Synopsis

We present a novel dual dipole coil concept for 7T ³¹P metabolic imaging in the body.

We constructed an array of 8x stacked ¹H/ ³¹P dipoles, evaluating their performance by bench measurements, EM modelling, phantom experiments and in vivo scans in human liver. Parameters such as coupling, transmission efficiency and SAR are compared to those of a whole-body ³¹P birdcage coil.

Introduction

The aim of the present study is to design and build a CP (circularly polarised) ¹H/³¹P body coil for use at 7T consisting of two separate dipole arrays, one for each nucleus, with minimal performance reduction for ³¹P. We demonstrate the feasibility of the stacked ¹H/³¹P dipole array coil for ³¹P-MRS in vitro by comparing the performance of the stacked dipoles to single-tuned dipoles in isolation for both frequencies. The stacked dipoles were constructed in an 8-channel wrap-around array and merged with additional 16 receive-only loops. Here, B₁⁺ efficiency is compared to an integrated body coil. B₁ values at liver depth of each dipole from this array coil have been acquired too on all 8 Tx elements and compared with simulations to ensure agreement to SAR guidelines. Subsequently, ³¹P spectra in a human liver are obtained with this setup at 7T.

Methods

The coil has eight ¹H-Tx/Rx dipoles and eight ³¹P-Tx/Rx dipoles (TeslaDC, Zaltbommel, the Netherlands) with 16 additional receive-only loops(fig.1).

In order to tune the 2 dipoles, the complex impedances and parallel inductances were chosen and calculated to maximise power transfer and make the dipoles resonate at 120 MHz for ³¹P and 298 MHz for ¹H by the use of an approximate element matching network (Smith v4.1;BUAS,Switzerland).
The ³¹P dipole tuned in isolation was then placed on the phantom along with a 1H dipole PCB not populated with any lumped elements above it.
The dipoles PCB and electronic circuits are shown in more details in fig.1.

The RF coil design efficiency has been evaluated at two sites: University Medical Center, Utrecht and at the University of Cambridge. In Utrecht, a Philips 7T Achieva Scanner (Best, the Netherlands) has been used for experiments and in Cambridge it was a 7T Terra Siemens (Erlangen, Germany).
The same phantom used in both sites (figure 1c) contains 26 litres of distilled water, 78g NaCl, and 150g H₂KO₄P. A 50ml tube with 0.5M phenylphosphonic acid (PPA) solution (f₀ at 17ppm from H₂KO₄P frequency) is placed in the centre of the shaft. We centred both our acquisition frequencies on PPA to do our B₁⁺ evaluation at liver depth.

³¹P performance evaluation:
An FID sequence has been used at both sites:
At Utrecht, the B₁⁺ in a single module setup (as opposed to the 8-channel array) is assessed by increasing the flip angle values from 5 through 40 degrees with a fixed block pulse duration of 3.23 ms and long TR. The distance from dipole to PPA tube was 10cm.
In Cambridge, the B₁⁺ was assessed by increasing the pulse duration from 1ms to 20ms and with a fixed voltage value of 400V at the Tx/Rx switch, where power losses of cabling and Tx/Rx switches were calibrated. The distances from the phantom surface to the PPA tube are as shown in figure 1c).
Three different configurations (single dipole, stacked dipole and a whole-body birdcage) have been compared in vitro in Utrecht in terms of SNR and B₁ per unit of square root of power at the centre of the phantom in the PPA tube.

Simulations of the electric and magnetic fields have been done (Sim4Life, Zurich MedTech, ZMT) using the exact geometry and mesh of the coil that has been built and with a self-written Matlab script for the RF shimmed field calculations.

In vivo ³¹P MRSI acquisitions in the liver were also performed in Cambridge with a 3D-UTE-CSI-FID sequence, a shaped pulse [4] with a duration of 2.3ms and 400V, a 16x16x8 matrix size and a 350mm FOV, a 1s TR and 6000Hz bandwidth.

¹H performance evaluation:
The ¹H performance of the single-module setup was evaluated by performing a dynamic scan – once with the single ¹H dipole in isolation and once with the stacked ¹H dipole with the ³¹P dipole. The SNR was calculated as the ratio of: the mean of the signal from the region corresponding to the PPA tube to the standard deviation from a region where no phantom was present.

Results and Discussion

The presence or absence of the ³¹P antenna causes an insignificant change on the ¹H SNR (fig.2) while the SNR of the ³¹P body array is 84% larger compared to the body coil (fig.2C).
The ³¹P B₁ maps resulting from simulations are shown in fig.3, as well as the peak SAR simulated values.
A comparison of B₁ values at liver depth from all the dipoles independently is shown in fig.4, and compared with simulations. While the absolute performance is lower than simulated (losses antenna not incorporated in simulation), the relative B₁⁺ matches well to simulations.
In fig.5, the in vivo results on the liver of a healthy adult male subject are shown.

Conclusion

A ³¹P/¹H coil array design, consisting of 8-stacked ³¹P/¹H dipole pairs, has been evaluated by the following parameters: influence of stacking ¹H dipoles on top of ³¹P dipoles, CP mode of the ³¹P coil antenna array, compared to an integrated body coil, and feasibility of the coil design for ³¹P-NMR spectroscopy in vivo in the liver at 7T. Results show that stacked dipoles are not compromised compared to isolated dipoles and the array design outperforms the integrated ³¹P body coil.