Threshold from vtk image data

I everyone I just approach vtk in python for my master thesis
I import a file.nii.gz from segmentation obtain through mri. Then I read it through vtkNIFTIImageReader and with get output I obtain a Vtk image data file
Now I have to use Vtk image threshold so to neglect the zero value voxels. How can I get the non zero value of the other voxel? I just cannot analyze vtk image data file