INTRODUCTION

Phased array principle (1) has been widely used in dual-tuned array designs at ultra-high fields (≥7 Tesla (T)). So far, these dual-tuned arrays have been constrained to use in conventional 1-channel ¹H transmit systems. However, ¹H B1+ inhomogeneity is a well-known issue at ultra-high fields, and parallel transmit (PTx) system is a must to tackle it. It is a tendency that the 7T PTx system will be available for clinical applications. Therefore, it is essential to support multinuclear functionality in 7T PTx systems. In this work, we present the first 7T ³¹P/¹H head array, consisting of 8 loops and 8 folded dipoles as ¹H transceivers, as well as a birdcage as ³¹P transceiver, for ³¹P/¹H MRS and MRI in a 7T PTx system (Philips Healthcare, 7T dSync). By leveraging RF shimming capability of the PTx, the array provides homogenous proton B₁⁺ and good SNR for both ¹H and ³¹P.

METHOD

Figure 1 shows the full-wave simulation modeling of the array and the coil prototype. The coil layout, capacitor distribution, and capacitance values were the same in both simulation and prototype. All the coil elements were sitting on a 276mm-diameter cylinder. A shield was placed on the top of the head to enhance the B₁⁺ in the dome. Coil layout: The ³¹P birdcage is a 22cm-long 8-rung bandpass birdcage, whose copper width is 1cm. Five capacitors (22.7pF) were evenly distributed on each rung, and eight capacitors (68pF) on each end-ring. Eight ¹H 14cm×7.5cm loops, made with 14-awg copper wire, were nested inside the ³¹P birdcage. 11 capacitors were used for tuning, and matching accomplished with a lattice balun. Eight ¹H 26cm-long 2-folded dipoles sit 1cm above the birdcage and the loops. The dipoles geometrically decouple with the loops. Eight inductors were used for tuning, and a lattice balun parallel to a variable capacitor for matching. Coil interface: Shielded cable traps were implemented on every ¼-λ coax cable. A ³¹P quadrature hybrid was used to create circularly polarized (CP) B1+ field, while the 0° and 90° ports receive individually via their own preamplifiers. Eight 1:2 ¹H Wilkinson power splitters were used to split 8 independent channels of ¹H transmit power into eight pairs of loop and dipole. All loops and dipoles connected to their own preamplifiers to receive, thus creating a 16-channel receive array. Preamplifier decoupling of the ¹H loops was achieved by adjusting the cable length. Experiments: All in vivo images were acquired using a 7T whole-body MRI scanner (Philips Healthcare, 7T dSync). Human subject studies adhered to an approved Institutional Review Board protocol with informed consent. Safety testing and SAR parameters were acquired in accordance with worst-case SAR guidelines (2). Phantom: ¹H B₁⁺ maps were acquired with a fast pre-saturated TurboFlash B₁⁺ mapping method (3), and ³¹P spectra were acquired with non-selective, ISIS and 2D CSI sequences. In vivo: 1) 1H images were acquired with high-resolution 3D MP2RAGE, and 31P spectra with FID and ISIS.

RESULT AND DISCUSSION

Simulation: Figure 1 shows that the 8 loops contribute most of SNR (82%) in the brain, while the 8 dipoles contribute to the longitudinal coverage. The loops have 54.7% higher SNR than the dipoles in the brain, and the SNR of the 16-channel array is dominated by the 8 loops; the ³¹P birdcage has homogenous signal in ROI. Workbench: Q ratio is 5.7 for the ³¹P birdcage, and 8.5 for the ¹H loop, loaded with a head phantom (ɛr=62, σ=0.45 S/m at 7T). S21<-13dB among the coil elements. Phantom experiments: Figure 3 shows B₁⁺ maps in CP and RF-shimmed modes in axial and sagittal planes respectively. The B₁⁺ in the null band of the CP mode in axial plane as well as the B₁⁺ in cortex in the sagittal plane were boosted with the RF shimming mechanism. Figure 4 demonstrated that the ³¹P birdcage has sufficient SNR and homogeneous field, by observing well resolved 17Hz J-coupling in a methyl phosphoric acid phantom acquired by the ³¹P ISIS spectra, as well as homogeneous ³¹P signal in the ROI over the same phantom by 2D CSI. The MP2RAGE images show nice contrast between grey and white matter (Figure 5, first row), consistent with the simulated SNR results. In vivo non-localized and ISIS ³¹P spectra shows sharp peaks of all typically ³¹P detected brain metabolites (Figure 5).

CONCLUSION

We presented the first ³¹P/¹H head array compatible and integrated with a 7T PTx system. Phantom and in vivo experimental results show that the coil can provide homogeneous B₁⁺ and good SNR for both ³¹P and ¹H in ROI.

Acknowledgements

This work was fully funded by Cancer Prevention and Research Institute of Texas (CPRIT) RR180056 and was a collaborative project between Advanced Imaging Research Center, UTSW and Philips Healthcare.