REAL-TIME MULTI-AXIS GRADIENT DISTORTION CORRECTION USING AN INTERACTIVE SHIM SET

摘要

A magnetic resonance apparatus includes a main magnet (12) which generates a main magnetic field through and surrounding an examination region (14). The main magnetic field contains inhomogeneities which affect image quality. A gradient coil assembly (22) generates gradient magnetic fields across the examination region, which contain higher order harmonics causing inhomogeneities in the gradient magnetic field. A multi-axis shim set (23) is selectively excited in order to cancel both the main magnetic and gradient magnetic field inhomogeneities. More particularly, DC currents are applied by a shim coil power supply (25) to cancel the main magnetic field inhomogeneities. AC current pulses are superimposed on the DC currents by the shim coil power supply (25) in order to correct the gradient magnetic field inhomogeneities. The existence of correctable higher order gradient magnetic field harmonics provides for last switching, high duty cycles, and high peak gradients, without adding distortions to the resulting magnetic resonance images.