Novel material identification method using three energy bins of a photon counting detector taking into consideration Z-dependent beam hardening effect correction with the aim of producing an X-ray image with information of effective atomic number

Energy resolved photon counting detectors have been developed and currently in use, we have proposed practical methods for producing an X-ray image having effective atomic number information by means of plain X-ray diagnosis. An effective atomic number image is hoped to be used to enhance its value and to carry out more precise clinical diagnosis. In this study, we aimed to propose a novel material identification method using three energy bins of a photon counting detector. First, we prepared theoretical X-ray spectra and X-ray spectra folded with the response function of a multi-pixel-type CdTe detector. The response function was simulated using the Monte-Carlo simulation code EGS5. Next, the X-ray spectra were divided into three energy bins and the products oflinear attenuation coefficient and material thickness for low, middle, and high energy bins were derived from the difference of counts before and after the X-ray penetrated the material. In order to accomplish accurate material identification, beam hardening corrections for derived attenuation factors were individually applied. In order to take consideration the dependence of atomic numbers on the beam hardening corrections, we propose a novel method in which the relationship between mass thickness multiplied by mass attenuation coefficient and attenuation factor was used. Then, using the corrected attenuation factors, normalized linear attenuation coefficients for lower and higher energy bins were calculated, and they were converted to effective atomic numbers using the theoretically determined relationships. In the present study, two different effective atomic numbers, $\mathrm {Z}_{\mathrm {Low}}$ and $\mathrm {Z}_{High}$, were individually determined from the analysis of lower and higher energy bins. Here, if beam hardening corrections were properly calculated, $\mathrm {Z}_{\mathrm {Low}}$ and $\mathrm {Z}_{High}$ become an equal value which is also consistent with the true value. Using virtual materials for $\mathrm {Z}=5-13$ with mass thicknesses of $1-10[\mathrm {g}/\mathrm {cm}^{2}]$, we confirmed that our method works properly.