Synopsis

A novel multi-layered radiofrequency coil for X-nuclei imaging is presented which implements stacked layers for improved B1+ and SNR. The multi-layer design increased B1+ by 27% in ²³Na phantom experiments and 19% in electromagnetic simulations compared to a single layer coil. Transmit-receive efficiency for a ¹³C multi-layer coil was double that of a quadrature coil, requiring half the power to achieve a 90° flip. An averaged SNR map from CSI indicated receive sensitivity gain of 33% from the quadrature to multi-layer design.

Introduction

X-nuclei imaging presents unique opportunities, for example dynamic nuclear polarization can be used to enhance the signal of ¹³C allowing monitoring of in-vivo metabolic changes in organs, such as the healthy and diseased heart¹. However, all X-nuclei coils are inherently power limited owing to the wideband requirements of X-nuclei radiofrequency (RF) amps. Improved RF coil transmit and receive performance would allow for more ambitious pulse sequence designs and increased SNR. Recent developments in X-nuclear coils have followed two general directions: either dense arrays with high SNR near the surface at the expense of depth penetration, or volume coils with excellent transmit homogeneity that are B1+ limited.

We present a multi-layer RF coil design that may replace standard single loop coils. The design principle follows the linear scaling of B1+ in a solenoid with the number of turns. To minimize resistance from both the skin and proximity effects at high frequency, conductive copper tracks from overlapping layers are offset and separated by a dielectric. The design superimposes the B1 field generated from each layer for overall improved transmit/receive (T/R) efficiency.

Coil Comparisons

A ²³Na multi-layer T/R coil was built for a 7T (Varian) preclinical MRI system at 79.5 MHz (Q loaded/Q unloaded: $$$\text{Q}_\text{L}/\text{Q}_\text{U}\,=\,0.805$$$). For this prototype, straight copper tracks tracing the path of an octagram were used; overlapping layers are separated by a $$$100\,\text{µm}$$$ polytetrafluoroethylene (PTFE) strip. Transmit improvements were quantified with a B1+ map of the ²³Na multi-layer coil, a small single-layer coil ( $$$\text{Q}_\text{L}/\text{Q}_\text{U}\,=\,0.891$$$) with a congruent local B1+ field profile and a large single-layer coil ( $$$\text{Q}_\text{L}/\text{Q}_\text{U}\,=\,0.785$$$) tracing the outer track for comparable volume coverage (Figure 1).

A second ¹³C multi-layer T/R coil was built at 75.5 MHz ( $$$\text{Q}_\text{L}/\text{Q}_\text{U}\,=\,0.723$$$) to demonstrate transmit-receive efficiency and receive chain improvements for hyperpolarized ¹³C imaging studies. Power-calibrations were performed to determine RF power required for a 90° flip-angle on a fiducial. SNR maps at this power for the multi-layer coil were compared to a commercially available preclinical quadrature surface coil (RAPID Biomedical GmbH, Rimpar, Germany).

Methods

Transmit efficiency was determined from phantom experiments and electromagnetic (EM) simulation. B1+ maps were obtained using Gradient Echo (GRE, $$$160\,\text{ms}\,\text{TR}$$$, $$$1.9\,\text{ms}\,\text{TE}$$$, $$$0.78\,\times0.78\,\times100\,\text{mm}$$$³ voxel size, $$$1\,\text{ms}$$$ Gaussian pulse, 1024 averages) images of aqueous NaCl (5 M) performed at increasing transmit powers supplied by the RF amplifier, from 0 to 90 W in steps of 10 W. 2D flip-angle projection maps at 100 W were calculated via sinusoidal curve fit for each voxel to produce B1+ field profiles² (Figure 2, top row).

EM simulations were performed on CST Microwave Studio, (Computer Simulation Technology AG, Darmstadt, Germany) using a frequency domain (FEM) solver³ with tetrahedral meshing to reproduce the curvature of the coil (≈ 400,000 mesh cells). The H-field was simulated until steady state and decomposed into right (B1+) and left (B1-) circularly polarized fields, (Figure 2, bottom row) assuming linear materials with constant permeabilities.

Receive sensitivity was determined by performing a power calibration on a 9 M [13C]urea fiducial for peak signal intensity representing a 90° flip-angle. Chemical Shift Imaging (CSI, $$$600\,\text{ms}\,\text{TR}$$$, $$$2\,\text{ms}\,\text{TE}$$$, $$$3.125\,\times\,3.125\,\times\,20\,\text{mm}$$$³ voxel size, 2 ms Gaussian pulse, 32 averages) at this optimal transmit power was run on natural abundance ethylene glycol (0.39 M of ¹³C) to provide an SNR map.

Results

The multi-layer design indicates B1+ improvements of 27% from phantom experiments and 19% from EM-simulations averaged over a specified ROI, when compared to the small single layer coil (Figure 2). Greater B1+ improvements of 66% from phantom experiments and 37% from EM-simulations are observed when compared to the large single layer coil. The 1D vertical plot though B1+ maps are consistent with expected $$$1/r^3$$$ drop off, where $$$r$$$ is distance from the coil (Figure 3). For receive efficiency, the multi-layer coil achieved a 90° flip-angle on the urea fiducial at 35 dB RF transmit power, with the quadrature coil requiring 41 dB transmit power. The multi-layer design requires half the power of a quadrature coil to achieve a 90° flip-angle, being twice as efficient. For receive sensitivity in the specified region, axial CSI data for the multi-layer coil and quadrature coil are converted into SNR maps. Quadrature coil designs theoretically improve SNR by 41.4% over a standard single loop; the multi-layer coil improves SNR by a further 33% compared to the quadrature coil (Figure 4).

Future Directions

Acknowledgements

References

1. Apps A, Lau J, Peterzan M, et al. Hyperpolarised magnetic resonance for in vivo real-time metabolic imaging. Heart. 104, 1484-1491 (2018).

2. Insko EK, Bolinger L. J Magn Reson. 103, 82-85 (1993).

3. Computer Simulation Technology AG. CST Microwave Studio Technical Specifications (2016).