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Hi Howard,<br>
<br>
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.
<br>
<br>
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).<br>
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.<br>
<br>
Best regards,<br>
Cyril<br>
<br>
<div class="moz-cite-prefix">On 09/18/2014 05:21 AM, Howard wrote:<br>
</div>
<blockquote
cite="mid:CAKr9h5WVwL18=DbX4-pxJj7ATW22bjCRX-m7RDYia0dvZETc=A@mail.gmail.com"
type="cite">
<div dir="ltr">
<div>Hi,</div>
<div> </div>
<div>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</div>
<div>the image. Please see the image:</div>
<div><a moz-do-not-send="true"
href="http://expirebox.com/download/2f55c9d8f0bc83138b954d2fec08e78c.html">http://expirebox.com/download/2f55c9d8f0bc83138b954d2fec08e78c.html</a></div>
<div> </div>
<div>I am using Ubuntu Linux box to reconstruct. Two sequential
commands were used:</div>
<div> </div>
<div>./rtkvarianobigeometry --xml_file ProjectionInfo.xml --path
Scan0/ --regexp Proj_.*.hnd -o varianGeometry<br>
</div>
<div>and </div>
<div> </div>
<div>rtkfdk \<br>
--geometry varianGeometry \<br>
--regexp .*.hnd \<br>
--path ../Scan0 \<br>
--output rec.mha \<br>
--verbose \<br>
--spacing 2.0,2.0,2.0 \<br>
--dimension 256,100,256 \<br>
</div>
<div>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!</div>
<div> </div>
<div>-howard</div>
</div>
<br>
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<br>
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</blockquote>
<br>
<pre class="moz-signature" cols="72">--
--
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</pre>
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