Reversal Trend of Hounsfield Unit Values of Substances with High and Low Effective Atomic Numbers

In dual-energy computed tomography (DECT), the Hounsfield values of a substance measured at two different energies are the basic data for finding the chemical properties of a substance. The trends of Hounsfield unit (HU) alterations following the changes in energy are different between the materials with high and low Zeff. The present study aimed to analyze the basic principles related to the attenuation coefficient of x-ray photons and a quantitative explanation is given for the mentioned behavior or trend.
A mathematical expression was derived for the HU difference between two different scanner voltages. Attenuation coefficients of diverse substances, such as methanol, glycerol, acetic acid, the aqueous solution of potassium hydroxide, and water were calculated for x-ray scanners operating differently at distinct applied voltages and with diverse inherent or added filters.
Findings of the current study demonstrated that the negative or positive outcome of HU(V1) - HU(V2) equation is not determined by the electron density of a substance. However, it is affected by the effective atomic number (Zeff) of the material and machine parameters specified by the source spectrum.
According to our results, the sign of HU difference [HU(V1) – HU(V2)] for the variable cases of V2 and V1 gives an indication of the effective atomic number of the material under study. The obtained results might be of diagnostic value in the DECT technique.