\bibitem{Fichele04}    S.~Fichele, M.N.J.~Paley, N.~Woodhouse, P.D.~Griffiths, E.J.R.~van Beek, J.M.~Wild,
                       {\it Investigating 3He Diffusion NMR in the Lungs using Finite Difference Simulations 
                       and in vivo PGSE Experiments},
                       JMR {\bf 167}, 1-11, 2004.