The Ancient and Honourable Art of Shimming

The fundamental physics underlying the art of shimming a high-resolution NMR superconducting magnet is described, and the rationale of the design of shim coils, together with their various appelations, is discussed. The effects of the various shims on a free induction decay and the ensuing spectrum are illustrated, and the use of a spinning sample is advocated for rapidly diagnosing and eliminating field inhomogeneity. The problems caused by impure shims and interactions among orders are highlighted, and the reasons for the interactions are given, together with strategies to help combat their time-consuming and destructive effects.