Introduction

Human multinuclear studies at high field have the potential to become powerful diagnostic tools. Dual-tuned multichannel excitation and reception have been proposed to increased SNR and B₁⁺ homogeneity in multinuclear imaging^1,2. To simplify the implementation of multinuclear multichannel hardware, we present an optically controlled dual-tuned on-coil current-mode amplifier for ¹H and X-nuclei excitation. On-coil amplification has been proposed for the practical implementation of ¹H parallel transmit (pTx) systems^3-5. We expect this new prototype will allow a more flexible multinuclear multichannel Tx hardware by eliminating the need of multiple cable traps, matching networks and coaxial connections needed at each resonance frequency.

Methods

The dual-tuned amplifier received the optical carrier signals through a broadband optical RX interface designed previously to control on-coil amplifiers for ¹H excitation^4,5, the input circuitry for the preamplification and Current-Mode Class-D (CMCD) amplification stage⁶ were designed to maximize the gate-source voltage to fully switched ON the FETs in both stages at ¹H and X-nuclei frequencies through dual-resonance LC networks (Fig. 1). For this implementation the networks were tuned for ¹H (297.2 MHz) and ³¹P (120.3 MHz) excitation. Initial network component values were selected based on SPICE simulations (Altium Designer). A path for harmonics currents, generated from the switch-mode amplification, was provided by an output filter. The dual-resonance amplifier was connected directly (not 50 Ω impedance matching) to a dual-tuned loop. Proton and phosphorus carrier signals were connected to the input ports of a hybrid combiner the output of which was connected to the RF signal input of the optical interface box ^4,5. Carrier signals were transmitted optically (not simultaneously) through a single fiber to the dual-tuned amplifier. Coil current amplitude and harmonic content was measured for both excitation frequencies with a calibrated probe coupled to the Tx coil and connected to a high-speed oscilloscope (2.5 GHz Infinium, Keysight Technologies). The performance of the dual-tuned on-coil amplifier was compared to a previous ¹H-tuned on-coil prototype. A preliminary multinuclear MR experiment was performed in a 7 T MRI scanner (Siemens, Erlangen). Carrier signals from the scanner control were connected to the in-house optically interface located in the scanner electronics room. Multinuclear excitation was performed with the new amplifier and Tx coil loaded with a ³¹P rich solution (50 mM potassium phosphate). Proton and phosphorous signals were detected with surface loops tuned to the corresponding frequencies (120.3 MHz and 297.2 MHz). The Rx coils were connected to a ³¹P /¹H interface box (Quality Electrodynamics, Ohio) plugged to the patient table in the 7 T scanner. A ¹H localization image was acquired (5 ms TE, 20 ms TR, 192 x 192 matrix size and 8 mm slice thickness) after which spectroscopy data was acquired with a Chemical Shift Imaging (CSI) sequence (0.5 us RF hard pulse, 3 s TR, 16 x 16 matrix size, 200 mm x 200 mm FOV, 10 average).

Results

Figure 2 shows simulated frequency response of the dual-resonance LC network (CMCD gate circuit) by sweeping a single component value at a time. Figure 3 shows the tuning of the dual-tuned, not 50 Ω matched, loop and corresponding load impedance for both frequencies. Coil current vs bias voltage of the amplifier power stage (V_DD) for both nuclei is shown in Figure 4. In this setup, maximum power delivered to the coil was 46 W and 81 W for ³¹P and ¹H respectively. Total harmonic distortion (THD) values, estimated from the FFT of the coil current at both frequencies, were THD ~ 1.2 % and THD~4.8 % for the ¹H and ³¹P current respectively. No degradation in performance was observed for ¹H excitation as shown from the power (delivered to the coil) measurement performed with the dual-tuned amplifier and a single-tuned ¹H prototype (Fig. 4). Figure 5 shows the result of the multi-resonant MRI experiment, ³¹P peak (left), CSI image (center) and metabolite image (right) were successfully obtained with this preliminary setup.

Discussion

The dual-tuned on-coil Tx setup allows to have multinuclear excitation without the need of additional matching networks, cable traps, and coaxial connections. Therefore, this approach can simplify the implementation of a multinuclear multichannel setup compared to those built based on 50 Ω broadband voltage amplifiers typically found in MRI systems with multinuclear capability. We presented preliminary data for ³¹P and ¹H excitation, however the dual-tuned amplifier can be tuned to excite other X-nuclei (²³Na, ¹³C, ¹⁹F etc.). In addition, the design of the gate circuit could be extended to allow more than 2 nuclei excitation. In this work we performed ³¹P and ¹H excitation through a single dual-tuned Tx coil, however the design can allow the combination of the amplifier with nested coils as presented elsewhere to improve efficiency². Finally, in a ¹H pTx system built with this technology each channel can be simply “transformed” by the control to excite a different nucleus as necessary for different applications. Therefore, we consider this technology will allow the implementation of a more versatile Tx hardware to be able to excite both ¹H and X-nuclei.

References

1- Ianniello C, et al. Magn Reson Med. 2019 82(4):1566-1575. 2- Brown R, et al. Neuroimage. 2016 124(Pt A):602-611. 3- Gudino N, et al. Magn Reson Med. 2013 70:276-89. 4- Gudino N, et al. Magn Reson Med. 2016 76:340-9. 5- Gudino N, et al. Magn Reson Med. 2018 79(5):2833-2841. 6- Kobayashi H, et al. IEEE Trans Microwave Theory Tech 2001;49:2480–2485.