Ria Forner1, Ettore Flavio Meliadò2,3, Martijn Lunenburg3, Catalina Arteaga de Castro3, Ladislav Valkovič4, Christopher T. Rodgers4,5, Alexander Raaijmakers2,6, and Dennis Klomp2
1Radiology, UMC Utrecht, Utrecht, Netherlands, 2UMC Utrecht, Utrecht, Netherlands, 3TeslaDC, Zaltbommel, Netherlands, 4Oxford Centre for Magnetic Resonance Research, University of Oxford, Oxford, United Kingdom, 5Dept of Clinical Neurosciences, University of Cambridge, Cambridge, United Kingdom, 6Biomedical Engineering, Eindhoven University of Technology, Utrecht, Netherlands
Synopsis
While an RF birdcage integrated
behind the bore-liner of a 7T MRI can provide relatively uniform 31P
excitation, it requires substantial system integration activities. Here we
present an alternative approach using an array of stacked dipole antennas where
one is tuned to 1H and the other to 31P. The setup is
significantly easier to configure and while 31P B1+
uniformity is somewhat compromised, B1+ is more efficient when
compared to an integrated birdcage.
INTRODUCTION
Metabolic imaging using X nuclei is an attractive technique for it provides
an unique insight into cellular metabolism, is not hindered by the three orders
of magnitude higher water and lipid signals and is less sensitive to B0
and B1 non-uniformities. While ideal X-nuclei transmit coils would,
similar to 1H transmit coils, be integrated behind the bore liner of
the MRI system to maximize patient comfort and provide uniform excitations, it
comes at a substantial integration effort, sometimes neither supported by the
MRI vendor nor the institute. Here we present an alternative approach for a 31P
and 1H setup that maintains patient comfort and can provide good B1+
throughout the human body at 7T. We show by simulations and bench top
measurements that the B1+ fields of dipoles tuned for 1H
do not affect the fields of dipoles tuned for 31P so that they can
be stacked together. When used as an 8 channel transceiver array, so far
without stacking, good MRI performance is demonstrated when operated at the
frequencies of 1H (demonstrated at 7T) and close to 31P
(demonstrated by 1H MRI 3T).METHODS
A 30cm fractionated dipole was tuned to 298MHz (1H at 7T).
Similarly, another 30cm dipole was tuned to 120MHz (31P at 7T) using
continuous meandering. Efficiency was measured on a large saline filled phantom
with 2cm separation between the phantom and closest antenna. The second antenna
was placed with 1cm spacer on top of the other antenna. A cylindrical hole
through the phantom was used to hold a calibrated pickup probe (fig1). B1+
and SAR simulations were performed using Sim4Life on Duke and S11
and S12 values were measured with the elements stacked, and for each
separately. 1H and 31P ceramic cable traps (one of each; 1H
closest to feed port) were placed on each cable to suppress shield currents.
The efficiency (S12) was compared to an integrated 31P
quadrature birdcage coil. To illustrate the potential in vivo performance of
the dipole array at 31P (120MHz), an 8 channel dipole array was
constructed and tuned for 3T 1H MRI (128MHz) and tuned for 1H
MRI at 7T upon which MRI was obtained from a healthy volunteer. RESULTS
B1+ and SAR simulations did not reveal any field
variations due to stacking at either the 31P or 1H
frequency (Fig 2). Stacking order made no difference. For the benchtop
measurements, the 31P element was positioned closest to the phantom.
The S11 on phantom remained good (-17dB to -20dB) for 1H when
comparing single element to the stacked element respectively, and S12
remained identical. For 31P, the S11 dropped from -15dB
to -6dB at 120MHz, which resulted in a small reduction in S12 of 2dB
when comparing the single element to the stacked element respectively. Here,
the presence of the 1H element shifted the matched condition (-18dB)
of the 31P element to 131MHz. The S12 of the single tuned
31P element was 2dB higher when compared to the S12 of a single port
of the birdcage with the same pickup probe placed in the isocenter. MRI (in
figure 3) that was obtained with the 8 channel dipole array reflects good
uniformity in the body when driven close to the 31P frequency and
fair uniformity when driven at the 1H frequency for 7T.DISCUSSION
A dipole array can be a good candidate for providing a relatively
uniform B1 field, not only for 1H but also for 31P.
While simulations show negligible effect of stacking the antennas, bench top
measurements did show a load dependence resulting in a subtle effect on the
matching conditions, which could be counteracted by adapting the matching
network to regain the subtle loss in efficiency (S12). In contrast
to double tuned setups that often compromise performance for one or both
nuclei, here we do not observe the compromise. In comparison to a 31P birdcage body
coil, the 31P dipole array has a 1.7-fold more favourable B1 and more
than 1.3-fold increased SAR efficiency (below 4.3μT/ for the birdcage vs 5.7μT/with B1 normalised per square-root peak local SAR based on
Duke simulations). However, the dipole array does require B1
shimming for ensuring uniform B1.CONCLUSION
An array of two stacked dipole antennas (one for 1H, one for 31P) can perform
equally well as the corresponding arrays of 1H or 31P alone. The 31P antennas
are more efficient than birdcages and therefore may be an attractive
alternative to integrate X-nuclei MRI in traditional 1H RF coil
arrays. Acknowledgements
This work was supported by: European H2020-FETOPEN: NICI
C. T. Rodgers is funded by a Sir Henry Dale Fellowship
from the Wellcome Trust and the Royal Society [098436/Z/12/B].
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