  For Markovian dynamics of field fluctuations we present here an extended
strong collision approximation, thereby putting our previous strong collision
approach (Phys. Rev. Lett. 83 (1999) 4215) into a systematic framework. Our new
approach provides expressions for the free induction and spin echo
magnetization decays that may be solved analytically or at least numerically.
It is tested for the generic cases of dephasing due to an Anderson-Weiss
process and due to restricted diffusion in a linear field gradient.
