[Insight-users] confusion about the coordinate system for the Image Orientation Patient tag in DICOM

[Steve M. Robbins]

On Wed, Jun 20, 2007 at 02:48:16PM +0100, Luca Pamparana wrote:
> Hi everyone,
> 
> Just been trying to use the Image Orientation Patient tag. Most of the
> emails that I have read in the forums say that the 'x' is increasing to the
> right side of the patient.
> 
> However, I thought that the patient coordinate system was that x was
> increasing to the left side of the patient.
> 
> Also, I looked at some code from Dr. Clunie and he does the following to
> determine the patient orientation:
> 
> String orientationX = x < 0 ? "R" : "L";
> 
> I interpret this as that x is notmally set to increase to the left of
> the patient. This is really confusing!
>
> I would be really grateful if someone can clarify this for me.

In matters of DICOM it's best to go straight to the source, though
David Clunie is nearly synonymous with DICOM ...  Look at
http://medical.nema.org/dicom/2007/07_03pu.pdf, page 301:


  C.7.6.2.1 Image Plane Attribute Descriptions
  C.7.6.2.1.1 Image Position And Image Orientation

  The Image Position (0020,0032) specifies the x, y, and z coordinates
  of the upper left hand corner of the image; it is the center of the
  first voxel transmitted. Image Orientation (0020,0037) specifies the
  direction cosines of the first row and the first column with respect
  to the patient.  These Attributes shall be provide as a pair. Row
  value for the x, y, and z axes respectively followed by the Column
  value for the x, y, and z axes respectively.  

  The direction of the axes is defined fully by the patient's
  orientation. The x-axis is increasing to the left hand side of the
  patient. The y-axis is increasing to the posterior side of the
  patient. The z-axis is increasing toward the head of the patient.

  The patient based coordinate system is a right handed system,
  i.e. the vector cross product of a unit vector along the positive
  x-axis and a unit vector along the positive y-axis is equal to a
  unit vector along the positive z-axis.

Cheers,
-Steve
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