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A Sixteen-Channel Array Coil for Carbon-13 Spectroscopy of the Breast at 7T
Matthew Wilcox1, Stephen Ogier2, Sergey Cheshkov3,4, Ivan Dimitrov3,5, Craig Malloy3,4,6, Steven M Wright1,2, and Mary McDougall1,2

1Biomedical Engineering, Texas A&M University, College Station, TX, United States, 2Electrical and Computer Engineering, Texas A&M University, College Station, TX, United States, 3Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, TX, United States, 4Radiology, UT Southwestern Medical Center, Dallas, TX, United States, 5Philips Medical Systems, Cleveland, OH, United States, 6Internal Medicine, UT Southwestern Medical Center, Dallas, TX, United States

Synopsis

Biomarkers detectable by carbon-13 NMR spectroscopy have known correlations with breast cancer characterization, but in vivo 13C spectroscopy has been limited by low SNR. To counteract this, a 16-element receive coil and isolating preamplifier box was constructed for carbon-13 spectroscopy of the breast at 7 Tesla. The array was characterized on the bench and showed good results in terms of ease of tuning, low element-to-element coupling and Q values. Scanner testing using the coil was preliminary but showed successful 1H and 13C transmission and that array elements were individually able to acquire spectra.

Purpose

Interest in non-1H imaging and spectroscopy is being driven by the ongoing and increasing attention to characterizing in vivo metabolics and is facilitated by the increased sensitivity provided by high field scanners. The detection of carbon-13 is of particular interest due to its use in hyperpolarized form, in [U-13C]glucose infusion studies, and as its potential as a sensitive naturally abundant biomarker. Specifically, previous research has demonstrated correlations between breast cancer risk/characterization and the lipid profile biomarkers found in 13C spectra [1,2], but in vivo work has been limited by low sensitivity. This work describes the design and construction of a 16-channel array coil and isolating preamplifier box to increase the SNR of carbon-13 spectroscopy of the breast at 7T.

Methods

The 16-channel carbon-13 array coil and 75 MHz preamplifier box were designed to be used with a “frequency-translation system” operated as an add-on to the 16-channel 1H interface for the Philips 7T Achieva and is described in detail in [3]. For the purposes of summarizing the system, a block diagram is shown in Fig. 1. Sixteen wire array elements were mounted on a 3D printed, cup-shaped former containing etchings for precise coil positioning based on the "soccer ball" geometry [4]. Each element contained three breaks for 50 pF segmenting capacitors, a 298 MHz LCC trap [5], and SMA connection to a detachable match/tune circuit board. Fine tuning, matching, and actively detuning the coils was accomplished using the network suggested by Reykowski [6]. S11 and Q values were recorded for each element, and the active detuning network functionality was verified using pickup probes.

The receive array was designed to insert into volume coils for 1H and 13C. The 1H transmit-receive volume coil was a 20 cm diameter version of a previously reported quadrature Helmholtz-saddle configuration driven with forced-current-excitation [7]. The 13C transmit coil was a 23cm diameter Helmholtz coil with LCC traps incorporated. The 16-channel 75MHz preamplifier box consisted of 16 coil input channels, each with a 125 MHz low-pass filter followed by an isolating preamplifier (WanTcom 74A). Fan-out boards distributed a single 10V power supply to all channels and distributed the PIN diode detuning signal to all connected coils.

Preliminary testing of the array was performed on a Philips 7T Achieva scanner. Using the 1H volume coil in T/R mode, images were taken of a spherical canola oil phantom using a multi-slice sequence (FOV - 450x450 mm, TR – 11 ms, resolution – 1.75x1.75 mm, slice thickness – 1.75 mm). Carbon-13 spectra were acquired using the 13C volume coil in T/R mode (BW – 8 kHz, TE – 325 µs, TR – 8 s, samples - 8192). Finally, spectra were acquired from select individual receive elements using the Philips single-channel 13C interface box and the same parameters.

Results

The 16-channel carbon-13 array coil is shown in Fig. 2, with insets to provide detail. Each element matched and tuned to better than -20 dB with an average Q of 55.4, as shown in Fig. 3. Additionally, active detuning better than -30 dB was measured for each element. The 16-channel 75MHz isolating preamplifier box is shown in the photo in Fig. 4 with details labeled. With all other receive coils plugged into the preamplifiers, individual receive element S11 measurements showed minimal amounts of coupling as compared to the disconnected case, indicating that preamplfier decoupling was effective.

