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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoNormal><span style='color:#1F497D'>Hi Rune, <o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>That is a great tip. Thank you so much. (We’ve recently installed XVI5.0 so I’m better off trying to use that new feature.)<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>My remaining question is, then, when you guys do a forward projection for clinical CT images (singed short) using rtk, which pre-processing do you do?<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>I think some “short to float” conversion is needed and that conversion formula should be also working with FDK to restore the CT image with same HU values.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Thanks.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Yang<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><div><div style='border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b>From:</b> Rune Slot Thing [mailto:Rune.Slot.Thing@rsyd.dk] <br><b>Sent:</b> Tuesday, March 03, 2015 3:29 AM<br><b>To:</b> 'Yang K Park'; rtk-users@openrtk.org<br><b>Subject:</b> SV: [Rtk-users] Obtaining HU number from RTK float data after CBCT reconstruction<o:p></o:p></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><span lang=DA style='color:#1F497D'>Hi Yang,<o:p></o:p></span></p><p class=MsoNormal><span lang=DA style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span 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 style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span 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 style='color:#1F497D'><o:p> </o:p></span></p><div><p class=MsoNormal><span style='color:#1F497D'>Hope this helps – otherwise feel free to ask again.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Best regards,<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Rune Slot Thing<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>PhD Student<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Institute of Clinical Research, University of Southern Denmark<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Laboratory of Radiation Physics, Odense University Hospital<o:p></o:p></span></p><p class=MsoNormal><span lang=DA style='color:#1F497D'><a href="mailto:rune.slot.thing@rsyd.dk"><span style='color:blue'>rune.slot.thing@rsyd.dk</span></a><o:p></o:p></span></p></div><p class=MsoNormal><span lang=DA style='color:#1F497D'><o:p> </o:p></span></p><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span lang=DA style='font-size:10.0pt;font-family:"Tahoma",sans-serif'>Fra:</span></b><span lang=DA style='font-size:10.0pt;font-family:"Tahoma",sans-serif'> Rtk-users [<a href="mailto:rtk-users-bounces@public.kitware.com">mailto:rtk-users-bounces@public.kitware.com</a>] <b>Pĺ vegne af </b>Yang K Park<span style='color:#1F497D'> </span><br><b>Sendt:</b> 3. marts 2015 02:05<br><b>Til:</b> <a href="mailto:rtk-users@openrtk.org">rtk-users@openrtk.org</a><br><b>Emne:</b> [Rtk-users] Obtaining HU number from RTK float data after CBCT reconstruction<o:p></o:p></span></p></div></div><p class=MsoNormal><span lang=DA><o:p> </o:p></span></p><p class=MsoNormal>Hi RTK users,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>This could be a class question but I couldn’t have any chance to ask it so far.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>After the RTK reconstruction, I can get float values for each voxel which might be linear attenuation coefficients (µ [mm-1]) coming from logarithm calculation.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>In my experience, this value is quite different from that of helical CT.<o:p></o:p></p><p class=MsoNormal>For example, µ 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 µ value is also affected by mAs value as well.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>My goal is to convert that float value to HU number with a reasonable explanation. <o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>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></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal style='text-indent:.5in'>Elekta CBCT number = µ * 65536 – 1024 (µ seems to be the float value identical to that comes from RTK reconstruction)<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>, which has no relationship to HU definition.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>And I’m wondering why they are using the above formula instead of using classic HU definition ( HU = (µ - µ_w / µ_w)*1000). <o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>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></p><p class=MsoNormal>I’m just trying to get some physically reasonable conversion method between CBCT number to HU number.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Thank you guys for your help in advance!<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Yang<o:p></o:p></p></div></body></html>