[Insight-users] Origin and direction in itk::GDCMImageIO
frankmiller at jhmi.edu
Wed Mar 1 13:09:09 EST 2006
I recently started to play with Dicom images with oblique orientation
and I just noticed that the origin that is stored in itk::Image is
different then the "Image Position (Patient)" tag in the Dicom header. I
thought this was a bug at first but when I dug around in the
itkGDCMImageIO class I discovered that this was done intentionally.
// DICOM specifies its origin in LPS coordinate, regardless of how
// the data is acquired. itk's origin must be in the same
// coordinate system as the data. This code transforms the DICOM
// origin into the itk origin. The itk origin is computed by
// multiplying the inverse(transpose) of the direction cosine times
// the dicom origin.
Can someone explain to me why the origin is mapped in this way? What is
the difference between the "LPS coordinate system" and the "coordinate
system of the data?"
Also, Im a newcomer to Dicom and especially to the spatial orientation
of medical images. Can someone can suggest a good reference? I have
taken a look at http://medical.nema.org/dicom/2004.html but those
documents read like tax code. I cant find where it describes what "Image
Position (Patient)" and "Image Position (Patient)" actually mean. Most
of my understanding comes from the notes in
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