Transmission line coupling and the resulting distortion of the RF magnetic field in nuclear magnetic resonance (NMR) imaging.

摘要

The signal to noise ratio and magnetic field homogeneity are the most important parameters for the probe circuit design in Nuclear Magnetic Resonance (NMR) applications. The efficiency of coupling a transmission line to the probe is directly related to the signal to noise ratio. However, transmission line coupling techniques are not yet completely understood by many NMR probe circuit designers. The magnetic field distribution is determined by a probe's geometry, and is affected by the transmission line coupling system. Unfortunately, the distortion due to the coupling system is usually ignored. This dissertation analyzes capacitive and inductive coupling circuits first. The results indicate that even for the simplest capacitive coupling circuits, the expressions of resonant frequency and input impedance are complicated and are interrelated. In reality, many iterations are necessary in order to obtain the proper resonant frequency tuning simultaneously with input impedance matching. A new method for decoupling tuning and matching was developed and proven both theoretically and experimentally. In principle, this method allows one to reach exact tuning and matching a probe without any iteration. Inductive coupling circuits were analyzed using equivalent circuits. Suggested procedures for the tuning and matching inductively coupled NMR probes have been presented. A theoretical model of magnetic field distribution for an inductively coupled asymmetric saddle coil was derived from the Biot-Savart law. Software has been developed which calculates the mutual inductance and field distortion from the model developed. Coupling coil position and size were optimized by this software package. Further confirmation of the results has been accomplished using specially designed automatic field mapping device. This device consisted of a Network Analyzer (used as a transmitter and a receiver), an automatic positioner and a PC (used as a controller). The measured data shows that this device can be successfully used for field mapping on different NMR probes. The field profiles in the experiments show fair agreement with the profiles predicted by the theoretical model.