Amorphous Selenium Flat Panel Detectors are direct conversion x-ray imagers, used in digital radiography for medical imaging applications. Direct conversion imagers utilize photoconductors, such as amorphous selenium (a-Se), to capture and convert incident x-ray photons directly into electric charge. X-ray photons incident upon a layer of a-Se generate electron-hole pairs via the internal photoelectric effect. A bias voltage applied to the depth of the selenium layer draw the electrons and holes to corresponding electrodes; the generated current is thus proportional to the intensity of the irradiation. Signal is then read-out using underlying readout electronics, typically by a thin-film transistor (TFT) array.
Indirect conversion detectors, the other leading digital radiography technology, utilize scintillators, typically either CsI (Cesium Iodide) or GOS (Gadolinium oxysulfide), to convert x-ray photons to optical photons, and then optical photons to charge. This two-step process, in which the signal profile is broadened due to optical scatter, leads to loss of spatial resolution and image degradation.
By eliminating the optical conversion step inherent to indirect conversion detectors, lateral spread of optical photons is eliminated, thus reducing blur in the resulting signal profile in direct conversion detectors. Coupled with the small pixel sizes achievable with TFT technology, a-Se direct conversion detectors can thus provide high spatial resolution. This high spatial resolution, coupled with a-Se's relative high quantum detection efficiency for low energy photons (< 30 keV), motivate the use of this detector configuration for mammography, in which high resolution is desirable to identify microcalcifications.