Energy-resolving photon counting detectors (ERPCDs) used for medical X-ray diagnosis have the potential to extract quantitative information concerning various organs. Previously, we proposed an analysis procedure to determine the effective atomic number (Zeff), but parameter optimization is required. The purpose of this study is to develop an imaging simulator and to derive optimal imaging conditions. First, we created an imaging simulator taking into consideration the following three elements: (1) X-ray attenuation in objects, (2) detector response of cadmium zinc telluride (CZT), and (3) statistical fluctuation. Second, virtual phantoms consisting of soft tissue (Zeff=6.5) and bone (Zeff=13) with mass thicknesses (ρt) of 0.1 to 80g/cm2 were made, and the Zeff images were generated under various imaging conditions related to tube voltages (60 to 120kV) and energy bin settings (10keV intervals). Then, the optimal imaging condition was derived from the accuracy analysis of Zeff images. Furthermore, a demonstration of clinical quantitative images was performed by using digital phantom which was created from voxel data of the CT image of human body phantom. The optimized imaging conditions were a tube voltage of 120kV with energy bin settings of 20–30–50–120keV. The applicable thickness ranges (achieving differences of +/- 1.0 from theoretical Zeffvalues) for Zeff=13 and 6.5 were ρt=0.8 to 10g/cm2 and 2 to 20g/cm2, respectively. Using the above imaging conditions, we succeeded in generating accurate quantitative images of the pelvic region.
A photon counting system has the ability of energy discrimination, therefore obtaining new information using X-rays for material identification is an expected goal to achieve precise diagnosis. The aim of our study is to propose a novel method for material identification based on a photon counting technique. First, X-ray spectra at 40-60 kV were constructed using a published database. Second, X-ray spectra penetrating different materials having atomic numbers from 5-13 were calculated. These spectra were divided into two energy regions, then linear attenuation factors concerning these regions were obtained. In addition, in order to accomplish highly accurate material identification, correction of beam hardening effects based on soft-tissue was applied to each linear attenuation factor. Then, using the linear attenuation factors, a normalized linear attenuation coefficient was derived. Finally, an effective atomic number was determined using the theoretical relationship between the normalized linear attenuation coefficient and atomic number. In order to demonstrate our method, four different phantoms (acrylic, soft-tissue equivalent, bone equivalent, and aluminum) were measured using a single-probe-type CdTe detector under the assumption that the response of the single-probe-type CdTe detector is equal to the response of one pixel of a multi-pixel-type photon counting detector. Each of these phantoms can be completely separated using our method. Furthermore, we evaluated an adoptive limit of beam hardening correction. We found that the adoptive limit depends on the mass thickness and atomic number. Our vision is to realize highly accurate identification for material with narrow range in atomic number.
Currently, an X-ray imaging system which can produced information used to identify various materials has been
developed based on photon counting. It is important to estimate the response function of the detector in order to
accomplish highly accurate material identification. Our aim is to simulate the response function of a CdTe detector using
Monte-Carlo simulation; at this time, the transportation of incident and scattered photons and secondary produced
electrons were precisely simulated without taking into consideration the charge spread in the collecting process of the
produced charges (charge sharing effect). First, we set pixel sizes of 50-500μm, the minimum irradiation fields which
produce equilibrium conditions were determined. Then, observed peaks in the response function were analyzed with
consideration paid to the interactions between incident X-rays and the detector components, Cd and Te. The secondary
produced characteristic X-rays play an important role. Accordingly ratios of full energy peak (FEP), scattering X-rays
and penetrating X-rays in the calculated response functions were analyzed. When the pixel size of 200μm was used the
scattered X-rays were saturated at equilibrium with relatively small fields and efficiency of FEP was kept at a high value
(<50%). Finally, we demonstrated the X-ray spectrum which is folded by the response function. Even if the charge
sharing effect is not completely corrected when using the electric circuit, there is a possibility that disturbed portions in
the measured X-ray spectra can be corrected by using proper calibration, in which the above considerations are taken
into account.
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