Feasibility study of photon counting detector for producing effective atomic number image

Currently, energy resolving photon counting detectors (ERPCD) are the focus of attention as a promising technique for material identification based on the analysis of X-ray energies. To accomplish highly accurate material identification, consideration of the response of a multi-pixel-type ERPCD is important. This is because the detector response distorts initial energy information which is strongly related to material information such as effective atomic number (Z
_eff
). In the analysis using the ERPCD, we should take into consideration the beam hardening effect depending on the Z
_eff
of an object. The aim of our study is to propose a novel material identification method with correction for both detector response and beam hardening effect. In order to derive the Z
_eff
of an object, we used the differences in attenuation factors which are calculated from the analysis of three different energy bins in the detected X-ray spectrum. For the detected X-ray spectrum, correction of detector response and beam hardening effect were applied to attenuation factors. Next, Z
_eff
of the object was derived from a novel procedure in which the theoretical relationship between attenuation factors and Z
_eff
s was parameterized as a reference curve. In order to demonstrate the accuracy of Z
_eff
determination, we evaluated our method by applying it to each pixel of an X-ray image measured with a proto-type imaging detector and the produced Z
_eff
image. In most of the cases, we verified that the Z
_eff
image can be obtained with an accuracy of Z
_eff
+/- 0.5. This study gives us the prospect to establish a novel quantitative diagnosis using an Z
_eff
image determined from plain X-ray diagnosis.