Quadrature 31P and single 1H dual-tune coil for cardiac 31P-MRS at 7T
Benoit Schaller1, Watcharaphol Paritmongkol1, Arthur W Magill2, Matthew D Robson1, and Christopher T Rodgers1

1RDM Cardiovascular Medicine, University of Oxford, OXFORD, United Kingdom, 2Institute of Neuroscience and Medicine - 4, Forschungszentrum Juelich GmbH, Juelich, Germany

Synopsis

Phosphorus spectroscopy is a powerful tool for cardiac metabolism studies. Working at 7T gives a significant (2.8x) gain in SNR compared to 3T. However, with existing RF coils, it is hard to excite the heart uniformly and with sufficient peak B1+. In this work we simulated 8 candidate coil designs and identified the best one for cardiac 31P-MRS. We built this quadrature pair of 15cm loops for 31P and a single 10cm loop for 1H, decoupled with LCC traps, performed safety tests, validated its B1+ in phantoms and recorded spectra in vivo in the human leg, liver and heart.

Introduction

Heart disease frequently involves impaired cardiac energy metabolism, which can be monitored non-invasively by phosphorous magnetic resonance spectroscopy (31P-MRS).1 We have used a 16-channel 31P receive array (Rapid Biomedical) which gives excellent receive SNR but which (on an 8kW Siemens 7T scanner) has a peak B1+ of only ~10µT at the depth of the heart, due to its 28x28cm2 transmit element.2 A more efficient transmit element would allow increased SNR for cardiac 31P-MRS by exciting at the Ernst angle; or even excitation with adiabatic pulses for absolute metabolite concentration measurements. This work aims to develop a 31P/1H coil with optimised transmit for cardiac 31P-MRS at 7T.

Materials and Methods

Simulations: The following coil designs were simulated in CST Studio 2014: the 28x28cm2 square transmit element in the 16-channel array2; 10cm, 15cm, 20cm and 30cm diameter circular loops; and 10cm, 15cm and 20cm diameter overlap-decoupled quadrature pairs.3 The “Laura” voxel model (CST) was used for loading, with FIT calculation using >1 million mesh cells, and co-simulation for the (balanced) matching networks. All Sii were <-30dB at 120.3MHz. We aimed to maximise B1+ across the heart, and also to minimise SAR10g / (B1+)2. To obtain safety parameters, we then simulated (Fig. 1) the 15cm 31P quadrature pair with an overlying 10cm 1H loop at both 297.8MHz (1H) and 120.3MHz (31P) frequencies on the Laura and Gustav virtual humans (each in two orientations) and on a large cube of muscle.

Coil: We fabricated the coil (Fig. 2) from 2mm copper wire, using Series 100E fixed capacitors (ATC Corp), and SGNMC1206E variable capacitors (Sprague Goodman), in a housing repurposed from a Siemens 1.5T cardiac coil. The two 31P loops were decoupled by adjusting their overlap. The 31P and 1H loops were decoupled by adding inductor-capacitor-capacitor (LCC) traps4 to the 31P loops (L ~100 nH, Cs = 9.2pF, Cp = 3.3pF). The coil was connected by coaxial cables to a “T/R switch box” (Virtumed LLC, MN, USA) containing 3x preamps for the loops, and a 90° quad-hybrid. Balanced matching was sufficient to suppress common mode currents5, although foam sleeves (FG265, Component Force Ltd) were fitted around the cables as a further precaution.

Phantom tests: Losses from the LCC trap circuits were tested by comparing B1+ and B1- maps made from a set of 31P FLASH images at 11 flip-angles on a 1L triphenylphosphite (TPP) bottle (Fisher Scientific UK Ltd, used as supplied). TPP has a 3.8M 31P concentration, 725ms T1 and a low conductivity and permittivity. TPP is a liquid at room temperature, is not toxic to humans, and costs <£50 per litre.

The SAR simulations were validated by heating turkey mince in the MRI scanner (Siemens 7T) while monitoring with fibre optic temperature probes (Neoptix).6,7

B1+ was measured using a phantom7 comprising 14L saline for loading and a moveable 2x2x2cm3 cube of KH2PO4(aq) inside. At each depth, non-localised spectra were recorded at several transmit voltages and the peaks integrals were fitted to a sinusoid to determine B1+.

