[Rtk-users] rtkadmmwavelets ring artifact

[Cyril Mory]

Hi Joel,

I've added the displaced detector filters where necessary, and on my 
data, the ring artifact has now disappeared. Just pushed the fix.
Would you be so kind to try the new version on your data and let us know 
whether it fixed the problem ?

Regarding your question on using the full-data FDK as initialization of 
an iterative reconstruction: if you have basic notions of linear 
algebra, I encourage you to read section V.6 of
https://tel.archives-ouvertes.fr/tel-00985728/document
And if you are doing 4D CBCT, I very strongly encourage you to try 
rtkfourdrooster

In short, it says that, in the absence of regularization:
- the information embedded in a given projection ends up in the Fourier 
transform of the reconstructed volume
- the Fourier coefficients affected are almost specific to each projection
- therefore not taking a projection into account in the iterative 
process means not modifying the corresponding Fourier coefficients of 
the reconstructed volume during the iterative reconstruction
- if you initialize with a zero-filled volume, that means the Fourier 
information corresponding to the projections you drop is assumed to be 
zero, and will remain so
- if you initialize with anything else, the Fourier information 
corresponding to the projections you drop will remain whatever it was in 
the input you gave

Regularization can make the correct Fourier coeffs "spread out" to the 
others, therefore the "dropped information" can be partially compensated 
by (very strong) regularization. In my experience, for cardiac 4D CBCT 
and pulmonary 4D CBCT, 3D wavelets regularization isn't enough to 
compensate for the loss of 80% or 90% of the data. Adding the 
regularization along time help a great deal (which is why I mentioned 
rtkfourdrooster).
So the question is: do you want data from the projections you dropped to 
end up in your reconstructed volume ?

Sorry for the long answer, despite the "in short" preamble.

Regards,
Cyril

On 03/18/2015 09:03 AM, Simon Rit wrote:
> Hi,
> It looks very much like a displaced detector artefact (the Cho 
> <http://www.sciencedirect.com/science/article/pii/0895611196000316> 
> situation). If I refer to the doc of ADMM wavelets 
> <http://www.openrtk.org/Doxygen/classrtk_1_1ADMMWaveletsConeBeamReconstructionFilter.html> 
> and TV 
> <http://www.openrtk.org/Doxygen/classrtk_1_1ADMMTotalVariationConeBeamReconstructionFilter.html>, 
> you have these nice graphs that document the pipelines used for the 
> computation and indeed, the displaced detector situation is accounted 
> for in TV but not in wavelets. Maybe you can try to dig in the code 
> and correct that in ADMM wavelets? Otherwise, I'll ask Cyril if he 
> agrees to do it next week.
> Simon
>
> On Mon, Mar 16, 2015 at 11:00 PM, Joel Beaudry <joelbeaudry at gmail.com 
> <mailto:joelbeaudry at gmail.com>> wrote:
>
>     Sure thing, thanks for the reply Simon. I've attached a Coronal
>     and Axial slice showing the ring. I'll wait for Cyril's reply.
>
>     The image quality is pretty poor but I'm using ~70 projections
>     (4DCBCT), so that's expected. Admittedly, I'm not using a high
>     number of iterations (n=10) but I suspect this ring artifact is
>     from something else. This does not occur when using
>     rtkadmmtotalvariation, but only the admmwavelets.
>
>     And just a general question regarding the nature of iterative
>     reconstructions. I'm currently using no input image, so I assume
>     the initial guess is just some uniform value but is it beneficial
>     to first do a full (so use all projections) FDK reconstruction and
>     use that as the input instead?
>
>     Thanks,
>     Joel
>
>     On Mon, Mar 16, 2015 at 1:27 AM, Simon Rit
>     <simon.rit at creatis.insa-lyon.fr
>     <mailto:simon.rit at creatis.insa-lyon.fr>> wrote:
>
>         Hi,
>         Could you provide a snapshot?
>         Cyril, the developer of this method, is away for a few days
>         but he'll
>         be able to answer next weeks.
>         Cheers,
>         Simon
>
>         On Fri, Mar 13, 2015 at 7:49 PM, Joel Beaudry
>         <joelbeaudry at gmail.com <mailto:joelbeaudry at gmail.com>> wrote:
>         > Hi RTK users/developers,
>         >
>         > I've been using RTK for CBCT reconstructions and found it to
>         be very useful,
>         > so thanks for all the work done on it!
>         >
>         > As for my question,when I use rtkadmmwavelets I get a ring
>         artifact in the
>         > center of the image. I'm using it for 4DCBCT
>         reconstructions, so I'm not
>         > using the full number of projections and not sure if this is
>         a contributing
>         > factor. From reading previous posts I know that the entire
>         object must be in
>         > the reconstruction, and so I have included both the patient
>         and couch in the
>         > reconstruction volume but still get the ring artifact. Has
>         anyone else
>         > encountered this? Any ideas/suggestions?
>         >
>         > Cheers,
>         > Joel
>         >
>         > PS. When trying rtkadmmtotalvariation I don't encounter such
>         issues.
>         >
>         > _______________________________________________
>         > Rtk-users mailing list
>         > Rtk-users at public.kitware.com
>         <mailto:Rtk-users at public.kitware.com>
>         > http://public.kitware.com/mailman/listinfo/rtk-users
>         >
>
>
>
>
>
> _______________________________________________
> Rtk-users mailing list
> Rtk-users at public.kitware.com
> http://public.kitware.com/mailman/listinfo/rtk-users

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.creatis.insa-lyon.fr/pipermail/rtk-users/attachments/20150324/7c260880/attachment.htm>