Design and test of a double-nuclear RF coil array for 1H MRI and 13C MRS at 7T
Omar Rutledge1, Tiffany Kwak1, Peng Cao1, and Xiaoliang Zhang1

1Radiology and Biomedical Imaging, University of California, San Francisco, San Francisco, CA, United States

Synopsis

RF coil operation at 7T is fraught with technical challenges, making expansion of 7T into clinical imaging difficult. In this work, a microstrip transmission line and a wire loop coil were combined to form a double-nuclear RF coil array for proton magnetic resonance imaging and carbon magnetic resonance spectroscopy at the ultrahigh magnetic field strength of 7T. Network analysis revealed a high Q-factor and excellent decoupling between the coils. Proton images and carbon spectra were acquired with high sensitivity. The successful testing of this novel double-coil array demonstrates the feasibility of this design for multi-nuclear studies at 7T.

Purpose

Acquiring MR data at ultrahigh magnetic field strengths (7T) offers higher signal-to-noise and better spatial and spectral resolution than what is available at clinically-approved field strengths (≤ 3T). However, there are many challenges to RF coil design when operating in the 7T environment, particularly in heteronuclear applications.[1] It has been shown that the distributed-element microstrip transmission line works well at the high frequency of hydrogen (~300 MHz).[2] In this work, we propose and test a novel double-nuclear surface coil design capable of hydrogen MRI and carbon-13 MRS at 7T.

Methods

The double-nuclear coil system was comprised of two individual surface coils. A symmetrical microstrip transmission line (MTL) surface coil was designed for hydrogen MRI at 7T (Figure 1a). Adhesive-backed copper tape was used as the strip conductor (12.5 mm × 141 mm × 36 µm) and the ground plane (97 mm × 141 mm × 36 µm) on a PTFE substrate (97 mm × 260 mm × 14.5 mm). A single-turn, segmented wire loop surface coil was created for carbon-13 spectroscopy at 7T (Figure 1b). Round copper wire (18 AWG, 1.02 mm diameter) was shaped into a rounded-rectangular coil and recessed into the PTFE dielectric substrate by 3 mm. This arrangement allows intrinsic electromagnetic decoupling between the two resonant elements. In both coils, single-loop trapping baluns were used to reduce common mode effects. Figure 2 shows the coil system with and without the protective PTFE cover sheet. Tuning, matching, and bench tests were performed using a network analyzer (Agilent Technologies, Model E5061A). Bench tests consisted of S11, S21, Q-factors, and loading frequency shift measurements. Q-factors were calculated using the 3 dB bandwidth. MR data were obtained using the GE 7T whole-body scanner (GE Healthcare, Model MR950). Two phantoms were imaged: a plastic syringe (200cc) filled with ethylene glycol and a plastic test tube (50cc) containing urea. A 2-D ultra fast gradient echo pulse sequence (FGRE) was used for proton imaging (TR = 6.9 ms, TE = 2.4 ms, NEX = 1, α = 30°, spatial resolution = 0.469 mm × 0.469 mm × 3 mm, matrix size = 256 × 256). Spectroscopic data were acquired using a 2-D free-induction decay chemical shift imaging (FID-CSI) sequence (TR = 1000 ms, TE = 2.09 ms, NEX = 1, α = 90°, reference point = 63.4 ppm, sweep width = 5 kHz, time points = 2048, dwell time = 0.2 ms, spatial resolution = 5 mm × 5 mm × 5 mm, matrix size = 8 × 8). AMIDE software was used to produce sensitivity line plots and SIVIC was used for spectrum preprocessing, plotting, and spectroscopic image generation.

Results

Bench test measurements are summarized in Table 1. The frequency shift experiment was performed independently of the S11 and S21 measurements. The MTL showed a frequency shift of ~4 MHz between loading conditions, while there was no noticeable frequency shift in the loop coil. Proton images and corresponding 1-D profiles in Figure 3 demonstrate the sensitivity of the decoupled 1H channel. Figure 4a shows the proton image with an 8 × 8 grid overlay and Figure 4b shows the grid with spectra plotted in each voxel. Spectroscopic data were further processed to a MRS image shown in Figure 4c. Spatial smoothing was applied using a sinc kernel in SIVIC.

Discussion

The double coil system performed well in our experiments. This relatively simple design was possible because the coils produced inherently perpendicular fields without significant coupling issues. The MTL performed as expected, with decreasing sensitivity as a function of distance from the strip conductor, while the wire loop coil obtained consistent signals throughout the volume. The double-coil array shows promise for the future where it could be used as a single element in a linear phased array, potentially offering spinal imaging (planar array) or head imaging (cylindrical array) at 7T.

Conclusion

We designed a double-nuclear surface coil array for proton imaging and carbon spectroscopic imaging with excellent decoupling performance and coverage of about 10 cm in diameter. Our work shows the combination of the MTL and wire loop coil is a feasible design for this purpose in the ultrahigh field of 7T.

Acknowledgements

Special thanks to Andrew Leynes for fabrication and technical assistance.

References

1. Adriany G. et al., MRM, 53(2):434-445, 2005.

2. Zhang X. et al., MRM, 46(3):443-450, 2001.

Figures

Figure 1. Diagrams of each RF resonator. a) Microstrip transmission line resonator. PTFE was used as the dielectric substrate. b) Wire-loop resonator. The rounded rectangular shape allowed a larger field of view with less wire than a rectangular coil.


Figure 2. Images of constructed double-nuclear coil system. a) Coil system with PTFE cover over the resonators. b) Coil system with the PTFE cover pulled off to the side. The wire coil is 3 mm below the plane of the strip conductor.


Table 1. Electrical characteristics and loading frequency shifts of microstrip transmission line (1H) and wire loop coil (13C). The unloaded condition has no phantom near the coil, while the loaded condition does. A shift in frequency can indicate coupling between the sample and the coil. f = Resonant Frequency, BW = Bandwidth, S11 = Reflection Coefficient, S21 = Transmission Coefficient, Q = Quality Factor


Figure 3. Proton images and line intensity profiles in each plane. A rainbow color lookup table has been used to accentuate intensity. (a) Axial view. (b) Sagittal view. (c) Coronal view. (d) Intensity profile across axial view image. Left to right in d) corresponds to bottom to top in a). (e) Intensity profile across sagittal image. Left to right in e) corresponds to top to bottom in b). (f) Intensity profile across coronal image. Left to right are analagous in c) and f).


Figure 4. Montage of image and spectroscopic data. a) Proton MRI with 8 × 8 grid overlay. The large phantom contains ethylene glycol and the small phantom contains urea. b) MRS data presented in each voxel, with peaks corresponding to 13C chemical shift. The two phantoms can be seen in the spectra. c) MRSI generated from spatial and spectral data representing ethylene glycol concentration. The color overlay was spatially smoothed using a sinc kernel in SIVIC.




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