3-D gradient coil design--initial theoretical framework.

An analytic inverse method is presented for the theoretical design of 3-D transverse gradient coils. Existing gradient coil design methods require the basic geometry of the coil to be predetermined before optimization. Typically, coil windings are constrained to lie on cylindrical, planar, spherical, or conical surfaces. In this paper, a fully 3-D region in the solution space is explored and the precise geometry of the gradient coils is obtained as part of the optimization process. Primary interest lies in minimizing the field error between induced and target gradient fields within a spherical target region. This is achieved using regularization, in which the field error is minimized along with the total coil power, to obtain a 3-D current density solution within the coil volume. A novel priority streamline technique is used to create 3-D coil windings that approximate this current density, and a secondary optimization is performed to obtain appropriate coil currents. The 3-D coil windings display an interesting general geometric form involving sets of closed loops plus spiral-type coils, and a number of examples are presented and discussed. The corresponding induced magnetic field is found to be highly linear within the region of interest, and a shielding constraint may be implemented to minimize the field outside the coil volume.