A PROGRAMMABLE PREEMPHASIS SYSTEM

MRI systems often use magnetic field gradient and shim pulse-shaping n etworks (pre-emphasis) to correct for magnetic field distortions cause d by eddy currents. A pre-emphasis system that uses up to 16 fixed res istor-capacitor (RC) time constants per channel with programmable ampl itude coefficients is described. The magnetic fields induced by the pr e-emphasis RC time constants serve as a set of basis functions for com pensating eddy-current fields induced by the gradient set. The resulta nt time-varying magnetic field gradient accurately reflects the gradie nt specified by the pulse programmer. Reductions in eddy-current field s are demonstrated for actively shielded and unshielded gradient sets.