Introduction

Hyperpolarised ¹²⁹Xe MRI when inhaled crosses the alveolar capillary barrier and dissolves in the red blood cells and plasma and its long T1 (~8s in oxygenated blood) allows its detection in other distal organs. Dissolved phase ¹²⁹Xe spectroscopy and imaging in perfused organs such as kidneys and brain has been investigated^1,2,3,4. In particular, the solubility of ¹²⁹Xe in tissue itself shows clinical potential as a biomarker of tissue viability in stroke⁵ and blood brain barrier permeability. As the dissolved ¹²⁹Xe signal is inherently weak when compared to the gas phase signal (~2%), RF coil sensitivity is critical for high SNR ¹²⁹Xe brain MRI. This abstract describes the simulation of three receive array designs and the preliminary results of a complete transmit and receive setup for 129Xe human brain MRI at 3T.

Material and Methods

Transmit coil: is an 8-legs low pass birdcage with octagonal end rings. A capacitance of 28.6 pF was placed in each leg to tune the coil at 35.33 MHz (¹²⁹Xe Larmor frequency at 3T). Two lattice baluns guarantee quadrature feeding of the coil. Active decoupling during the receive phase was achieved with high power PIN diodes, switched by DC trough an RF choke. Along the legs, proton traps enable the acquisition of proton images using the system Q-body coil (Acheiva, Philips). Receiver Array: Inspired by a previous work at 1.5T⁶, the array is built on an in-house 3D printed head former (Figure 1). The array consists of 6 elements matched to the available channels on the receiver chain. Preliminary simulations of three different designs were simulated in Ansys HFSS in order to evaluate the design with the highest intrinsic SNR. All single elements of the array were tuned at 35.33 MHz and fed with 1A current. The magnetic fields were then exported in MATLAB and combined to evaluate the array with the best sensitivity. The coil was then built in the workbench with high quality copper tubes and the circuit composes one-proton trap, a lattice balun for matching and a PIN diode for active decoupling with the transmit coil. The array was connected to the receive chain using λ/2 length cable to assure no change of phase in the input impedance for optimal preamp decoupling. Scanner experiments: All scans were performed on a 3T Acheiva Philips scanner. The coil was tested with a ¹²⁹Xe gas phase phantom consisting of a bag hyperpolarized ¹²⁹Xe that was imaged using a 3D spoiled gradient echo, TR/TE=6.7ms/2.5ms, acquisition matrix: 96x72x32, 8mm slice thickness, FA=10^o, Receiver bandwidth=8.6 KHz. Before the scan, a flip angle calibration was performed: TR=2000ms; BW=4096 Hz, N of sample 256; 30 dynamics; nominal FA=120^o. The decay of dynamic spectra of each element was then fitted according to , and the resulting FA was obtained from the average of all channels. Dynamic in vivo brain spectroscopy was acquired with only five channels. Scan was performed on a healthy male volunteer (27 years old, 75Kg), with a pulse acquire sequence: TR=2000ms, BW=4096 Hz, Number of sampling points 256 (spectra resolution of 16Hz/point), 25 dynamic scans; FA=16^o; inhaled ¹²⁹Xe Dose=1L (~30% of polarization), breathold time ~ 20 sec, total scan time ~ 50 sec.

Results

The transmit coil has an unloaded quality factor (Q) of 175 and a loaded Q of 61, resulting in a ratio Q_load/Q_unload of 2.89. The Results of EM simulations are shown in Figure 2. The optimum array design consists of two orthogonal saddle coil and 4 loops, which offer the best overall higher sensitivity along 3 planes and the best B1^- field penetration along axial and sagittal planes. Phantom gas phase ¹²⁹Xe images have a mean SNR~120 in the central slices. The average of the acquired dynamic spectra from the brain of the volunteer is shown in Figure 3 with the reference for all the peaks. Dynamic spectra show a maximum grey matter peak at 22 seconds (Figure 4).

Conclusion

A novel RF setup for dissolved phase ¹²⁹Xe brain MRI at 3T is presented consisting of a transmit birdcage and a 6-channel receiver RF coil. Trough preliminary EM simulation was found a design that guarantees an optimal coil sensitivity. The in vivo brain dissolved phase ¹²⁹Xe spectra and ¹²⁹Xe gas phantom images indicate high SNR for future dissolved 129Xe brain imaging.

Acknowledgements

Acknowledgments: This work was funded by the Medical Research Council and the NIHR.