核磁共振高信噪比弱信号检测的理论与实验研究

摘要

基于小型射频线圈的核磁共振检测探头在波谱分析和成像研究中具有广泛的应用,如化学位移波谱分析、磁共振成像和勘探测井等技术领域.但是,由于外加静磁场作用下,自旋体系发生塞曼能级分裂后,高低能态之间的核自旋数量之差很小,普遍存在检测信噪比很低的问题,而且初级磁共振接收信号的质量受所用探头线圈电气参数的影响较大.研究结果表明,在特定的被测样品和接收线圈占空比以及静磁场等条件不变的情况下,检测信噪比与单位电流产生的射频磁场成正比,而与线圈高频电阻的平方根成反比.在永磁0.39Tesla主磁场条件下,研究了趋肤效应影响下小型螺线管线圈几何参数的优化设计方法.理论仿真和实际的测量结果表明,几何参数为线径0.5 mm、直径5.5 mm的10匝微螺线管线圈,在16.9 M Hz谐振频率上,相对信噪比取得一个极大值点,对应的Q值约为199.8,与阻抗分析仪测得结果有较好的吻合,验证了该核磁共振检测线圈设计新方法是合理的.本文提出的基于线圈电磁特性的高信噪比检测探头设计方法,可推广到目前的质子密度成像、岩心弛豫谱分析等应用中.%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 sig-nal-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 frequen-cy.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 M Hz,i.e.the operation con-dition,the quality factor of a modeled micro solenoid coil is scanned in a frequency span.It is in good agreement that the simula-tion 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.