Gradient coil design method for handy MRI

Abstract

Our goal is to develop a handy MRI system which enables us to image an inner body distribution near a body surface region noninvasively. This system consists of several permanent magnets, small gradient coils, and radio-frequency coils to generate a static magnetic field, gradient magnetic field, and RF field distributions, respectively. In order to obtain an appropriate image, the gradient coils in particular should be optimized to produce highly linear gradient field along large region. Although the target field method, which estimates a current distribution on the coil plane based on the desired magnetic field, is one of the well known gradient coil design methods, this method has a oscillating problem of the current distribution. This oscillation complicates the coil pattern and, as a result, makes the miniaturization of a coil difficult. We revealed the cause of this problem is higher order harmonics of the target field, and proposed a novel method to prevent the oscillation by approximating the target field with lower order harmonics. By using this concept, a small gradient coil which produces the target field can be designed. In this study, a planar gradient coil (12.8 × 12.8 cm) was designed with proposed method, and the validity of this method was confirmed through the numerical analysis and the fundamental experiments. As results, the linear gradient field was obtained in the 4 cm square region on an image plane 1.5 cm away from the coil surface. This linear region was larger than that generated from a pair of rectangular coil, which is used as gradient coil in a conventional handy MRI system. © 2009 Springer-Verlag.