[Rtk-users] Reconstructing from fan-beam projections

Wed May 17 16:41:34 CEST 2017

I guess the extreme negative values are due to truncation. I have never
worked on the interior problem before so I don't have experience with this
but I'm not surprised that you reconstruct bad values.
The implemented correction is
Ohnesorge, B.; Flohr, T.; Schwarz, K.; Heiken, J. & Bae, K.
Efficient correction for CT image artifacts caused by objects extending
outside the scan field of view
*Med Phys, **2000**, 27*, 39-46
The parameter is the ratio of extension (between 0 and 1).
Simon

On Wed, May 17, 2017 at 3:41 PM, Jacob Frøsig <jafs at force.dk> wrote:

> Hi,
>
>
>
> Thank you, it is a great help! Attached is a report describing the project
> where the beginning of chapter 5 describes the data acquisition.
> The investigated object is an oil pipe. Yes, the reconstruction contains
> the expected interior part of the oil pipe along with the ring artefact
>
> which is characteristic for the interior problem.
> What puzzles me is that I get a few extreme negative values,  -1.5884e+38
> , in the bottom of the reconstruction using FDKConeBeamReconstructionFilte
> r. I have attached the truncated reconstruction without negative values
> and the reconstruction with negative values.  What exactly is your
> truncation correction?
>
> Have you seen something similar before?
>
>
>
> Best regards,
>
>
>
> Jacob
>
>
>
> *From:* simon.rit at gmail.com [mailto:simon.rit at gmail.com] *On Behalf Of *Simon
> Rit
> *Sent:* 17. maj 2017 15:03
>
> *To:* Jacob Frøsig <jafs at force.dk>
> *Cc:* rtk-users at openrtk.org
> *Subject:* Re: [Rtk-users] Reconstructing from fan-beam projections
>
>
>
> Hi,
>
> Here is what I obtain. I have no clue what you expect but that's how I
> would translate the geometric information that you provided. The interior
> problem is obviously an issue here (our truncation correction does not help
> for interior CT) but maybe you can tell us if you recognize your object.
> There are clear ring artefacts as well in your sinogram.
>
> I hope this helps,
>
> Simon
>
>
>
> On Tue, May 16, 2017 at 7:22 PM, Jacob Frøsig <jafs at force.dk> wrote:
>
> Hi,
>
>
>
> yes, it is indeed an interior- / region of interest problem in the sense
> that we only are interested in the fully illuminated centre region.
> I have previously had success in getting good reconstructions from
> interior data using variational formulation with tailored penalty term.
> However, this is implemented in Matlab and the aim is to use your
> implementation to get a C++ implementation of this method.
>
> But for now, I am just figuring how to do a simple reconstruction to get a
> grasp on the library.
>
>
>
> Best regards,
>
>
>
> Jacob
>
>
>
>
> ------------------------------
>
> *Fra:* simon.rit at gmail.com <simon.rit at gmail.com> på vegne af Simon Rit <
> simon.rit at creatis.insa-lyon.fr>
> *Sendt:* 16. maj 2017 16:59:12
> *Til:* Jacob Frøsig
> *Cc:* rtk-users at openrtk.org
> *Emne:* Re: [Rtk-users] Reconstructing from fan-beam projections
>
>
>
> Hi,
>
> If this is true, then the sinogram indicates that you are dealing with an
> interior problem since you have high line integrals at the border (see
> enclosed profile). Is that indeed the case? If yes, that's a difficult
> problem...
>
> Simon
>
>
>
> On Tue, May 16, 2017 at 3:16 PM, Jacob Frøsig <jafs at force.dk> wrote:
>
> Hi,
>
>
>
> The attached sinogram consist of line integral values, i.e., sinogram(i,j)
> = -ln[ *I(i,j)/I_0(i)* ] where *I *and ***I_0 *are the measured
> intensities with and without an object, respectively.
> Since we are measuring with a line detector, the “zero” (*I_0*) is in
> this case just an array.
>
> *j* is the index for the projections, i.e., *j* ∈  {1,2,…,360} since we
> take one projection for each whole angle.
>
> I hope this explains the sinogram and again; thank you for your time!
>
> Best regards,
>
> Jacob
>
>
>
> *From:* simon.rit at gmail.com [mailto:simon.rit at gmail.com] *On Behalf Of *Simon
