Image‐based method to measure and characterize shim‐induced eddy current fields

Abstract

ABSTRACTDynamic magnetic field shimming is gaining interest for field sensitive MRI acquisitions. Using slice based or real‐time shim updating, significant improvements in static field (B0) uniformity can be obtained. While the ability to rapidly switch shim fields can improve overall B0 homogeneity, it induces eddy current fields that must be characterized and compensated for. Methods used to achieve this have thus far been based on linear projection spin echo sequences or field probe assemblies. Here, a novel image‐based method is presented to measure and characterize eddy current fields without the need for field probes or projection based measurements. This technique can be extended to characterize very high order spherical harmonic fields, making it a useful tool to calibrate next‐generation shim systems implementing dynamic field steering with greater than third order shim terms. Results are used to calibrate a Dynamic Shim Updating unit for pre‐emphasis and eddy current compensation. Three‐dimensional datasets are acquired at multiple MR facilities containing complete spatiotemporal field information to compensate eddy current field self‐ and cross‐terms for up to third order. Furthermore, simulation studies are performed to investigate the effect of scan resolution and phantom size with respect to accurate eddy current field characterization. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part A 42A: 245–260, 2013.