Shen Sheng, Guo Pan, Wu Jiamin, Ding Ye, Chen Fangge, Meng Fanqin, Xu Zheng
State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing 400044, China.
School of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 401331, China.
J Magn Reson. 2019 Oct;307:106565. doi: 10.1016/j.jmr.2019.07.052. Epub 2019 Jul 25.
We report a novel inside-out NMR (nuclear magnetic resonance) probe for the measurement of soil moisture. The probe consists of a dumbbell-shape magnet and an opposed-solenoid RF (radio frequency) coil. Optimization methods for the structure of the magnet and RF coil that maximize the SNR (signal-to-noise ratio) of NMR measurements are also described. The dumbbell-shape magnet consists of three cylindrical magnets in series whose magnetic field was calculated with analytic expression deduced by converting magnet to equivalent magnetization current on its cylindrical surface. Based on the analytic expression, a nonlinear optimization mathematical model was built to determine the optimal structure parameters automatically. The opposed-solenoid is a pair of reverse-connected solenoids and used as RF coil on inside-out NMR probe in this work. Its structure-parameter optimization was carried out based on FEM (finite element method) simulation, and UD (uniform design) was applied to increase the optimization efficiency. A prototype was designed and built consisting of a magnet with length of 100 mm and a diameter of 40 mm. An NMR-based soil moisture measuring experiment was conducted by this prototype, with NMR performed using the CPMG (Carr-Purcell-Meiboom-Gill) pulse sequence for soil sample in different moisture content. The T distribution spectrum reveals that there are two compartments of water in the soil sample: free water and bound water.
我们报道了一种用于测量土壤湿度的新型外-内式核磁共振（NMR）探头。该探头由一个哑铃形磁体和一个对置螺线管射频（RF）线圈组成。还描述了用于优化磁体和RF线圈结构以最大化NMR测量信噪比（SNR）的方法。哑铃形磁体由三个串联的圆柱形磁体组成，其磁场通过将磁体转换为其圆柱表面上的等效磁化电流推导得到的解析表达式来计算。基于该解析表达式，建立了非线性优化数学模型以自动确定最佳结构参数。对置螺线管是一对反向连接的螺线管，在本工作中用作外-内式NMR探头上的RF线圈。其结构参数优化基于有限元方法（FEM）模拟进行，并应用均匀设计（UD）来提高优化效率。设计并制造了一个原型，其磁体长100 mm，直径40 mm。使用该原型进行了基于NMR的土壤湿度测量实验，对不同含水量的土壤样品使用Carr-Purcell-Meiboom-Gill（CPMG）脉冲序列进行NMR测量。T分布谱表明土壤样品中有两个水的隔室：自由水和结合水。
