This review consists on the progress of flat-panel detectors for digital radiography based on amorphous materials; flat panel detectors remain the essential element for radiography improvement. The X-ray converter and the readout pixel array consists the most important components of flat-panel detectors where their parameters and developments will be described. This review also presents the innovative development perceptions of new detectors. In addition, we will define the cascaded linear systems method because it is a very influential tool for improving the design and assessment of X-ray imaging detector systems.