Synopsis
RF coil design for human ultra-high-field
scanners is an area of intense development, to address difficult challenges
including RF excitation spatial heterogeneity and low RF efficiency. We present
the development and testing of a novel category of multi-channel RF volume coil
structures at both 7T and 10.5T based on a subject-loaded multifilar
helical-antenna RF coil. Phantom data show excellent consistency between
numerical simulations and experimental results with 4- and 8-channel helical-antenna
coil prototypes. This
design shows capability for multi-channel RF-transmit technology and parallel
imaging. This work may help decide which coil structure should
be used for future studies at 10.5T.Purpose
Magnetic resonance (MR) scanners
operating at ultra-high magnetic field (UHF) (7T and above) provide substantial
gains in signal-to-noise ratio and spatial resolution compared with 1.5–3T
clinical scanners
1. However, a fundamental issue at UHF, with increasing
Larmor frequency (300 MHz at 7T), comes from the reduced RF wavelengths, down
to the order of or smaller than the size of the imaged samples, and the main
challenges include RF excitation spatial heterogeneity and low RF efficiency. RF
coil design for human UHF scanners is an area of intense development,
especially regarding the most challenging targets such as torso or body imaging.
One example is twisting a birdcage coil toward a
spiral shape
2, for head RF excitation at 4T, thus with RF interactions still
dominated by a near-field or quasi-static regime. Another example are helical antennas as
traveling-wave sources at 7T with the imaged sample being positioned outside
the helix
3.
Methods
We use a novel category of multi-channel RF coil
structures with volume coverage for UHF MR imaging (MRI) based on helix conducting elements,
the helix coil, namely, subject-loaded multifilar helical-antenna RF volume
coil, with its inner volume being utilized to image a sample, first proposed in
4,5. The coil has been dramatically advanced since; the size is reduced by
more than threefold, the power efficiency is very significantly increased, 8-channel
coils were built (Fig. 1), designs were extended to 10.5T, and MRI experiments on
prototypes at both 7T and 10.5T at the Center for Magnetic Resonance Research
(CMRR), University of Minnesota, were performed (Fig. 2). Simulations are done
using the higher-order method of moments
6. The helix coil allows for
uniquely combining traveling-wave behavior through the overall coil wire
structure while preserving near-field RF interaction between the inner side of
the conducting elements and the imaged tissues. Single channel — or monofilar — helical-antenna coil is the simplest design
7.
Taking advantage of the multi-channel RF technology available at CMRR at 7T and
10.5T, we have developed quadrifilar (4-channel) and octafilar
(8-channel) helical-antenna volume coils, as shown in Figs. 1 and 2. The goal is to enable
multi-channel RF methods (e.g., B
1
shimming) to further mitigate B
1+
field heterogeneity
8.
Results
Fig.
3 shows measurements at the CMRR and simulations at 7T of a 4-channel helical-antenna
RF coil. Shown in Fig. 4 are CMRR-measured results for the 8-channel coil at 7T. Fig. 5 shows the results from CMRR
experiments with the 4-channel helical-antenna RF coil prototype at 10.5T (note
that 10.5T experiments are done without proper phasing of the four excitation
ports of the coil, with which circular polarization would be further enhanced).
Discussion
We
observe from Fig. 3 an excellent agreement between the measurements and
simulations. Good circular polarization and B
1+ field strength and uniformity are observed, as well as a diverse interleaved field/phase pattern due to the four helices (four channels),
and thus capability for parallel imaging.
The power efficiency is good and the amount of power delivered to the
imaged phantom is sufficiently high for all experiments and MRI processing.
Data are obtained for absolute B
1 maps, g-factor, and
GRAPPA X3 acceleration. Similar observations are
made, from Fig. 4, for the 8-channel
helical-antenna 7T coil, with a diverse
interleaved pattern corresponding to eight channels and the resulting capability
for parallel imaging and acceleration. Experimental and simulation results at 10.5T
have demonstrated the scalability and versatility of the coil design. From Fig.
5, we observe that the received signal from the multiple channels presents an
interleaved pattern of dominant channels, which indicates spatial encoding
capability and is desirable for parallel imaging.
Conclusion
The main goal
of developing a subject-loaded multi-channel helical-antenna coil is to, when
loaded with a subject,
provide improved RF performance for UHF MRI while preserving the easiness of
use of a volume coverage coil. This design benefits from the congruence
of far- and near-field regimes. Multiple channels are utilized (4 and 8 in the prototype configuration) to enable all multi-transmit channel RF technology,
which is expected to expand the capability to mitigate B
1+ heterogeneity, while also providing parallel imaging
capability. The presented phantom data
obtained at 7T show excellent consistency between numerical simulations and
experimental results with 4- and 8-channel helix coils. The 10.5T machine at
CMRR used for this work is the first (and only, as of today) operational
human-size MR scanner reaching 10.5T. This work may also help decide which
coil structure and parameters should be used as a starting point to maximize
the chances for successful future studies at 10.5T.
Acknowledgements
This work was supported by the National Science
Foundation under grant ECCS-1307863 and by the Serbian Ministry of Education,
Science, and Technological Development under grant TR-32005.References
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