  A perturbation method for computing quick estimates of the echo decay in
pulsed spin echo gradient NMR diffusion experiments in the short gradient pulse
limit is presented. The perturbation basis involves (relatively few) dipole
distributions on the boundaries generating a small perturbation matrix in
O(s^2) time, where s denotes the number of boundary elements. Several
approximate eigenvalues and eigenfunctions to the diffusion operator are
retrieved. The method is applied to 1-D and 2-D systems with Neumann boundary
conditions.
