Eigenmode Analysis of Eddy Currents and Eigenmode Coil Design

Introduction Eddy currents have long been the bane of MRI. Particularly since the mid 1980s when imaging sequences became more rapid [1] and superconducting magnets became routinely used. When gradient coils are switched rapidly, eddy currents are induced in the cryostat vessel and other surrounding conducting structures. Two principal solutions have been adopted into standard practice for dealing with the spatiotemporal eddy current field problem: “pre-emphasis” aims to temporally modify the gradient pulse such that the primary gradient field and it’s secondary field combine to produce the desired pulse shape [2]; and “active shielding” or “active screening” in which an additional coil is interposed between primary gradient coil and cryostat to reduce the fringe field to almost zero [3]. Preemphasis only works well for one point in space since the eddy current field is not identical to the primary field and changes in time. Active shielding is never perfect with some residual field present in the ROI with complex spatiotemporal behaviour. Both approaches are often used in combination in most MRI scanners. In this work we investigated the nature of the eddy currents induced in surfaces and their relation to the currents in the gradient coil by simulated eigenmode analysis [4]. Additionally, we study a simple solution to improving pre-emphasis by reducing the problem to a purely temporal one.