chapter[shorttitle]{title} and toc

[Nico Mock]

Hi All,

I'm using the package fancyhdr with pagestyle fancy to have some nice 
headers on top of my theses, but unfortunately the titles are quite long.
I used the command 
\chapter[shorttitle]{title} 
to have a shorttitle in the header but this changes the toc - entry, too.

Is there a way to use the shorttitle in the header and the long title in toc ?

Thanks 

Nico