1. A photon-counting X-ray CT apparatus comprising: an X-ray tube configured to irradiate a subject with X-rays; an X-ray detector including an X-ray detection element configured to detect photons of the X-rays and to convert the photon into an electrical signal; data collection circuitry configured to collect a count value by counting a number of electrical signals in each of a plurality of energy bins output by the X-ray detection element; correction circuitry configured to determine a true count value for each X-ray detection element in each of the plurality of energy bins, based on at least one of probability of escape in the X-ray detection element and probability of crosstalk in the X-ray detection element; and processing circuitry configured to generate a reconstruction image, based on the true count value.