The NMR Pulsed Field Gradient Spin-Echo method (PFG-SE) may be conveniently applied to the study of water self-diffusion in lyotropic liquid crystals, giving insight on their molecular organization. The technique has been widely used and mathematical descriptions of the experiments have been worked out which, although simple to visualize, may involve computer calculations rather cumbersome and time consuming. Here we derive a mathematical expression giving the echo attenuation in a PFG-SE experiment which is formally compact and easy to handle by computer simulations technique. Its application to hexagonal mesophases is discussed as an example of how to handle complex situations.