World coordinate do not give me the correct dicom coordinate

Dear all,

I use itk to perform 2d segmentation of a 3d volume given by a dicom series
and i need to add some seeds with the mouse for some 2d slices. So i am
forced to show a 2d slice in vtk obtained with itk (using extractfilter) in
a vtk windows flipping the slide as it is upside down. Then i pick a point
with the mouse to choose the seeds. But i found that it does not give the
exact coordinate. At least it is different frm the one given in slicer 3d.
As i need it for itk segmentation and accuracy is very important. Is there
any way to get the same coordinates fro itk and vtk?