
the tube was made into an elbow with a 90° turn (curvature 5/m) at
the end. Tube 2 had an i.d of 11.4 cm
(allowing the lowest HE, TM and TE modes), a wall thickness of 0.6 cm, and a length
of 67 cm. Tube 3 had an i.d. of 2.5 cm (HE11 mode only), a wall thickness of 0.2 cm, and a length
of 157 cm.
MRI
experiments were conducted at 7T (Philips Healthcare) on Tubes 1 and 2. A transmit/receive one turn
surface coil was placed at one end of the tube, and the other end of the tube was placed next to a prostate phantom at
the scanner’s center. The wave propagation in Tube 3 was investigated in an open
space using a network analyzer. Both transmit and receive used a 2-turn, 1 cm
diameter coil placed at each end of the tube in an orientation perpendicular to
the end surface. The signal gain when the tube is inserted in between transmit
and receive loops was recorded.
Results
Figure
1 shows an MR image of the phantom together with the end section of Tube 1 (2D
FFE, slice thickness 20mm, pixel size 3.1mm, TR/TE/flip=1000ms/2ms/45°). The image shows that the elbow section of
Tube 1 guided the RF traveling wave making a 90° turn.
Figure 2
shows a sagittal 3D FFE MR image of a prostate phantom using Tube 2 as a wave-guide (acquisition matrix size 150×150×15, fov
150×150×60mm3, pixel bandwidth 253 Hz, TR/TE/flip angle=100ms/2ms/20°,
NSA=2). The scanner was unable to detect signal when the tube was
removed from the setup.
In
the open space RF transmission test using Tube 3, when
the tube was placed between the transmit and receive coils, the signal gain at 128
and 298 MHz was 25 and 30 dB, respectively.
Discussion
The use of a dielectric wave-guide may allow RF transmission to a localized volume through a direct path in traveling wave MRI. This appproach may avoid E-field hot
spots caused by T/R surface coils positioned near the skin.
Further optimization is needed regarding RF-loop size and positioning, as well
as the length and diameter of the dielectric wave-guide.
Conclusion
A
dielectric rod can be used as a wave-guide for traveling wave MRI.
Further studies are needed to explore this approach in MRI of
small tissue volumes.
Acknowledgements
No acknowledgement found.References
1. Brunner
DO, et al: Nature 2009; 457(7232): 994-8.
2. Vazquez F, et al: J Appl Phys 2013;
114: 064906.
3. Brunner DO, et al: MRM 2011; 66(1): 290-230.
4. Andreychenko A,
et al: MRM 2013; 70(3): 885-94.
5. Jackson JD: Classical Electrodynamics, 3rd
Edition, John Wiley & Sons, 1999.
6. Huang K-C and Edwards DJ: Millimetre
Wave Antennas for Gigabit Wireless Communications, Wiley, 2008.