> Rit
> *Sent:* 16. maj 2017 14:54
> *To:* Jacob Frøsig <jafs at force.dk>
> *Cc:* rtk-users at openrtk.org
>
>
> *Subject:* Re: [Rtk-users] Reconstructing from fan-beam projections
>
>
>
> Hi Jacob,
>
> Thanks for the image of the sinogram. I don't understand this sinogram. Do
> you know how to convert each pixel value to a line integral? I.e., if it's
> x-ray imaging, do you have an image without object?
>
> Thanks,
>
> Simon
>
>
>
> On Tue, May 16, 2017 at 2:24 PM, Jacob Frøsig <jafs at force.dk> wrote:
>
> Hi again,
>
> and thank you for the example!
>
> I have some trouble getting a proper reconstruction. If you have the time,
> any help would be appreciated.
>
> Attached is my fan-beam sinogram as a .tif file. Here, projections are
> taken for 360 degrees and the detector has 507 pixels.
> I believe the issue lies in setting the geometry and maybe the spacing
> between the pixels. The measurement geometry is as follows
>
> Source to centre: 590 mm
>
> Source to detector: 1000 mm
>
> Detector length: 411mm
>
>
>
> Best regards,
>
>
>
> Jacob
>
>
>
>
>
> *From:* Rtk-users [mailto:rtk-users-bounces at public.kitware.com] *On
> Behalf Of *Simon Rit
> *Sent:* 15. maj 2017 17:32
> *To:* rtk-users at openrtk.org
>
>
> *Subject:* Re: [Rtk-users] Reconstructing from fan-beam projections
>
>
>
> Hi,
>
> I have added an example here:
> http://wiki.openrtk.org/index.php/FanBeam
>
> The values in the first and the third rows are not used, as illustrated in
> this example. We add them to do a 2D interpolation but we do this 2D
> interpolation exactly on the second row so that's not a problem.
>
> Note that it's a bit more tricky for iterative recon but we also have a
> solution for this if you need it.
>
> Best regards,
>
> Simon
>
>
>
> On Mon, May 15, 2017 at 4:31 PM, Jacob Frøsig <jafs at force.dk> wrote:
>
> Hi again,
>
>
>
> Thank you for the quick answer! An example would be great, thanks. I have
> one concern about copying the fan-beam data:
>
>
> Then we would have 2D projections for which each column is the same
> fan-beam projection. By this, values of the same row are equal even though
> the cone-beam geometry indicates the outer columns correspond to
> intensities of rays with  longer travel time through the object and hence
> should have attenuated more. Is this neglectable?
>
>
>
> Again, thanks!
>
>
>
> Best regards,
>
> Jacob
>
>
>
>
>
>
>
> *From:* simon.rit at gmail.com [mailto:simon.rit at gmail.com] *On Behalf Of *Simon
> Rit
> *Sent:* 15. maj 2017 16:05
> *To:* louie L <ghostcz at hotmail.com>
> *Cc:* Jacob Frøsig <jafs at force.dk>; rtk-users at public.kitware.com
> *Subject:* Re: [Rtk-users] Reconstructing from fan-beam projections
>
>
>
> Hi,
>
> I agree. What I do is do a 3 slice sinogram from the fan-beam projections
> by copying the same data in each slice but reconstruct one 2D slice. If you
> need an example, I can quickly demonstrate this in a short Python script.
>
> Simon
>
>
>
> On Mon, May 15, 2017 at 4:01 PM, louie L <ghostcz at hotmail.com> wrote:
>
> Hi,
>
>
>
> I think you can carefully pad zeros to your projections. Reconstruct as if
> it is a 3d object. Take the middle slice as your fanbeam result.
>
> Let me know if it helps.
>
>
>
> Best regards,
>
> Louie
>
> Sent from my iOS
>
>
> Am 15.05.2017 um 15:55 schrieb Jacob Frøsig <jafs at force.dk>:
>
> Dear Rtk-users
>
>
>
> I was wandering if Rtk includes an implementation of a reconstruction
> method to reconstruct a 2D representation of an object given  fan-beam
> projections.
>
> Or, if anyone has experience using e.g. FDKConeBeamReconstructionFilter
> on projections from line-detectors (fan-beam instead of cone-beam).
>
>
>
> I hope you guys can help.
>
> Yours sincerely
>
> *Jacob Frøsig *
>
> Project Manager
> Sensor Innovation
>
> FORCE Technology
> Park Allé 345
> 2605 Brøndby
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>
> Phone: +45 43 25 00 00 <+45%2043%2025%2000%2000>
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