world coordinates do not give the correct dicom coordinate

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Dear all,

I want to do segmentation with itk and i would like to select a seed using
vtk with the mouse. I get the coordinates in world coordinates

                    int x = this->Interactor->GetEventPosition()[0];
                    int y = this->Interactor->GetEventPosition()[1];
                    std::cout << "(x,y) = (" << x << "," << y << ")" << std::endl;

                    seeds[0] = x;
                    seeds[1] = y;

but when i want to use it as a seed for itk i see it is the wrong place. I
tried the transformphysicalpointoindex but the result is even worst

            bool isInside =
                    extractFilter->GetOutput()->TransformPhysicalPointToIndex(coordinate_vtk,

coordinate_itk);
if (isInside) {

                    ImageType::PixelType pixelValue =

extractFilter->GetOutput()->GetPixel(coordinate_itk);
}

Any help about the coordinates in vtk?

Thanks