[Rtk-users] Obtaining HU number from RTK float data after CBCT reconstruction

[Yang K Park]

Hi RTK users,

 

This could be a class question but  I couldn’t have any chance to ask it so
far.

 

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.

 

In my experience, this value is quite different from that of helical CT.

For example, µ for water ~= 0.015 [mm-1] in RTK (measured in regions without
scatter artifacts) vs ~0.027 [mm-1] in CT. And that µ value is also affected
by mAs value as well.

 

My goal is to convert that float value to HU number with a reasonable
explanation. 

 

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:

 

Elekta CBCT number  = µ * 65536 – 1024  (µ seems to be the float value
identical to that comes from RTK reconstruction)

 

, which has no relationship to HU definition.

 

And I’m wondering why they are using the above formula instead of using
classic HU definition ( HU =  (µ - µ_w / µ_w)*1000). 

 

Of course I’m aware of that CBCT number should be quite different from CT
number due to many artifact sources.

I’m just trying to get some physically reasonable conversion method between
CBCT number to HU number.

 

Thank you guys for your help in advance!

 

Yang

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