[Rtk-users] bright circle on the reconstructed image

[Cyril Mory]

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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