低场核磁共振仪器振铃抑制新方法及其电路实现
收稿日期: 2022-05-12
网络出版日期: 2022-08-26
基金资助
中国博士后科学基金资助项目(2021M700172)
A New Method and Circuit of Ringing Suppression for Low-field NMR Instruments
Received date: 2022-05-12
Online published: 2022-08-26
加快天线残余能量释放以减弱天线振铃信号有利于缩短低场核磁共振仪器的回波间隔（TE），从而提高快弛豫组分的测量分辨率和信噪比（SNR）．而天线Q值对能量的发射效率和泄放速度起着相反的作用．为此，我们首先设计了一种新型Q转换电路，在保证发射效率的同时，可以大大缩短能量泄放时间．在此基础上，应用了一种优化的脉冲序列以弥补传统相位交替对脉冲序列（PAPs）不能消除90°脉冲振铃的缺陷，通过相位循环的方法进一步提高了信噪比．最后，在2 MHz岩心分析仪上测试了新型Q转换电路，当天线Q值降为发射期间的约1/5时，天线恢复时间由280.0 μs降为18.2 μs；而且，使用新型Q转换电路和优化的脉冲序列后，TE=60 μs时，可以有效获得快弛豫组分的T2信号．
关键词： 低场核磁共振（low-field NMR）; Q转换; 振铃; 信噪比（SNR）; 脉冲序列
师光辉 , 肖立志 , 廖广志 , 罗嗣慧 , 侯学理 , 卢亚普 . 低场核磁共振仪器振铃抑制新方法及其电路实现[J]. 波谱学杂志, 2023 , 40(1) : 68 -78 . DOI: 10.11938/cjmr20223002
Accelerating the release of the antenna residual energy to weaken the antenna ringing signal is beneficial to shorten the echo time (TE) of low-field nuclear magnetic resonance (NMR) instruments, thus improving the measurement resolution and signal-to-noise ratio (SNR) of fast relaxation components. The antenna Q value has an opposite effect on the energy emission efficiency and discharge speed. For this reason, we first designed a new Q-switch circuit, which can greatly shorten the energy discharge time while ensuring the transmission efficiency. On this basis, an optimized pulse sequence was applied to overcome the defect that traditional phase alternated pair stacking (PAPs) cannot eliminate 90° pulse ringing, and the SNR was further improved by the method of phase cycling. Finally, the new Q-switch circuit was tested on a 2 MHz core analyzer, the Q value of the antenna was reduced to about 1/5 of the transmit period, and the antenna recovery time was reduced from 280.0 μs to 18.2 μs. Moreover, with the new Q-switch circuit and optimized pulse sequence, the T2 signal of the fast relaxation component can be effectively obtained when TE=60 μs.
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