[Rtk-users] projection normalization

[Howard]

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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