[Rtk-users] rtk user : FDKConeBeamReconstructionFilter

[Simon Rit]

Hi Elena,
Please post your questions to the mailing list, they can interest everyone
in the community and you may get (better) answers before mine.
FDK is very simple: it takes the projections, weight them with the cosine +
angular gaps, apply the ramp filter with a kernel adapted to the number of
pixels and backproject. The backprojection is voxel based so it does a
linear interpolation between pixels. There is never any attemps to "adjust"
one of these steps according to the pixel and the voxel spacings so the
user must manage it. There are different solutions to do it:
- do a binning of the projections (average 4 or 16 pixels as you maybe
did?). In the command line tool, you can do it with the --binning option.
- cut high frequencies during ramp filtering using --hann and --hannY
options
This is assuming larger voxels than pixels (there is no reason to have
smaller voxels than pixels).
Let me know if this does not answer your question.
Simon

On Tue, Jan 16, 2018 at 11:08 AM, Elena Padovani <e.padovani at r4p.it> wrote:

> Dear Simon,
> I am a new user of the Reconstruction toolkit. I've been testing some
> tools lately and i was able to reconstruct using
> FDKConeBeamReconstructionFilter. I am now trying to figure out something
> more about it and i was wondering if you could help me understand how
> rtkFDKConeBeamReconstructionFilter works.
>
> I am now reading raw images and importing them into the pipeline through
> the importFilter and then a tilerFilter is used to create the stack of
> projections. i was testing if the reconstructed volume would change using
> as input to rtkFDKConeBeamReconstructionFilter images of different
> resolutions. More precisely, i tested 2 cases:
> - 1) the input images are 1536x1536 with spacing 0.2,0.2 and the
> ConstantImageSource volume is 384,384,384 with spacing 0.8,0.8,0.8
> - 2) the input images are scaled-down to 384,384 with spacing 0.8,0.8 and
> the constantImageSource is again 384,384,384 with spacing 0.8,0.8,0.8
> The reconstructed volumes appear to be different. Indeed, the range of
> values is quite different and in the first case the image does not seem
> totally right (even tough the volume is reconstructed). Can you explain me
> how  rtkFDKConeBeamReconstructionFilter (or maybe other filters used
> inside, such as ExtractImageFIlter) manages different resolution in input
> and output ? Does it average the pixel value over the bigger voxel/pixel
> area ?
>
> Thank you in advance for any hint,
> Kind regards,
>
> Elena
>
>
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