  An approximative method for solving the Bloch-Torrey equation in general
porous media is presented. The method expand the boundaries defining the porous
media using electrostatic charges. As a result the eigenvalue problem of the
Laplace operator in a confined geometry can approximately solved. Importantly
the approximative solution is orthogonal in the low-frequent region of Fourier
space. This gives a natural approach for studying spin magnetization in
presence of magnetic fields. The error in the approximation scales with N^{-2}
times the magnitude of each eigenvalue, where N is the size of the expansion
matrix. From a computational point of view, the calculations scale
quadratically with the number of basis functions using fast multipole methods.
