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<p class="MsoNormal"><span style="color:#1F497D">Hi Yang,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">The short answer to your question is that the projection images you acquire on an Elekta CBCT unit are not normalized properly. If you perform the same scan with two different exposures (e.g. double
the mAs value), you will get different projection images and hence different grey values in the reconstructed images, regardless of whether you use the XVI reconstruction or RTK. Since the Elekta system does not record your unattenuated signal anywhere, it
is not possible to do the proper I/I_0 normalization that you need in order to get to the attenuation coefficients required for the HU definition, and hence you cannot get to proper HU values from the projection images alone. Due to detector drift and all
the different sources of artefacts in CBCT imaging, it is a non-trivial problem to get this normalization right.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">With the latest release of the Elekta XVI 5.0 software, there is some calibration added on the software side to allow a pseudo-HU calibration of the CBCT grey values. Since we do not have the software
in our clinic yet, I have not investigated how exactly this is incorporated. My guess is that it is more of an empirical mapping which works well on objects of similar size, shape and composition as the calibration phantom used, than it is a proper calibration
of all projection images to the open field images that would give the correct normalization.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">Hope this helps – otherwise feel free to ask again.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">Best regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">Rune Slot Thing</span><span lang="EN-US" style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">PhD Student</span><span lang="EN-US" style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">Institute of Clinical Research, University of Southern Denmark</span><span lang="EN-US" style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">Laboratory of Radiation Physics, Odense University Hospital</span><span lang="EN-US" style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><a href="mailto:rune.slot.thing@rsyd.dk"><span style="color:blue">rune.slot.thing@rsyd.dk</span></a></span><span style="color:#1F497D"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">Fra:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Rtk-users [mailto:rtk-users-bounces@public.kitware.com]
<b>Pć vegne af </b>Yang K Park<br>
<b>Sendt:</b> 3. marts 2015 02:05<br>
<b>Til:</b> rtk-users@openrtk.org<br>
<b>Emne:</b> [Rtk-users] Obtaining HU number from RTK float data after CBCT reconstruction<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US">Hi RTK users,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">This could be a class question but I couldn’t have any chance to ask it so far.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">After the RTK reconstruction, I can get float values for each voxel which might be linear attenuation coefficients (” [mm-1]) coming from logarithm calculation.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">In my experience, this value is quite different from that of helical CT.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">For example, ” for water ~= 0.015 [mm-1] in RTK (measured in regions without scatter artifacts)
<i>vs</i> ~0.027 [mm-1] in CT. And that ” value is also affected by mAs value as well.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">My goal is to convert that float value to HU number with a reasonable explanation.
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">In our Elekta system, they seem to convert those float values to their own CBCT number ranging from “0-65535” by using a following formula:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-indent:36.0pt"><span lang="EN-US">Elekta CBCT number = ” * 65536 – 1024 (” seems to be the float value identical to that comes from RTK reconstruction)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">, which has no relationship to HU definition.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">And I’m wondering why they are using the above formula instead of using classic HU definition ( HU = (” - ”_w / ”_w)*1000).
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Of course I’m aware of that CBCT number should be quite different from CT number due to many artifact sources.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I’m just trying to get some physically reasonable conversion method between CBCT number to HU number.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Thank you guys for your help in advance!<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Yang<o:p></o:p></span></p>
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