Introduction

Parallel transmission (pTx) was developed for UHF MRI to improve the B1 homogeneity. To facilitate pTx, the number of available Tx channels are usually 8~16, the peak power per channel was not more than 2kW with total available RF power not more than 32kW ^2-4, some studies on body imaging reported limitation on available RF power^2,5. MRI scanners are used with different RF transmit coil array configurations, there were also studies on the impact of number of pTx channels or different shimming modes⁶. These applications place different demands on the number of channels or peak RF power per channel.
This work presents a RFPA design method that supports pTx with highest peak power capability (64kW) known for UHF MRI. High utilization of RF power can be obtained when number of Tx channels are configured into 8/4/2. Body images are acquired to demonstrate the functionality of this new technology.

Method

In this study, the RFPA consists of power supply unit, RF power units(RFPU), control unit, Tx selection unit and power combiner unit¹. Figure 1(A) shows the picture of the RFPA.
The power supply unit converts the three-phase AC power into the DC power required by control unit and RFPUs. The control unit configures and monitors the operation status of RFPUs and power supply unit. Each RFPU contains 2 independent amplifier modules, each with power capability of 8kW. The final amplification stage of each amplifier module contains 4 Metal Oxide Semiconductor Field-Effect Transistors (MOSFETs), whose power is combined by a 4-to-1 Wilkinson combiner. The matching circuit of each MOSFET is shown in Figure 1(B).
The RFPU output power flow is configured by Tx selection unit, which can optionally connect the RFPU outputs to the power combiner unit or directly to the RFPA output terminals. Tx selection unit contains 8 identical RF switch modules, The schematic of RF switch module are shown in Figure 1(C).
The power combiner unit consists of six 2-to-1 Wilkinson combiner modules, the connection between them are variable. The introduction of the Tx selection unit and power combiner unit makes the number of RFPA’s output channels configurable between 8/4/2.
MOSFETs and the corresponding impedance matching circuits are critical to RFPA power capability. Figure 2 shows an equivalent circuit model of RF MOSFET and the load impedance.
When designing impedance matching circuit, the theoretical load impedance seen from the transistor current generator can be calculated using equations below.
$$R_{\text{Load}} = \frac{{(\frac{{V_{\text{DC}} - V}_{K}}{\sqrt{2}})}^{2}}{P_{\text{sat}}}$$
$$X_{\text{Load}} = \frac{1}{\omega \bullet C_{\text{oss}}}$$
Where V_dc is the DC drain-source voltage and V_k is turn-on voltage, P_sat is the MOFET’s saturation output power.
The load impedance seem from transistor’s current generator can’t be measured directly. The equations can serve as a good start point for load-pull and optimization procedure if power capability or efficiency can’t meet the requirement. In this work, the peak power for each MOSFET (MRFX1K80H, NXP, The Netherlands) is tuned to be 2.5kW with output impedance of 1.4-j0.3Ohm and drain voltage of 75Vdc, the actual peak power is higher than rated power(1800W) declared in the device manual.

Results

A. Bench test
The RFPA peak power capability is tested when configured to 8/4/2 channels modes, the test results are shown in table 1.
A digital pre-distortion(DPD) algorithm is developed to compensate the nonlinear characteristic of the RFPA. Figure 3 shows the RFPA linearity when RFPA is configured into different modes. Though the gain fluctuation is slightly influenced by the operation modes, the gain fluctuation per channel is less than 0.3dB respectively, high linearity is accomplished over wide dynamic range.
B. Scanner test
Some studies on body imaging reported limitation on peak B1 field, higher RF power is needed even for local transmission. In this work, Images of the abdomen are acquired on a 5T whole-body scanner (uMR Jupiter, United Imaging Healthcare, Shanghai, China) with bore sized 8-channel(8 ch) volume transmit coil and 24ch local receive coil.
Body images are acquired using FSE sequence. Figure 4 shows images of abdomen when RFPA peak power is set to 64kW(8kW*8ch), 32kW(4kW*8ch) and 16kW(2kW*8ch).

Conclusion

RFPA with highest power capability known for UHF MRI is designed and demonstrated in a 5T whole-body scanner. The number of RFPA output channels can be configured between 8/4/2. High linearity over a wide dynamic range is demonstrated when RFPA is configured into different modes. Body images acquired in the 5T scanner demonstrate the feasibility of the new method proposed in this study, this new method can also be applied in other UHF systems such as 7T scanner, the addition of more channels can be achieved straightforwardly.

Acknowledgements

This work is supported by National Key Research and Development Program of China, 2021YFE0204400; the Strategic Priority Research Program of Chinese Academy of Sciences, XDB25000000; National Natural Science Foundation of China, U22A20344; Youth Innovation Promotion Association of CAS No. Y2021098; Key Laboratory Project of Guangdong Province, 2020B1212060051; Shenzhen city grant, RCYX20200714114735123.