Recently,spectral Computed Tomography( SCT) based on photon counting detectors( PCDs) has become a hot topic in the field of radiation imaging. By setting different energy thresholds in PCD,incident photons are counted into different energy bins,which provides us with signals carrying energy information. By processing CT data from multiple energy bins,material composition of scanned objects can be estimated from spectral CT. This PCD-based SCT images can be of high contrast-to-noise ratio( CNR) and high spa...