[vtkusers] display part of an actor

[Xu Zhong Hao]

Hi,

Please advice me on how to display part of an actor, for example, like 
medical 3 example, i have put a cone into the virtual skull and the 
cone can navigate through the skull by keyboard command, now i need to 
what is beyond the cone when the cone stops in a particular place of a 
skull.

please give me some ideas

thanks

Zhong Hao