Digital Detectors and Image Processing

Direct and Indirect Capture

A digital detector turns the remnant beam into an electronic signal in one of two ways. In a direct-conversion flat panel, a layer of amorphous selenium (a-Se), a photoconductor, absorbs x-ray photons and converts them straight into electrical charge, and an electric field pulls the charge down to the collecting electrodes with very little sideways spread. In an indirect-conversion detector, a scintillator first turns x-ray photons into light, and photodiodes then turn that light into charge. The usual scintillator is cesium iodide (CsI), grown as needle-shaped columns that channel light toward the photodiodes and limit its spread. Most indirect flat panels use an amorphous silicon (a-Si) photodiode and transistor array, while CCD and CMOS systems pass the scintillator light to a small chip through lenses or fiber optics.