K-edge imaging with a photon counting CT system

The purpose of our research is to measure the amount of contrast agents used in x-ray CT accurately. The use of contrast agents such as gold nano-particles enables molecular imaging with the x-ray CT system. To realize an energy discriminating measurement, we have developed a photon counting detector with a CdTe semiconductor that is able to measure x-rays with the count rate of 10⁷ cps/mm² with four energy windows. In order to measure the concentration ratio of the contrast agent, we used a k-edge imaging technique. The materials used in this experiment were Au-colloid, Pt-colloid and gadolinium solution. The results of experiments showed linearity between the linear attenuation coefficients of k-edge images that were calculated using the two energy bins near each k-edge, and the concentration ratios of the contrast agents. For Au-colloid, the measurement error was less than 4 %, while for the gadolinium agent, the accuracy depended on the window width. This was caused by the electrical noise in the lower energy window and the beam-hardening effect in the measured data.