[Rtk-users] bright circle on the reconstructed image

[Simon Rit]

Hi,
I fully agree with Cyril. Something else : you might want to mask out what
is outside the field-of-view using the rtkfieldofview executable. Your ring
corresponds to that border. See an example here:
http://wiki.openrtk.org/index.php/RTK/Examples/VarianReconstruction
Simon

On Thu, Sep 18, 2014 at 10:12 AM, Cyril Mory <
cyril.mory at creatis.insa-lyon.fr> wrote:

>  Hi Howard,
>
> The bright ring artifact is typically caused by high-pass filtering (the
> ramp filter used in FDK) of truncated projection data (when projections
> have non-zero values on the sides, the edge of the projection makes a sharp
> transition to zero). A classical answer to this problem is to pad the
> projections, for example with values starting from the values on the side
> and slowly decreasing to zero.
>
> rtkfdk can do this padding for you. Just add "--pad 0.1" to your options
> (or more than 0.1 if it isn't enough).
> Also, I noticed that your reconstructed image seems quite noisy. You might
> want to use a Hann windowing of the ramp filter to reduce the noise
> amplification effect of high-pass filtering. You can do that by adding
> "--hann 0.5" to your options. Note that the argument is a cut-off
> frequency, so smaller values mean more smoothing.
>
> Best regards,
> Cyril
>
>
> On 09/18/2014 05:21 AM, Howard wrote:
>
>  Hi,
>
> I use rtkfdk to reconstruct varian cbct images following the example on
> rtk wiki page. The projections were obtained with half fan mode because it
> is an abdominal region with the patient lateral x AP dimension of 550x550.
> The reconstructed cbct images have a big bright circle around
> the image. Please see the image:
> http://expirebox.com/download/2f55c9d8f0bc83138b954d2fec08e78c.html
>
> I am using Ubuntu Linux box to reconstruct. Two sequential commands were
> used:
>
> ./rtkvarianobigeometry --xml_file ProjectionInfo.xml --path Scan0/
> --regexp Proj_.*.hnd -o varianGeometry
>  and
>
> rtkfdk \
>    --geometry varianGeometry \
>    --regexp .*.hnd \
>    --path ../Scan0 \
>    --output rec.mha \
>    --verbose \
>    --spacing 2.0,2.0,2.0 \
>    --dimension 256,100,256 \
>  The projections are stored in the standard varian format: Scan0 and
> ProjectionInfo.xml with 644 projection images.  I assume that rtkfdk
> handled the half fan geometry properly. What did I do wrong then?  Many
> thanks!
>
> -howard
>
>
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>
> --
> --
> Cyril Mory, Post-doc
> CREATIS
> Leon Berard cancer treatment center
> 28 rue Laënnec
> 69373 Lyon cedex 08 FRANCE
>
> Mobile: +33 6 69 46 73 79
>
>
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