Active magnetic screening of coils for static and time-dependent magnetic field generation in NMR imaging

A new method is introduced for screening or reducing extraneous magnetic fields outside coil structures. The method utilises a set of current-carrying conductors or a discrete wire array to simulate the induced surface currents which occur in superconducting magnetic screens or which occur in high conductivity thick metal screens when placed around coils producing time-dependent fields or field gradients. The active screens have the advantage that they work at any frequency including DC. Field gradient coil structures utilising the principle can be compact and of low inductance. Strong external fields can be made arbitrarily low thus making it feasible to generate large rapidly switched gradients within and in close proximity to a superconductive magnet. Such gradients can be especially useful in the implementation of high speed NMR imaging methods.