In Catphan504’s offsets setting
‘Uniformity’: -65, line587
but in Catphan504 user manual write Solid image uniformity module is 80mm from the center of CTP404. (page.7)
65mm is near the junction of CTP515 and CTP486.
There may be a greater mistake
Indeed, the manual indicates the center of the uniformity phantom is 80mm away from the 404 slice. However, during the testing of the catphan module, it turned out a lot of people’s CBCT data didn’t include the full uniformity module. This is because Varian’s CBCT is somewhere around16cm long. If the user aligns the catphan to the white dots near the high resolutions module, the center of the uniformity slice is outside the scan. Although aligning to the HU module is recommended, many users align to the white dots. The 65mm shift rather than 80mm was a pragmatic decision to allow those users who had different alignment procedures to still use pylinac. In my internal testing the HU values at 65mm were never significantly different than those at 80mm.
If you want to use 80mm just set that value to -80 and you’re good to go. For pylinac 2.0 this would look like:
from pylinac import CatPhan504 CatPhan504.offsets['Uniformity'] = -80 cbct = CatPhan504(...)
It would look similar but not exactly the same for previous versions.
Let me know if you find any issues. Thanks!
James