How to disable Root node or avoid selection on treeview in wpf?

Hi all,

i am using treeview in my app i want to disable the Root node selection means i dont want to selection on root node.
for this i hide my root node by using some style in XAML but when im using Arrow keys from keyboard selection is also goin on root node.

plz tell me how to avoid this selection on root node.

thanx in advanced