[Rtk-users] projection normalization

[Howard]

Hi Simon,

Thanks very much for your help in answering my questions. I have further
follow up comments/questions (see below) and hope to understand the issues
I encountered better. I will also include the offline discussion with
another CBCT reconstruction expert.

Regards,
Howard

On Tue, Jun 28, 2022 at 5:41 PM Simon Rit <simon.rit at creatis.insa-lyon.fr>
wrote:

> 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.
>

Yes, what I did before using the maximum intensity to scale the measured
projection was incorrect. However, there are ring artifacts on the
reconstructed images when using the flood field to do pixel by pixel
correction. I learned from the offline expert that to correct for the ring
artifacts I will need to do "flat field correction" or other techniques.
For the projection data acquired from GATE simulation, are there any
available RTK tools to do this correction? Any other suggestions would also
be appreciated.

2) yes! But with a spectrum, it's hard to anticipate the ref value due to
> beam hardening.
>

For monochromatic X-ray, we know the linear attenuation for the known
materials at the given energy, therefore I should be able to rescale the
grayscale values on the reconstructed images. However, there will still be
beam hardening problems?


> 3) It's "normal" to have negative values due to scatter,  not beam
> hardening. But one should try to correct for scatter.
>

I see. This might be a bit tricky then. We'll have to explore the
correction methods.

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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>>
>
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