\bibitem{Hurlimann95}   M.D.~Hurlimann, K.G.~Helmer, T.M.~de Swiet, P.N.~Sen, C.H.~Sotak,
                        {\it Spin Echoes in a Constant Gradient and in the Presence of Simple Restriction},
                        JMR A {\bf 113}, 260, 1995.