[vtkusers] how to get continuous voxel indices of my C-arm isocenter

[baby]

dear all,
  
 these days i read a paper in insight journal, the title of this paper is "ITK-Based Implementation of Two-Projection 2D/3D Registration Method with an Application in Patient Setup for External Beam Radiotherapy "
  
 and  i have a problem about this paper 
  
 in this paper, the author says that "To align a 3D image with our coordinate system, the user needs to provide the continuous voxel  indices of the isocenter  location with respect to the  image volume." so what should i do to get continuous voxel  indices of my C-arm  isocenter  location with respect to my CT image volume?
 do i need to ask a doctor in hospital?
  
 thank you in advence,
  
 michael
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