Why CT number to electron density calibration curve is not straight line?

The CT number to electron density calibration curve is not a straight line due to the nonlinear relationship between the attenuation of X-rays in a material and its electron density. This nonlinearity arises from factors such as variations in composition, atomic number, and the energy of the X-rays used in the CT scan. Additionally, different materials exhibit different scattering and absorption characteristics, leading to deviations from a linear correlation. As a result, the calibration curve often requires polynomial or piecewise functions to accurately represent the relationship across varying densities.