Photon counting apparatus

A photon counting apparatus, comprising:

• an X-ray tube configured to generate X-rays;
• an X-ray detector configured to detect the X-rays generated by the X-ray tube and transmitted through a subject;
• data acquisition circuitry configured to acquire count data concerning a count number of the detected X-rays for a plurality of energy bands, based on an output signal from the X-ray detector;
• memory circuitry configured to store data of a response function that associates an energy of incident X-rays on the X-ray detector with a response characteristic of a system including at least one of the X-ray detector and the data acquisition circuitry;
• and
• processing circuitry configured to calculate an X-ray absorption amount of each of a plurality of base substances based on the count data concerning the plurality of energy bands acquired by the data acquisition circuitry, an energy spectrum of the incident X-rays generated by the X-ray tube, and the data of the response function stored in the memory circuitry.