[Insight-developers] Image::DirectionType should be Nx3 rather than NxN?

[Bill Lorensen]

Steve,

I see your point. I'll have to think about how we could address this problem
and still maintain backward compatibility. The same issue applies to the
origin of a DICOM (Image Position Patient) if we only use one DICOM image.

Bill

On 7/4/07, Steve M. Robbins <steve at sumost.ca> wrote:
>
> Bill,
>
> Perhaps my original message was too oblique.  Upon reflection, I
> would phrase my objection this way: the dimension of the physical
> space is independent of the image dimension.  The two examples I
> cited should prove this: you can take a 2D image in 3D space, and
> you can take a 4D (volumetric time series) in a 3D space.
>
> As it stands now, the GDCM ImageIO class effectively throws away
> information when you read a 2D image: you get only the first two
> components of the 3-vectors.
>
> I understand that ITK is intended to be generic, and yes I can
> conceive of taking an nD image inside an nD space.  But is there any
> way that the DirectionType could specify the row and column dimensions
> independently?  Well, the number of columns should be the image
> dimension, but the number of rows should be specifiable independently
> somehow and arguably default to 3.
>
> -Steve
>
>
> On Wed, Jul 04, 2007 at 08:02:18AM -0400, Bill Lorensen wrote:
> > Steve,
> >
> > nD direction cosines should be fine for nD images I think.  DICOM is a
> bit
> > special since the 2D image has 2 3D direction cosines. The third is
> derived
> > if more than one DICOM image exists in the series.
> >
> > The named directions for medical images (axial, coronal, sagittal) don't
> > have meaning in higher dimensions. As you pointed out, the direction
> cosines
> > for a time series are hard to interpret. One could argue that the
> meaning of
> > the directions is up to the application. itk just supports the concept.
> >
> > Bill
> >
> >
> > On 7/4/07, Steve M. Robbins <steve at sumost.ca> wrote:
> > >
> > >Howdy,
> > >
> > >In the DICOM view of things, the world coordinate system is always 3
> > >dimensional.  Therefore a 2D DICOM image has two 3D direction vectors
> > >(a.k.a. the direction cosines).
> > >
> > >The itk::ImageBase::DirectionType, however, is defined as
> > >
> > >/** Direction typedef support.  The Direction is a matix of
> > >  * direction cosines that specify the direction between samples. */
> > >typedef Matrix<double, VImageDimension, VImageDimension> DirectionType;
> > >
> > >So a 2D image is provided with two 2D direction vectors.  That's a bit
> > >odd, I think.  Is it a bug or is there some subtlety I don't
> understand.
> > >How does one interpret a 4D direction cosine for volumetric time series
> > >data?
> > >
> > >I have read
> > >
> > >    http://www.itk.org/Wiki/Proposals:Orientation
> > >and  http://www.insight-journal.org/dspace/handle/1926/145
> > >
> > >without gaining any enlightenment.
> > >
> > >Thanks,
> > >-Steve
> > >
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