An approximate method for solving the Bloch-Torrey equation by surface integrals is developed. The method presents a fast means for calculating pulsed-gradient spin-echo nuclear magnetic resonance signals in porous systems, and it is especially efficient when the surface-to-volume ratio is low. The number of operations for retrieving echo decays scale as O(k2), where k is the number of surface elements. The theory is numerically validated for pulsed-gradient spin-echo sequences on two-dimensional and three-dimensional examples.
- Laplace operator