INTRODUCTION

Sodium MRI provides novel information on energy metabolism, and has the potential to elucidate physiological processes [1] in kidneys, brains, heart, knees, skins, and etc. However, clinical sodium MR imaging remains expensive because of high cost of broadband multi-nuclear transmit/receive spectrometer and dedicated RF coils for sodium imaging. These high costs and poor availability are the large impediment for clinical application of sodium MRI. In this study, we developed a novel add-on ²³Na MRI platform (crossband repeater, CBR) consisting of a crossband convertor and receive/transmit coils, that could be built into an existing ¹H MRI scanner without the need for modifying the scanner configuration. This study presents an initial proof-of-concept for the add-on ²³Na MRI CBR to a 1.5T ¹H scanner, followed by its application to in vivo imaging of live mouse. The concept of the proposed system is applicable to other ¹H scanners, and would expand the opportunity for researchers to assess clinical applications of sodium MRI.

METHOD

Concept of add-on ²³Na MRI crossband repeater
We focused on the CBR technology used in wireless communication, and applied it to transmit signals between the ¹H and ²³Na resonance frequencies. The overview and diagram of the add-on ²³Na MRI CBR operated at 1.5T are shown in Figs. 1 and 2. The 64MHz excitation pulses transmitted from the 1.5T ¹H-MRI system are received by a ¹H pick-up coil in the gantry, downconverted to 17MHz ²³Na excitation pulse through frequency mixer I, amplified by an RF transmitter, and transmitted to a ²³Na RF coil to excite a sample and acquire NMR signals. The acquired 17MHz ²³Na signal is amplified, upconverted to 64 MHz through frequency mixer II, and transmitted back to the ¹H-MRI by a ¹H transmit coil. In this system, the MRI signal received by the ²³Na coil is passed to the ¹H receiver coil by electromagnetic coupling; The ¹H-MRI system operates as if it is receiving the ¹H signal, but what is actually acquired is ²³Na signal modulated to 64MHz.
Apparatus
The ¹H-MRI system consisted of a 1.5T/280mm horizontal-bore superconducting magnet (JASTEC, Japan), gradient coil, transmit/receive RF coil (birdcage coil, 8-elements, 110mm in diameter, 130mm in length), and MRI console. The add-on ²³Na platform consisted of a crossband convertor (DSTechnology, Saitama, Japan) ²³Na transmit/receive coil (17 MHz, birdcage coil, 8-elements, 42mm in diameter, 120mm in length), ¹H pick-up receive/transmit coils (64MHz, loop coils, 50mm in diameter), RF amplifier (500kHz - 150MHz, 2kW, BTO2000-AlphaSA, TOMCO, Australia), RF shield between a sample and ¹H transmit coil, and TX/RX switch and ²³Na preamplifier (17MHz). All the coils were homebuilt.
Bench test of control signals
We measured the fidelity and time response of the control signals using an oscilloscope (2GHz, ViewGo II, IWATSU, Japan).
Sensitivity measurement
To evaluate the sensitivity of the ²³Na signals, the noise and FID signals from phantoms (saturated saline solution and purified water) were measured in the following three cases: (A) using a conventional ²³Na transceiver (as a reference) (Fig. 2(b)), (B) using the CBR (Fig. 2(c)), and (C) using the CBR with direct, bypass electrical connection from the crossband convertor to the ¹H-MRI transceiver using a BNC cable (dot-dashed line in Fig. 2(c)), instead of connecting via electromagnetic coupling between the ¹H transmit coil of the CBR and the receive coil of the ¹H system. The experiment C was performed to evaluate the signal loss in electromagnetic coupling between the two ¹H RF coils.
In vivo imaging
A mouse under gas anesthesia (C57BL/6(M), 6 weeks) was imaged using 3D gradient echo sequences. The sequence parameters for ¹H imaging were; matrix size = 128×64×32, repetition time (TR) / echo time (TE) = 100ms/15ms, and scan time = 13min 40sec. Those of ²³Na imaging were; matrix size = 64×32×32, TR/TE = 40ms/2.8ms, number of excitations = 80, compressed sensing with acceleration factor of 3, and scan time = 18min 12sec.

RESULT

The bench tests (Fig. 3) of the RF excitation pulses show that the output signals tracked the input signals with high fidelity and short rise/fall times.
From the sensitivity measurement of the MRI signals (Fig. 4), the total signal loss in the CBR system (B) was 21.1dB (equals to a summation of 9.1dB loss in the crossband convertor and 12.0dB loss in the electromagnetic coupling), compared with the conventional ²³Na system (A). The signal-to-noise ratios (SNRs) were 37.9dB (A), 34.1dB (B), and 35.2dB (C).
Figure 5 shows the results of ¹H/²³Na imaging. The ²³Na MRI’s SNR in the bladder were 21.7dB, comparable to that of the reference saline solution phantom (18.6dB).

DISCUSSSION

The control signals generated in the CBR had high fidelity, allowing the acquisition of ²³Na signals in synchronization with the existing ¹H system. Despite the large signal loss compared with the conventional ²³Na system, the CBR system provided the ¹H/²³Na images with the high SNR comparable to the conventional system. The performance of the system was verified by live mouse imaging.

CONCLUSION

We have developed the prototype of the add-on ²³Na platform using the CBR, that can realize ²³Na-MRI by simply attaching it to an existing ¹H-MRI system.

Acknowledgements

No acknowledgement found.

References

[1] Hu R, Kleimaier D, Malzacher M, Hoesl MAU, Paschke NK, Schad LR. X‐nuclei imaging: current state, technical challenges, and future directions. J Magn Reson Imaging. 2020;51:355‐376.