Referring now to the drawings, FIG. 1A is a perspective view partially in section illustrating coil apparatus in an NMR imaging system, and FIGS. 1B-1D illustrate field gradients which can be produced in the apparatus of FIG. 1A. This apparatus is discussed by Hinshaw and Lent, An Introduction to NMR Imaging: From the Bloch Equation to the Imaging Equation, Proceedings of the IEEE, Vol. 71, No.