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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link="#0563C1" vlink="#954F72"><div class=WordSection1><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>