Multi-Channel Helical-Antenna Inner-Volume RF Coils for Ultra-High-Field MR Scanners
Pranav S. Athalye1, Milan M. Ilic1,2, Pierre-Francois Van de Moortele3, Andrew J. M. Kiruluta4, and Branislav M. Notaros1

1Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, CO, United States, 2School of Electrical Engineering, University of Belgrade, Belgrade, Yugoslavia, 3Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota, Minneapolis, MN, United States, 4Radiology Department, Massachusetts General Hospital, Harvard Medical School, Boston, MA, United States

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., B1 shimming) to further mitigate B1+ 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 B1+ 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 B1 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 B1+ 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

1. Ugurbil K. Magnetic Resonance Imaging at Ultrahigh Fields. IEEE Transaction on Biomedical Engineering, vol. 61, no. 5, pp. 1364–1379, May 2014.

2. Alsop D C, Connick T J, Mizsei G. A spiral volume coil for improved RF field homogeneity at high static magnetic field strength. Magn Reson Med. 1998;40(1):49-54.

3. Raaijmakers J E, Van der Werf A, Kroeze H, Luijten P R, Van den Berg C A T, Klomp D W J. Helix antennas: approaching the target from a different angle. Proc of the Joint Annual Meeting ISMRM-ESMRMB 2014, Milan. 2014.

4. Notaros B M, Ilic M M, Tonyushkin A A, Sekeljic N J, Athalye P. Quadrifilar Helical Antenna as a Whole-Body Traveling-Wave RF Coil for 3T and 7T MRI. Proceedings of the 23th Scientific Meeting of the International Society for Magnetic Resonance in Medicine, ISMRM 2015, Toronto, Canada. 2015, pp.1825.

5. Athalye P S, Sekeljic N J, Ilic M M, Tonyushkin A A, Kiruluta A J M, Van de Moortele P F, Notaros B M. Long and Short Monofilar and Quadrifilar Helical Antenna RF Coils at 7 T. Invited Presentation. 10th Biennial 2015 Minnesota Workshop on High and Ultra-High Field Imaging. 2015.

6. Djordjevic M, Notaros B M. Double higher order method of moments for surface integral equation modeling of metallic and dielectric antennas and scatterers. IEEE Trans Antennas Propagat. 2004;52(8):2118-2129.

7. Ilic M M, Tonyushkin A A, Sekeljic N J, Athalye P, Notaros B M. RF excitation in 7 T MRI systems using monofilar axial-mode helical antenna. Proc. of the 2015 IEEE International Symposium on Antennas and Propagation, Vancouver, Canada, 2015.

8. Van De Moortele PF, Akgun C, Adriany G, Moeller S, Ritter J, Collins CM, Smith MB, Vaughan JT, Ugurbil K. B-1 destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil. Magnetic Resonance in Medicine, 2005;54(6):1503-18.

Figures

Fig. 1. Sketch of the phantom-loaded octafilar (8-channel) helical-antenna RF volume-coil prototype, with M helices wound coaxially and fed with 360°/M phase increments (M = 8) against the common back plate (ports P1–P8), used in CMRR experiments; saline-water cylindrical “bottle” phantom is at the far end inside the coil.

Fig. 2. Four-channel helical-antenna RF volume coil prototypes (Fig. 1, M = 4) during CMRR experiments in August 2015: (a) 7T, 300-MHz, prototype, (b) 10.5T, 443-MHz, prototype, and (c) specially designed matching plates - internally matched antennas in free space (return loss better than 10 dB for all ports, all coils).

Fig. 3. Results for coronal and axial B1-maps for 4-channel helical-antenna RF coil at 7T in Figs. 1 and 2(a) (M = 4, Lbore = 336 cm, Dhelix = 32 cm, helix pitch 10.7 cm): (a) higher-order-MoM simulations, (b) CMRR measurements, (c) all channels together, 4 different coronal slices (measured).

Fig. 4. Results for B1-maps for 8-channel helical-antenna 7T RF coil (Figs. 1, 2(a), M = 8, Lbore = 336 cm, Dhelix = 32 cm, helix pitch 10.7 cm): (a) higher-order-MoM simulations and (b) CMRR measurements in coronal and axial cross-sections; (c) all channels together, 10 different coronal slices (measured).

Fig. 5. CMRR-measured results for 4-channel helical-antenna RF coil at 10.5T in Figs. 1 and 2(b) (Lbore = 410 cm, Dhelix = 21 cm, pitch 16 cm): (a)–(d) magnitude image (GRE, sagittal slice) from each of the 4 receive channels (RF transmission on all channels without specific transmit phase adjustment).



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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