1. 中国科学院电工研究所, 北京 100190
2. 中国科学院大学电子电气与通信工程学院, 北京 100049

Research on Nuclear Magnetic Resonance High-Quality Detection
LI Xiao-nan1, LIU Guo-qiang1,2, HU Li-li1
1. Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China
2. School of Electronic, Electrical and Communication Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
Abstract

There are many applications which are based on the nuclear magnetic resonance with microcoils detection, such as MR imaging, spectroscopy and well logging. But due to the low Zeeman splitting difference in populations, usually the specific signal-to-noise ratio associated with the micro solenoid coils is very low. The geometry of the detection coil has numerous influences on the detected MR signals. Under a given duty ratio of the sample volume to the coil size, as well as a certain static magnetic field and so on, it is concluded that the signal-to-noise ratio is in proportion to the radio-frequency magnetic field by unit current flowing through the RF coil, and is inversely proportional to the squared root of the resistance of the coil under certain frequency. In the paper, for a 0.39 Tesla main field the relationship between the optimization on coil geometry parameters and the skin-depth effect of bulk conductor is addressed. Compared with sophisticated processes in MEMS technologies, the micro solenoid coils was winded with lacquered wires on polytetrafluoroethylene holding. After electronic measurement, the signal-to-noise ratio on a certain frequency for the coils made is benefit from the proper number of turns of coil. On the other hand, this fabrication is quite simplely relative to a couple of masks, lithography and electroplating. In the content of 16.9 MHz, i.e. the operation condition, the quality factor of a modeled micro solenoid coil is scanned in a frequency span. It is in good agreement that the simulation predicted a maximum SNR of 199.8 when the number of turns is 11, under the condition of wire and coil diameter 0.5 and 5.5 mm, respectively. So the investigated principle is well verified from the practical point of view. In the future, this method can be further used in the proton density MR imaging, relaxation spectrum analysis on rock fluidics.

Keyword: Nuclear magnetic resonance; Radio frequency field; Signal-to-noise ratio; Quality factor; Microcoil

1 理论方法

$s(t)=-∫Vs∂∂t[B→u(r→)M→(t, r→)]dVs(1)$

 Figure Option 图1 静磁场下核磁共振检测线圈示意图Fig.1 In static field the induction between the RF microcoil and the sample element

$SNR∝BuRc(2)$

 Figure Option 图2 趋肤效应下导体的有效载流横截面(a)和螺线管几何计算模型(b)Fig.2 The effective area of the conductor for a micro solenoid (a) and (b)

$SNR=C0N(2R)2+L21lC0=μ0ρSeff(3)$

$Seff(r, f)=π[r2-(r-reff)2](4)$

$dSNRdN∝12N1/2[R2+4(Nr)2]-1/2+N1/2-12[R2+4(Nr)2]-328(Nr)r(5)$

$N-12[R2+4(Nr)2]-12=N12[R2+4(Nr)2]-328(Nr)r(6)$

$N'=R/(2r)(7)$

2 结果与讨论

B0=0.39T的磁场条件下, 质子的共振频率约为16.9 MHz, 以半径5 mm、 导线直径0.5 mm的微螺线管线圈为原型, 进行提高信噪比的参数优化设计, 通过改变线圈匝数N来寻找信噪比最大的情形。 由于漆包线的直径应大于或等于两倍的趋肤深度, 参考上述计算结果, 选择的导线合适。 利用Matlab工程计算软件, 根据式(2), 仿真得到的结果如图3所示。

 Figure Option 图3 微螺线管线圈变匝数下检测信噪比仿真Fig.3 The signal-to-noise ratio for a micro solenoid when scanning the number of turns

 Figure Option 图4 微螺线管线圈阻抗多参数测量Fig.4 The RF impedance measurement for Rc, L and Q-factor of the microcoil

4 结 论

The authors have declared that no competing interests exist.

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