[Insight-users] [Gdcm2] Matching CT and RT dose coordinates

[Karthik Krishnan]

As others already mentioned, you need to re-inteprolate the dose volume onto
the CT data. To do that you must use the grid frame offset vectors.

For all dose images I've encountered, the spacing between these grid planes
(as specified by the grid frame offset vectors) is a constant, but this need
not always be the case. If you want to be thorough you need to account for
this. Several folks don't (for instance below is an excerpt from Varian's
conformance statement):

*"Dose Plane Spacing:
If Grid Frame Offset Vector (3004,000C) is present, it must contain offset
values yielding equally spaced dose planes. Therefore, those values follow
option a. in C.8.8.3.1 in Part 3 of [1]. RT Dose Instances having unequally
spaced dose planes are not supported.*"

So for this specific case, where the grid plane spacing is the same, you
could use the using the starting grid frame offset vector and the grid plane
spacing and use the itk::ResampleImageFilter. Simply use GDCM to convert the
images to MHD. Then set the spacing and origin to match the starting grid
plane offset vector and the grid plane spacing..

If you load the images in to 3DSlicer, I believe they should already be
re-interpolated and placed in the right coordinate system.


--
karthik




On Thu, Feb 24, 2011 at 2:36 AM, Mark Roden <mmroden at gmail.com> wrote:

> Hi Jothy,
>
> What are the direction cosines?
>
> There is a problem where VTK reinterprets the direction cosines to
> change the directions of images produced by the dicom reader.
>
> DICOM specifies that the image patient position is always the upper
> left pixel (if you look at the image as a 2D image on a plane like a
> monitor).  VTK will take the direction cosine and potentially flip the
> image along those cosines.
>
> Mark
>
> On Wed, Feb 23, 2011 at 9:38 AM, Jothy <jothybasu at gmail.com> wrote:
> > Hi all,
> >
> > I am trying to overlay an dose shade on a CT slice.I am using
> > vtkDICOMImageReader to read the CT and vtkGDCMImageReader to display the
> > dose shade.But I am having problem with the coordinates.How to relate the
> > coordinates? For e.g: I  get ImagePositionPatient of CT image for central
> > slice as [-275,-524,-128.25] and ImagePositionPatient for dose as
> > [-228.65,-419.25,-122.5] adn their spacings are [1.074,1.074,3] and
> > [2.52.5.2.5] respectively for CT and Rt dose.
> >
> > The image looks as in the attached figure.
> >
> > Any hint
> >
> > Thanks
> >
> > Jothy
> >
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