In-vivo tests: Three healthy volunteers were scanned using our standard cardiac 31P-MRS protocol (3D UTE-CSI, 1s TR, 16x16x8 matrix over a 24x24x20 cm3 field of view) with the new coil in the heart, leg and liver. Localisers were acquired with single-shot FLASH and pulse-oximeter gated CINE FLASH. In the liver, B0 shimming was performed using a dual-echo GRE method.8

Results and Discussion

Simulations: Table 1 and Fig. 1 show the simulation results. The coil with the greatest B1+ and the lowest SAR/(B1+)2 over the heart was the overlapped 15cm quadrature pair (Fig. 2).

Phantom: Losses due to the LCC trap were <10%, consistent with previous findings.4 Defining 1=31P_A, 2=31P_B, 3=1H, S-parameters on the 14L phantom in the magnet were S11 -30dB, S22 -28dB, S12 -24dB at 120.3MHz, and S33 -32dB, S13 -32dB, S23 -25dB at 297.8MHz. Similar values were obtained in vivo. In the 14L phantom, peak B1+ varied between 16 – 25µT at the depth of the heart (Fig. 3).

In-vivo: Localization was straightforward and good quality spectra were recorded in the leg, liver and heart. Fig. 4 shows representative cardiac spectra and localizer images with acceptable SNR right across the septum. In another abstract, we show in 5 normal volunteers, that this coil enables adiabatic excitation across the heart for the first time at 7T.

Conclusion

This transmit coil delivers the B1+ needed for cardiac 31P-MRS at 7T. Adding a 1H loop makes localization and per-subject B0 shimming straightforward. This transmit coil should now be combined with our receive array to optimise both B1+ and B1-.

Acknowledgements

Funded by a Sir Henry Dale Fellowship to CTR from the Royal Society and the Wellcome Trust [098436/Z/12/Z].

References

[1] P. A. van Ewijk et al., NMR in Biomed., 2015, DOI: 10.1002/nbm.3320.

[2] C. T. Rodgers et al., Proc. ISMRM 2014, #2896.

[3] G. Adriany and R. Gruetter, J. Magn. Reson, 125:178-184, 1997.

[4] M. Meyerspeer et al., Magn. Reson. Med., 2014.

[5] Peterson et al., Concepts in Magnetic Resonance 19B(1) 1-8, 2003.

[6] El-Sharkawy AM et al, Magn Reson Med, 61(4):785-795, 2009.

[7] C. T. Rodgers et al., Magn. Reson. Med., 72: 304–315, 2014.

[8] L. delaBarre et al., Proc ISMRM 2015, #3152.

Figures

Figure 1: EM simulations for (a) the final coil design on the Gustav virtual human at 120.3MHz (31P). (b) S-parameters (c) transverse plot of B1+ / uT at Pin=1W; (d) SAR10g (worst-case = 0.76kg‑1). Then at 297.8MHz (1H): (e) B1+; and (f) SAR10g (worst-case = 0.52kg‑1).

Figure 2: Photograph of the coil.

Figure 3: B1+ vs depth in a phantom comprising a moveable 2x2x2cm3 cube of KH2PO4(aq) submerged in 14L saline, for the quadrature coil, a 10cm loop coil (with its existing T/R switch rated at 270V and at the scanner’s full output), and the 16 channel array coil from Rapid Biomedical.

Figure 4: Example spectra from the heart of a healthy volunteer. Left: Localizer images acquired with the 10cm 1H loop in the mid-short axis and horizontal long-axis views. Right: Spectra from voxels along the interventricular septum as marked on the short-axis localizer.

Table 1: EM simulation results on the Laura virtual human. Total input power was 1W RMS. The cardiac segment depths were 8.5, 12.5, 16.5cm from the coil (and somewhat less in a male). The 15cm quadrature design gives maximal B1+ and acceptable SAR10g/(B1+)2 in the mid cardiac segment.



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