[Rtk-users] projection normalization

[Simon Rit]

Hi,
1) I think I would divide the measured projection by the flood field, no
reason to limit it to one scalar. The rest seems correct.
2) yes! But with a spectrum, it's hard to anticipate the ref value due to
beam hardening.
3) It's "normal" to have negative values due to scatter,  not beam
hardening. But one should try to correct for scatter.
Simon



On Tue, Jun 28, 2022, 05:50 Howard <lomahu at gmail.com> wrote:

> Dear RTK Users,
>
> I am having some questions on the correct way of normalizing/calibrating
> the projection images. My projections were generated using GATE (geant4
> simulation for those who may not know what GATE is) with cone beam X-ray
> and flat panel detector. Here are the steps I followed.
>
> 1. generate projections (proj) with a full rotation (360 degree, say 1
> projection/degree) with the imaging object in the X-ray beam path
> 2. remove the imaging object to generate an air projection (flood field)
> 3. Get the maximum intensity (maxInt_air) of the air projection obtained
> in step 2
> 4. Correct the projections obtained in step 1 with the formula proj_corr =
> -log(proj / maxInt_air) (I did see before some discussions using the
> maximum intensity to correct projection images.)
> 5. Reconstruct the CBCT using rtkfdk with the corrected projections
> obtained in step 4
>
> So here are my questions:
>
> 1)  Is the above procedure the correct way of reconstructing CBCT?
> 2) Are the grayscale values in the reconstructed CBCT correspond to the
> attenuation coefficients of those materials?
> 3) There are some negative grayscale values in the reconstructed CBCT due
> to artifacts such as the beam hardening. Is this normal?
>
> Many thanks for any feedback or suggestions.
>
> Howard
>
>
>
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