if you would like to do this entirely in SPM, the SPM Diffusion Toolbox
may be a good place to start. On the SPM extensions web page you will
find a link to the sourceforge download site. In the forums on this
site, there are also a few hints about how to use this toolbox.
If you have any further questions about the toolbox, please send them to
the sourceforge forums or the trac system on sourceforge.