[vtkusers] 'Picking' and 'vtkFixedPointVolumeRaycastMapper' image volume

[Kall, Bruce A.]

What is the correct way to pick a point that would be on the outermost
surface of an image volume rendered with a vtkFixedPointVolumeRaycastMapper
and to get the coordinates in the image volume(model) space?

I've tried using vtkCellPicker and can get a cellid,  but don't know how 
to look up
the cell closest to the outside surface of the volume. I've also tried 
vtkPointPicker.
Both are returning a Z value that is the same for points on the surface 
that should have a
different Z value as there is a small hump on the top of my volume that 
should not have
the same Z value as that on te same surface but not on the hump.

Also as I spin the camera around with an interactor the x,y,z 
coordinates for the same
point (on the hump) on the volume surface returns different x,y,z 
coordinates and I need to get coordinates
in my model (image volume) coordinates.

Is there a good example of doing this somewhere?  I've seemingly looked 
everywhere
and haven't found one, and I'm sure this has been implemented by someone.

Thanks in advance
Bruce