Ultrashort shielded gradient coil design with 3D geometry

Abstract

Ultrashort gradient coils for ultrashort cylindrical MRI systems require new design methods. The challenge is to reduce system length while maintaining performance, e.g., to maintain acceptable linearity and uniformity over a large field of view (FoV). Trading MR system performance to achieve short length is in itself not a challenge. As a system is made shorter, the increasing leakage of the magnetic flux outside the gradient coil enlarges the eddy current effect. We present in this article a new approach to design of short cylindrical gradient coils with 3D current geometry. In contrast to past methods, in this approach the lengths of the gradient primary and shield are arbitrary and genuinely finite throughout the design process, and therefore the process does not require truncation of the current density. We show how to incorporate the residual eddy current effect into the design and control it at any reasonably low level. For a transverse gradient coil we show that the stored magnetic energy is significantly reduced by allowing the current to flow off the primary coil surface toward the shield coil surface along a conical surface that connects the two. For an axial gradient coil we show that allowing the current density to be nonzero at the edges of the primary and shield coils reduces the stored energy as well. We explore the strength of this approach in the example of an ultrashort gradient coil design intended for use in an ultrashort yet whole-body capable notional 1.2-m-long main magnet. © 2005 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 26B: 1–15, 2005