Fig 2. Gray value vs X-ray tube voltage after CT reconstruction.

The gray values have been extracted from reconstructed CT slices at different regions of the sample shown in Fig 8. This corresponds to different materials. The data in the right plot is fitted with: $W\left(U\right)=e^{a_{ct}{U}^{-2}+0.01a_{ct}{U}^{-1}+c_{ct}}$.