[Insight-users] Problems with Digitally Reconstructed Radiographs

[Luis Ibanez]

Hi Christina,

You must take into account that when generating a DRR,
the scale of the projection depends on the distance of
the volume to the focus of the radiation, and on the
focal distance.

ITK projection calculations are done in real physical
units (eg.mm), so you must use those unit in order to
interpret the results that you are seeing.

Note that you can always fine tune the parameters of the
image grid where you are doing the mapping. (e.g. pixel
spacing of the output image).


About the fact that the image seems to get off center
when you rotate it, this is because the default location
for the center of rotation is the origin of the image
which is the 0,0,0 and it is usually located in a corner
of the image, not the center of the image.

You should use the SetCenter() method in the transform
to be the geometrical center of the image, in order to
get rotations that look natural to you.

You will find a detailed discussion on the issues of
centered transforms and rotations in the ITK Software
Guide:

        http://www.itk.org/ItkSoftwareGuide.pdf

both in the "Image Registration" chapter and in the
section on "Geometrical Transformations".


Please read these two sections before proceeding with
your experiments. Understanding how the transforms
work will save you a lot of time and unnecessary pain.



    Regards


       Luis


------------------------
Christina Lacalli wrote:
> Hello,
> 
> I have problems with generating DRRs as it is described in the ITK 
> example code 'DigitallyReconstructedRadiograph1.cxx'.
> Especially I do not understand how to set the parameters correctly.
> 
> I use a real data set with size [256, 256, 17], pixel spacing [1.48, 
> 1.48, 5]mm and origin [0,0,0]. According to the instructions from a 
> previous thread I have changed the translation parameters to [-190, 
> -190, -42,5] to center the DRR. Indeed, the resulting DRR is centered in 
> x and y but it appears too close to the
> camera so that I can only see a small region.
> However, setting the z-part of the translation to -242,5 - that is the 
> z-position
> of the DRR - the resulting DRR seems to be correct?! How do I
> calculate the right z-parameter?
> 
> Furthermore, when I change the rotation parameters, for example the 
> x-part to
> 45 and leaving the translation unchanged (z = -242,5), I do not obtain 
> the expected result.
> The resulting DRR is identical to the one without rotation, except that 
> it is not centered anymore.
> But when I set the z-part again to -42,5 the rotation seems to work, but 
> the
> resulting DRR again does not show the whole image.
> 
> Can anyone give me some advice how to figure out this problem?!
> 
> Thank you
> 
> Christina
> 
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