### 1544

A Novel and Efficient No-tuning Inductive-coupling Q-damping Circuit for a Low-field Portable MRI System
Zhi Hua Ren1, Guang Yang2, Pengde Wu3, Sergei Obruchkov2, Robin Dykstra2, and Shao Ying Huang1

1Singapore University of Technology and Design, Singapore, Singapore, 2Victoria University of Wellington, Wellington, New Zealand, 3Sichuan University, Chengdu, China

### Synopsis

In a low-field portable MRI system, e.g. a permanent-magnet-based system, $B_0$ is less homogeneous thus $T_2^*$ is short. However, the ring-down of the transmit RF energy is slow, which dramatically decreases the strength of acquired signals or limits the minimum echo time for a CPMG type experiment. In this abstract, we present the design of a no-tuning inductive-coupling Q-damping circuit for transmit-coils in a low-field MRI/NMR system. The proposed circuit can effectively and quickly damp RF energy yet simple to be implemented and constructed. The effectiveness is successfully demonstrated in simulations and experimentally. In the experiment, it shows that the ring-down time is reduced by about 45%.

### INTRODUCTION

In a low-field MRI system, e.g. a Halbach-array-based portable MRI scanner, a $B_0$ field with a quadrupole pattern is used for encoding [1-3]. $B_0$ fields are less homogeneous thus $T_2^*$ is short. On the other hand, a solenoid is used as a transmit-coil [1-3]. The ring-down time of a solenoid is long due to a high inductance of a solenoid, L, and a low working frequency (< 3 MHz) (the decay time constant, $\tau= (2Q)/ \omega_0$, and Q is the quality factor, $Q = \omega_0 (L/r)$, where r is the resistance, $\omega_0$ is the working frequency of the coil). This dramatically decreases the strength of acquired signals. To deal with this issue, an effective damping circuit is needed to accelerate the damping process.

Available solutions to this problem include those based on a resonant circuit that need to be tuned, e.g. those using quarter wave network with a diode expander [4], and those using coupling transformer [5]. There are tuning-free solutions as well, such as those using a short phase-inverted pulse following the main RF pulse [6], and that using a MOSFET switch with a resistive load [7]. Although tuning is not needed, the former method is complex to implement, and the latter only works up to 2MHz.

### METHOD

For a low-field system based on permanent magnet array, the working frequency is more than 2MHz. Here, we proposed a tuning-free inductive-coupling Q-damping circuit working at 2.83MHz. It provides efficient Q-damping and is easy to implement. It is possible to be applied at a frequency up to 10MHz. A photo and the schematic diagram of the proposed circuit is shown in Fig.1(a) and (b), respectively. It consists of a damping-coil (a short solenoid) in series with a MOSFETs switch. As shown in Fig.1(a), the damping-coil has a diameter slightly larger than the solenoid, and is placed in the middle of the solenoid to achieve a maximum inductive coupling. Due to a low working frequency, two back-to-back connected MOSFET switches are used as the switch rather than PIN-Diodes which are difficult to drive, and switch slowly at low frequencies (<10 MHz). The proposed circuit is controlled directly by the Kea spectrometer [8] using a TTL digital logic line. When the transmit-coil is ON, the switch is OFF, and the damping-coil are open with negligible effects on $B_1^+$ fields. When the RF pulse is off, the switch is ON, and the damping-coil is inductively coupled to the transmit-coil through mutual inductance. The Q factor of the transmit-coil is effectively damped. Moreover, the parasitic resistance is loaded to the transmit-coil, accelerating the energy dissipation.

### RESULTS

The effectiveness of the proposed Q-damping circuit was tested by both simulations and experiments. The response of a single pulse in a solenoid is simulated in ADS [9]. Fig.2 shows the simulation result. As shown, with the proposed circuit, the ring-down time is successfully reduced by about 60% compared to that without the circuit. The proposed circuit was tested in a Halbach-array-based portable MRI system working at 2.83MHz [2-3]. Fig.3 shows the damping-pulse (green) and the signal at the receive-coil with and without the proposed circuit (yellow). As shown, the damping time of the received signal with the proposed circuit is reduced from over 20us to about 11us, about 45% compared to that without the damping circuit. Both the simulation and experimental measurement shows the effectiveness in terms of reducing the ring-down time of a transmit solenoid by using the proposed circuit.

### DISCUSSION & CONCLUSION

We successfully show the effectiveness of the propose Q-damping circuit in terms of reducing the ring-down time for a low-field MRI/NMR system. Both simulations and experiments were conducted. Experimentally, the ring-down time is effectively reduced by about 45%. The proposed method can be applied to a higher frequency. Compared to other damping techniques [4,5,6], the proposed tuning-free method is much easier to implement. Although the reduction in ring-down time using the proposed method (about twofold reduction) is lower compared to that using techniques in [4,5] (up to sevenfold reduction), the performance of the proposed circuit can further be improved by optimizing the inductance of the damping-coil. This will be investigated next.

### Acknowledgements

Zhi Hua Ren would like to thank the support of Singapore University of Technology and Design President Fellowship.

### References

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