Introduction

The inter-coil coupling in the MRI deteriorates the isolation between pTx channels [4], making it difficult to maintain a constant gain at the RFPA output. In a UHF whole-body MRI system, high power components are usually selected in building the RF transmission chain in order to supply high voltage and current required by body-imaging. Negative impact, coming from the parasitic parameters in these high-power components under UHF, to the circuit’s performance shall be completely considered and attenuated. Especially in the T/R circuits, the forward/reverse biasing (ON/OFF) of the high-voltage PIN diode cannot be reliably controlled because of its relatively higher equivalent series inductance and capacitance comparing to those in 1.5T/3T systems, which therefore deteriorates the system’s isolation degree. This paper presents an 8-channel pTx hardware architecture and proposes a method of effectively suppressing the negative effect caused by the parasitic parameters in the high-voltage PIN diode. It realizes the high isolation among pTx channels and between T/R circuits when the system is providing high power outputs.

Methods

The difficulty in controlling output gain of each RFPA independently is due to the coupling effect among the coils used in pTx system. The output impedance of the RFPA in one channel is highly coupled with the status of the other channels. The issue can be solved by inserting a circulator at the output of each RFPA as illustrated in Figure 1. The use of circulator can adjust the output impedance of each RFPA to 50 ohm, which realizes the high isolation among different RFPA channels.
By controlling the ON/OFF state of the PIN diodes in the T/R switch, the circuit can be operated in either Tx or Rx modes. The schematic and equivalent circuits are shown in Figure 2. The PIN diodes are forward biased when the system is regulated in Tx mode. The PIN diodes are reverse biased when the system is regulated in Rx mode. Due to the relatively high parametric inductance and capacitance, the selected PIN diodes with high-voltage tolerance and high current capability could not be considered as an ideal ON/OFF switch. As shown in Figure 2(b) and 2(c), the diode’s ON status shall be equivalent to an inductor Ld instead of short-circuit while its OFF status a capacitor Cd instead of open-circuit. Furthermore, in the UHF RF circuit, the operating frequency is much higher than that in the conventional 1.5T and 3T systems, the isolation of the T/R circuits is more susceptible to parasitic parameters, causing more noises transmitted from Tx circuit to Rx circuit through T/R switch, it may deteriorate SNR of images or even damage the Rx module.
The circuit topology shown in Figure 3 has been proposed to improve the isolation degree in UHF MRI T/R circuits. A capacitor Cs has been series connected to the PIN diode, Cs resonates with Ld, the λ/4 transmission line can be converted to equivalent Cx-Lx-Cx circuit. The capacitance of Cs in series with Cd is used as a partial capacitance of Cx. Therefore, the capacitance of Cy can be calculated by using Eq. (1).
Cy = Cx – Cs* Cd /( Cs + Cd) (1)
The ideal resonance and matching can be realized in diode’s ON/OFF status, leading to high isolation between T/R circuits.

Results

The pTx system is designed and tested on a whole body 5T scanner (United Imaging Healthcare, Shanghai, China). The RFPA has 8 independent outputs with 8kW (69dBm) peak power output per channel.
The circuit topology to attenuate the PIN diode’s parasitic parameters has been validated functional. Figure 4 shows the T/R isolation when the system is regulated in Tx mode. The Rx module is easily damaged by high Tx power with low T/R isolation. By using the new circuit topology, the isolation degree of the system has been improved from 21.85dB to 54.86dB, no evidence of module damage has been detected during or after the high power testing, which has proven the validation of the proposed circuit for high T/R isolation and protection.
Figure 5 shows the T/R isolation when the system is in Rx mode. The T/R isolation degree has been improved from 13.1dB to 40.2dB, RF interference that worsens SNR can be blocked from entering the Rx module.

Conclusion

The hardware architecture of 8-channel pTx system is presented, circulator is inserted at the output of each RFPA, making the RFPA output impedance constant at 50 Ohms, the RF output of each channel can be controlled independently.
Topology for improving T/R isolation is proposed and tested. The advantage over the conventional topology is verified.

Acknowledgements

No acknowledgement found.

References

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