NMR-SSC Magnetic Field Profiler Applied to Magnetic Field Shimming

Abstract

A new methodology aimed to control the spatial dependence of a magnetic field within a volume of interest is described. The designed hardware uses Nuclear Magnetic Resonance (NMR) to sense the magnetic field. A modified version of the Statistical Signal Characterization (SSC) method was used to analyze the NMR signal, providing a dynamical definition of the characterization parameters. The information is used to control the currents driving a set of coils that introduce corrections in the spatial dependence of the main field generated by an independent unit (magnet or electromagnet). As a practical matter, we deal with the particular case of magnetic field shimming. 1

Using Proton Nuclear Magnetic Resonance (NMR) as a calibrating reference for magnetic field measurement instruments: Sensitive volume and magnetic field homogeneity

“…In this way, the MFP device has four shimming channels, each of them connected to a current source which is managed by the system controller that executes the control algorithm tasks. The compensation coils generate a magnetic field profile that is additive with the NMR Instrument Zeeman field, resulting in a more homogeneous magnetic field as compared to the results obtained in [36].…”

“…Profiler") to obtain a symmetric distribution. The experimental situation has already been described in an earlier publication [36]. We consider a distribution g Bj ( i ) to be symmetric when it can be…”

“…The control of the magnetic field spatial dependence was implemented using an improved version of the hardware presented in reference [36]. A block diagram of this device is shown in Figure 4.…”

“…The system uses an algorithm to optimize the currents driving the set of coils which runs continuously until the DFFT of the NMR signals show-up a symmetric distribution. In [36] a generic coil configuration was used just to test the concept. In this work, the correcting coils were specifically designed after mapping the magnetic field of the NMR instrument.…”

“…Assuming that it takes 1ms among NMR experiments of different current sets, it would take 1193 hours to go over the whole search space, which is clearly unpractical. A successive approximation method was used in reference [36] for the optimization of the current intensities. The step by step correction of each current value was based on the optimization of a discriminant function (f d ), whose terms were obtained from a statistical characterization of the FID signal.…”