  In this paper commutator expansions for solving the Bloch-Torrey's equation
are derived. An exact solution for free diffusion in a constant magnetic field
gradient is found. Furthermore the moments of the signal in the short gradient
pulse limit for spins confined to a restricted geometry are derived. The
moments are tested numerically against the known solution of diffusing spins
between two perpendicular plates in the short gradient pulse limit.

Chemical Physics