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

[Bill Lorensen]

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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