CT-density table affecting the calculation when all the densities are overriden

[Pedro]

Hello, I have seen in Pinnacle something I do not understand:

We have a synthetic CT of a phantom and we override the density to assign it manually. When we change the CT-density table by a slightly different one, the dose calculated in the phantom changes despite the density is overriden (it is a small change, but not totally negligible: from 158.4 cGy to 157.6 cGy).

No other parameter is changed, only the CT table.

My understanding is that the CT-density table is used to get the density (and mass atenuation coefficients) from the CT images, but if we override the density we are already providing the density and we are telling the system "trust this density, not the original CT number". Therefore, if the density is overriden in all the pixels which are not air, a change in the CT table shouldn't have any impact in the calculation.

Please tell my if I am obfuscated or missing something. Is this change expected or it could be a bug?

My version is 16.2

Thanks in advance!

Pedro Almendral 

[Mohr, Dr. Peter]

Dear Pedro,


in principle you are right: a change of the density table should not change the dose calculation with density-override. However, in reality there are some numerical problems:


1) Depending on the size of the ROI with density-override, you may observe minor changes in the source-surface distance. (This is also sensitive to the chosen OutsideAirThreshold.)


2) At least in earlier versions, the "density override" function worked very indirectly. For a voxel with "density override", first Pinnacle calculates the corresponding HU from the density table and sets this voxel to the override-HU. Then Pinnacle calculates the dose using the the original HU outside the override region and the override-HU in the override region. All these HU are converted to density by the density table; i.e., the "density override" voxels are converted from the override density to override-HU in a first step and from override-HU to density in a second step. This back-and-forth procedure depends on the interpolation between the calibration points in the chosen density table. (Tou may check this behavior by setting a density outside your density table - this should lead to an error message because the above back-and-forth procedure cannot work.)


I have found very minor changes for the calculated dose (<< 1%) as long as the density override occurs for densities close to 1.0 (where many well-defined points are found in my density tables), but somewhat larger changes (but still below 1%) for low densities (e.g., 0.5) or high densities (e.g., 2.5) where my density tables have only few points.


From my point of view, the 0.5% change in your case seems to indicate that at least one of your density tables has very few points (maybe, with some scatter) around your override density.


Best regards,

Peter


Dr. Peter Mohr

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From: pinnacl...@googlegroups.com <pinnacl...@googlegroups.com> on behalf of Pedro <palme...@gmail.com>
Sent: Tuesday, March 19, 2019 12:50 PM
To: pinnacle3-users
Subject: [p3rtp] CT-density table affecting the calculation when all the densities are overriden

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