[vtkusers] Get imageData value from axis coordinates

[Tony]

Dear list,
 
I would like to find the ijk cordinates of a cell in an image data from the xyz 'world' coordinates, that is the cordinates the origin and the spacing are defined in. I can do the reverse, that is getting the physical coordinates from ijk cordinates, using FindCell. However I have been unable to do the reverse. Is this possible?
 
Thanks