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Simultaneous transmit/receive for Bloch-Siegert encoding: a feasibility study
Baosong Wu1, Sajad Hosseinnezhadian2, Yonghyun Ha2, Kartiga Selvaganesan2, Charles Rogers III2, Kasey Hancock2, Gigi Galiana2, and R. Todd Constable2
1Department of Radiology and Biomedical Imaging, Yale School of Medicine, NEW HAVEN, CT, United States, 2Department of Radiology and Biomedical Imaging, Yale School of Medicine, New Haven, CT, United States

Synopsis

This work presents the feasibility of Bloch-Siegert (BS) frequency encoding for magnetic resonance (MR) applications requiring a simultaneous transmit and receive system. RF frequency encoding technique is referred to receiving MR signal while existing one offset-field radiation, which can be considered as a radio frequency interference(RFI). A prototype was built demonstrating first MR experiment and preliminary data using this method. Our system is currently a research technology with the goal of generating a low-field and point-of-care medical imaging system using novel RF encoding.

INTRODUCTION

Recently, our group1-3 has shown the progress of developing BS encoding using an array of coils that would simultaneously irradiate off-resonance at 870 kHz and detect signal on-resonance at 1MHz. Even the array coils3 maintained high isolation with inductive decoupling, strong RFI still was marked when a large RF power was applied in order to obtain enough frequency encoding. MR signal mixed with induced irradiation voltage would feed through the cross-diode of passive duplexer4 into ground before it passes the filter3 in the receive chain. In addition, RF harmonics still bring the issue of pre-amplifier saturation with using a low-pass filter in the transmit chain.

METHODS

The RF system was built as shown in Figure 1(a). The schematic diagram in Figure 1(b) includes two coils and a phantom, which implies the challenge in the hardware design: one coil was used to pick up MR signal from phantom while the other was transmitting offset RF pulse. The duplexer and RF filter were furtherly investigated in order to migrate the challenge. The active duplexer5 was used to take the place of the switch in Figure 2(a) to avoid MR signal loss during such experiments. PIN diodes are difficult to drive at low frequency, even if they have very high figure of merit. We therefore focused on semiconductor switches, as shown in Figure 2(b). The key figure of merit for such switch is 1/(Ron*Coff), where Ron is the resistance in the on-state and Coff is the capacitance in the off-state. The value of Ron should be minimum to prevent further noise contribution, while that of Coff should be minimized to keep the isolation between the source and drain in the off-state. In this study, Ron and Coff in our selected (GS66508T) were 0.05Ohm and 65pF, respectively. In the design of drive circuit, DC noise should be carefully considered. We also employed a band-pass filter in Figure 3 replacing the previous low-pass filter2 that removes the issue of receiver saturation enabling simultaneous transmit/receive.

RESULTS

First, simultaneous transmit/receive experiment (Figure (1b)) was performed using the passive duplexer shown in Figure 2(a). Signal amplitude decreases as the offset field irradiation improves. This demonstrates that signal losses in critical conduction or ON state of the cross-diode when the induced voltage increases on the receive coil. The similar experiments were performed using the sequence shown in Figure 5(a). For comparison, each acquisition has 10 echoes in Figure 5(b). The front five acquisitions accompany offset irradiation. The peak-to-peak voltage (Vpp) applied on the coil are 0V, 30V and 77V, respectively, which are corresponding to three echo trains. Experimental results demonstrate that the SNR is not affected by increasing RFI. This shows the feasibility of the system in separating the NMR signal in nanovolt level from the strong coupling power in the level of a few hundred millivolts. The filter also successfully removes RF harmonics.

DISCUSSION and CONCLUSION

The feasibility of simultaneous transmit/receive was studied at 1MHz. This study demonstrates survival of the NMR signal in strong RFI. The system employed a GaNFET based duplexer with a high figure of merit to avoid signal loss. The band-pass filter in transmit chain helped to better remove harmonics and resulted in a more efficient decoupling. The prospect of this study is that frequency encoding with Bloch-Siegert Shift can be achieved for low field imaging.

Acknowledgements

No acknowledgement found.

References

[1] Wu B., et al., RF encoding using a simultaneous transmit and receive system. In: ENC.; 2019.

[2] Constable R.T., et al. Design of a novel class of open MRI devices with nonuniform Bo, field cycling, and RF spatial encoding. In: ISMRM.;2019. p. 1546.

[3] Wu B., et al. Strategies to reduce coupling for continuous acquisition of a Bloch-Siegert spatial encoding in low-field MRI device. In: ISMRM.;2020. p. 4044.

[4] Fukushima E. and Roeder S. B. W., Experimental pulse NMR: a nuts and bolts approach. pp.400-407,1981.

[5] Zhen, J. Z., et al. A resistive Q-switch for low-field NMR systems. Journal of Magnetic Resonance, 287:33-40, 2018.

Figures

Figure 1. (a) The open MRI system. (b) The diagram of simultaneous transmit and receive. The phantom is above the coils. The coil on the right side is for the use of picking up NMR signal at 1MHz, and the coil on the right side is irradiating offset field.

Figure 2. Schematic diagram of two duplexers. (a) π-typed duplexer and (b) GaNFET based duplexer. (a) is a two-port network. One is connected to transmit chain and the other is to receive chain. (b) consists of two FET switches connected back-to-back that isolate the receiver when the transmitter is ON. Gate port is controlled by a driver circuit to switch the duplexer ON/OFF. Two drain ports are connected to transmit and receive chains, respectively.

Figure 3. The schematic diagram of a butterworth typed filter (bandwidth 200kHz and centre frequency at 870kHz), which is used in transmit chain to remove harmonics.

Figure 4. Echo trains were acquired using the passive duplexer when different amplitudes of offset field were applied from 0 to 10V. The offset field lasted through the whole echo train. 12 echo peaks are shown here.

Figure 5. Experiments for simultaneous transmit and receive. (a) The diagram of a multiple spin echo sequence with continuous offset irradiation in the front five acquisition window. (b) Experimental data acquired with the sequence above with echo spacing (TE) 1ms. It shows three echo trains under different offset-field irradiations.

Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)
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