How to get voxel coordinates(as in ITK snap ) from vtkimage viewer from mouse click

I am trying to use ITK’s region growing algorithm for medical image segmentation. So to put the seedpoint i need exact voxel coordinates of the mouseclick position in my vtkimageviewer. So how can I get those voxel coordinates(as in ITK snap tool) from my vtkimage viewer. Currently I am able to get VTK World Coordinates using pick point picker . Now how to convert this vtk world coordinates to voxel coordinates as in ITK snap tool …
Please help me with this information.