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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'>In addition to my previous question,<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'>When I do a forward projection/FDK for my CT image (HU value), what I do is:<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoListParagraph style='text-indent:-.25in;mso-list:l1 level1 lfo2'><![if !supportLists]><span style='color:#1F497D'><span style='mso-list:Ignore'>1)<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='color:#1F497D'>Add 1024 to CT HU (to make signed short to unsigned short)<o:p></o:p></span></p><p class=MsoListParagraph style='text-indent:-.25in;mso-list:l1 level1 lfo2'><![if !supportLists]><span style='color:#1F497D'><span style='mso-list:Ignore'>2)<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='color:#1F497D'>Multiply 1/65536 for each voxel value (ushort to float)<o:p></o:p></span></p><p class=MsoListParagraph style='text-indent:-.25in;mso-list:l1 level1 lfo2'><![if !supportLists]><span style='color:#1F497D'><span style='mso-list:Ignore'>3)<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='color:#1F497D'>Forward projection<o:p></o:p></span></p><p class=MsoListParagraph style='text-indent:-.25in;mso-list:l1 level1 lfo2'><![if !supportLists]><span style='color:#1F497D'><span style='mso-list:Ignore'>4)<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='color:#1F497D'>FDK reconstruction<o:p></o:p></span></p><p class=MsoListParagraph style='text-indent:-.25in;mso-list:l1 level1 lfo2'><![if !supportLists]><span style='color:#1F497D'><span style='mso-list:Ignore'>5)<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='color:#1F497D'>Multiply 65536 (float to ushort)<o:p></o:p></span></p><p class=MsoListParagraph style='text-indent:-.25in;mso-list:l1 level1 lfo2'><![if !supportLists]><span style='color:#1F497D'><span style='mso-list:Ignore'>6)<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='color:#1F497D'>Subtract 1024 (ushort to short)<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'>Then, I can get the exact same CT volume as used for input.<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'>Is this a reasonable method or am I doing a kind of silly thing?<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> Yang K Park [mailto:theday79@gmail.com] <br><b>Sent:</b> Monday, March 02, 2015 8:05 PM<br><b>To:</b> rtk-users@openrtk.org<br><b>Subject:</b> 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>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>