Figure 5 includes a 1H image, 13C spectrum from the volume coil in T/R mode, and example single-channel 13C spectra from the array. The data is preliminary only, indicating the functionality of the coils, an increase in SNR of the array elements over the volume coil, and needed modifications to the current design. Immediate future work includes making the 13C transmit coil quadrature, quantifying and verifying the expected increase in SNR, and using the coil and preamplifiers in conjunction with the frequency translation system to acquire simultaneous 16-channel carbon-13 data.

Conclusions

This work has reported the design and construction of a 16-channel carbon-13 array coil for unilateral breast spectroscopy. Good benchtop performance was demonstrated as indicated by ease of tuning and low inter-coil coupling and preliminary scanner results are promising, with the expected SNR increase offering exciting possibilities for future in vivo studies.

Acknowledgements

The authors gratefully acknowledge funding for the project given through NIH R21 EB 016394 and P41EB015908 as well as funding through CPRIT RP150456.

References

1. Bougnoux, P., Giraudeau, B., Couet, C., "Diet, cancer, and the lipidome," Cancer Epidemiology Biomarkers and Prevention, vol. 15, no. 3, pp. 416-421, 2006.

2. Chajes V., Thiebaut ACM., Rotival, M., Gauthier, E., Maillard, V., Boutron-Ruault, M., Joulin, V., Lenoir, GM., Clavel-Chapelon, F., "Association between serum trans-monounsaturated fatty acids and breast cancer risk in the E3N-EPIC study," American Journal of Epidemiology, vol. 167, no. 11, pp. 1312-1320, 2008.

3. Ogier, S., Wright, S., "A frequency translation approach for multichannel 13 C spectroscopy." 2015 37th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC). IEEE, 2015.

4. Wiggins GC, Triantafyllou C, Potthast A, Reykowski A, Nittka M, Wald LL. "32-channel 3 Tesla receive-only phased-array head coil with soccer-ball element geometry." Magnetic Resonance in Medicine 56.1 (2006): 216-223.

5. Meyerspeer, M; Roig, ES; Gruetter, R; Magill, “An improved trap design for decoupling multinuclear RF coils”. Aw. Magnetic Resonance in Medicine; Aug. 2014; 72; 2; p584-p590

6. Reykowski A, Wright S, Porter J. "Design of matching networks for low noise preamplifiers." Magnatic Resonance in Medicine 33.6 (1995): 848-852.

7. McDougall, M.P.; Cheshkov, S; Rispoli, J; Malloy, C; Dimitrov, I; Wright, S; “Quadrature transmit coil for breast imaging at 7 tesla using forced current excitation for improved homogeneity.” Journal of Magnetic Resonance Imaging, Nov. 2014; 40(5): 1165-1173.

Figures

Figure 1: the signal from each receive coil is first filtered to ensure no contamination from the 298 MHz 1H signal before being amplified; the signal is then translated up to the 1H frequency to be received directly by the scanner; additional steps are taken to ensure phase stability, and the positioning of the frequency translation unit after the preamplifier ensures no noticeable loss in SNR from the translation

Figure 2: the coil array is designed for imaging and spectroscopy studies with the patient lying in a prone position; semi-rigid coaxial lines lead to individually shielded match/tune boxes for 1H and 13C transmit; SMA connectors attach each match/tune/detune board to corresponding receive elements mounted on the 3D printed hemispherical former which is designed to accommodate 80% of breast sizes

Figure 3: S11 measurements taken of individual receive elements while all other elements were detuned showed that all elements could be simultaneously matched/tuned at the 75 MHz 13C frequency; all elements showed better than -20 dB matching and generally clean S11 curves, indicating low coupling between coils; Q values were typically high and consistent with only elements 3 and 5 indicating lower values

Figure 4: the modular preamplifier box consists of individually shielded enclosures with custom-made low-pass filters (rejection > 50 dB at 298 MHz, insertion loss < 0.12 dB) and 1.5Ω low-noise preamplifiers (WanTcom 74A); the preamplifier power supply is individually voltage-regulated and protected on each channel, and the detune bias fan out board overlays the detuning signal onto each coil’s coaxial line using a bias-T circuit on each channel

Figure 5: images taken with the 1H transmit coil show fairly homogenous excitation throughout the phantom (coronal image shown); 13C spectra from individual receive elements compare favorably to spectra receive from the 13C volume coil with SNR improvements over the volume coil of 26% and 47% for elements 1 and 2 respectively

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)
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