Passive screening of switched magnetic field gradients

Abstract

In NMR imaging the rapid switching of magnetic field gradients can induce currents in surrounding conductors, thus creating undesirable reflected fields which are spatially and temporally inhomogeneous. For certain types of gradient switching it is possible to screen the magnetic fields completely, using a thick cylindrical conductor, within which the gradient uniformity may be re-established using simple changes in coil separations. Details are presented for the analytic treatment of this problem, with results for the cases of the Maxwell pair, a modified Maxwell pair and saddle coils. These results are also applicable when active screening is used.