how to get the mouse coordinate on dicom

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1

hi friends:
I am trying to run the offically provide demo https://kitware.github.io/vtk-examples/site/Cxx/Images/PickPixel2/, for JPEG image(use vtkJPEGReader to load it), the mouse coordinate and pixel value get successfully, it as below:

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but for dicom (use vtkDICOMReader to load it), the mouse coordinate and ct number get failed.

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as my debug, vtkAssemblyPath* path = this->Picker->GetPath(); the path is always null

anybody had encounter this issue ? or other better solution to get the mouse coordinate ?

many thanks in advance!!!