[Rtk-users] Correction for detector in-plane rotation?

[Chao Wu]

Hi Simon,

Thanks for the confirmation.
The rotation in my projections are less than 0.2 degrees, so the ramp
filter along image line or along true horizontal direction may leads to
almost identical reconstructions.

I got into this subject because I was working with a displaced detector and
found dark and bright regions along the axis of rotation at two side of the
mid plane when the IPR is set to non-zero.
After reading the document I realized that this would be due to that the
IPR is not accounted when the weighting [G Wang] is performed, then later
it is accounted during the FDK, so that Wang's weight function applied to
opposite projections does not sum to 1. Before I make any changes to the
DisplacedDetectorImageFilter I would like to see how you dealt with IPR in
FDK, and suddenly I found IPR is not accounted for either in the ramp
filter which is not 'mathematically correct' (but may be sufficient and
efficient in practice).

So now I am thinking how to solve the problem.
The most straightforward way could be that I perform a pre-correction for
IPR in all my displaced-detector projections, and send them to rtkfdk with
an IPR=0 geometry, since this can make the behaviour of both
DisplacedDetectorImageFilter and FFTRampImageFilter ideal (and less
workload in coding, of course).
Or modify the DisplacedDetectorImageFilter to account for IPR while leave
the FFTRampImageFilter as it is.
Do you have any other comments and suggestions? Thank you.

Best regards,
Chao



2014-02-14 15:10 GMT+01:00 Simon Rit <simon.rit at creatis.insa-lyon.fr>:

> Hi,
> Yes, this is correct. In plane rotations are accounted for during
> backprojection but not in the FFT ramp filter which is done along lines. It
> should be modifiable if you want to. The weighting in the projection image
> should not sensitive to in plane rotation since it uses the distance to the
> center.
> How large are your rotations?
> Simon
>
>
>
> On Fri, Feb 14, 2014 at 2:26 PM, Chao Wu <wuchao04 at gmail.com> wrote:
>
>> Hi,
>>
>> First thanks for the hard work on this nice toolkit.
>>
>> I am working on FDK reconstruction.
>> I noticed that the InPlaneRotation seems not to be handled in some
>> situations, such as in the DisplacedDetectorImageFilter, which is
>> explicitly stated in the document.
>> Furthermore, when I went through the FDKConeBeamReconstructionFilter,
>> there seems to be no correction for the in-plane rotation to the projection
>> images before they are send to the FFTRampImageFilter. Thus the filtering
>> seems to be performed along the rows of the projection image but not the
>> true horizontal direction of the projection.
>> Did I miss anything around here? In which steps and to which extent the
>> in-plane rotation has been taken into account?
>>
>> Best regards,
>> Chao
>>
>> TUDelft, NL
>>
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>>
>>
>
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