[MINC-users] FW: min and max coordinates in minc vs dicom

[Audette, Michel]

Hi all, 

I'm playing with a MINC volume that was converted from DICOM, which has some unusual direction cosines,  and entering both volumes, MINC and DICOM, with ITK importers. I'm finding that there is a difference in min and max coordinates, and I'm having trouble ascertaining where the trouble is. 

For example, when typing mincheader, one sees the line:
dicom_0x0020:el_0x0032 = "-26.902344\\-197.40057\\-45.887484" ;

which mostly coincides with what I see when we load the data in DICOM format:
minx: -26.9 maxx: 75.5
miny: -197.4 maxy: -95.2
minz: -51.89 maxz: -12.59

Other data in mincheader include:
                xspace:step = -0.1953125 ;
                xspace:start = 26.80468775 ;
                xspace:direction_cosines = 1., 0., 0. ;

                yspace:step = -0.1953125 ;
                yspace:start = 193.094103137385 ;
                yspace:direction_cosines = 0., 0.99691733368886, 0.0784590962903196 ;

                zspace:step = 0.299075200106658 ;
                zspace:start = -67.2154025304974 ;
                zspace:direction_cosines = 0., -0.0784590962903196, 0.99691733368886 ;


doing a mincinfo produces
maudette at icaw164201:~/data/work/earWork/goodEarData> mincinfo etzold_emma_12792115_003002_mri_xyz.mnc
file: etzold_emma_12792115_003002_mri_xyz.mnc
image: signed__ short -32768 to 32767
image dimensions: xspace yspace zspace
    dimension name         length         step        start
    --------------         ------         ----        -----
    xspace                    512    -0.195312      26.8047
    yspace                    512    -0.195312      193.094
    zspace                    129     0.299075     -67.2154

We implemented a program that inverts the sign of step and start if the step is negative, from a minc import, and it produces 
minx: -26.8 maxx: 73
miny: -197.77 maxy: -97.97
minz: -51.86 maxz: -13.58

Can anyone help me make heads or tails of this discrepancy? 

How does a dicom to minc conversion produce negative spacing in x and y, and can this converter make a mistake with start and spacing size? Does it round off anything, or is it sensitive to non-standard cosine angles? 

Best wishes, 

Michel


Michel Audette, Ph.D.   
Innovation Center Computer Assisted Surgery (ICCAS)   
Semmelweisstraße 14  
Leipzig, Germany  
Phone: ++49 (0) 341 / 97 - 1 20 13  
Fax: ++49 (0) 341 / 97 - 1 20 09