[Insight-developers] [GDCM] ITK Origin and coordinate system

[Simon Warfield]

Gordon Kindlmann wrote:
> hello,
>
> On Jan 18, 2006, at 6:08 AM, Luis Ibanez wrote:
>
>> It seems that it will be helpful to add a couple of definitions here ...
>
> Thank you for the self-contained explanation!
>
>> 1) Image Grid ...
>> 2) Image Coordinate System ...
>> 3) Physical Coordinate System ...
>>
>> ...
>>
>> Our question at this point is whether DICOM specifies the Image Origin
>> in the "Image Coordinate System" or in the "Spatial Coordinate System".
>>
>>    ... and the answer to that question
>>        is still to be determined with certainty...
>
> Really?  Reading this:
> http://www.vtk.org/Wiki/Proposals:Orientation#Some_notes_on_the_DICOM_convention_and_current_ITK_usage 
>
>
> which quotes from:
> http://medical.nema.org/dicom/2003/03_03PU.PDF
> Section C.7.6.2.1.1, page 237:
>
> "
> 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.
> "
>
> That means unambiguously that "Image Position (0020,0032)", which is 
> the only possible source of origin information, is giving the origin 
> coordinates in the same LPS coordinate system as the direction cosines 
> are measured in, which is consistent with the simple statement of 
> image coordinates to physical coordinates you defined previously:
>
>>>>        Point = M * S * Index + Origin


The standard doesn't specify which of the three tuple is the x 
coordinate, which the y coordinate and which the z coordinate.
The ordering is not specified.

I wonder if some vendors provide the x,y,z coordinates of the origin in 
LPS space in the order (L,P,S) and some, perverse though it would be, in 
a different order (maybe , z, y, x).

-- 
Simon