1. A method for determining calibration factors for calibrating photon-counting detectors of a spectral imaging system, comprising: combining, with a processor, a heuristic calibration of the photon-counting detectors and an analytical calibration of the photon-counting detectors, wherein the analytical calibration is based on an analytical model; and generating, with the processor, a set of photon-counting calibration factors based on the combining of the heuristic calibration and the analytical calibration applying, with the processor, the calibration factors to measured computed tomography (CT) energy-resolved data from the photon-counting detectors of the spectral imaging system, generating corrected data; decomposing, with the processor, the corrected data into energy dependent components using the analytical model; and performing a spectral CT scan of a subject or an object with the spectral imaging system, generating the measured CT energy-resolved data.