Radiographic imaging characteristics of a direct conversion detector using selenium and thin film transistor array

Progress on the development of a semiconductor-based, direct-detection, flat-panel digital radiographic imaging device will be discussed. The device consists of a 500 mu m thick amorphous selenium sensor coupled to an amorphous silicon thin-film-transistor (TFT) readout matrix. This detector has an active imaging area of 14 '' x17 '', 3072x2560 pixels with dimensions 139 mu m x 139 mu m and a geometrical fill factor of 86 percent. Charges generated primarily as a consequence of photoelectric interaction between the incoming x-rays and Se are integrated on storage capacitors that are located at each pixel. The high electric field applied across the Se minimizes the lateral spreading of the signal resulting in a significantly higher spatial resolution when compared to conventional film/screen systems used for general radiography. The sensor array is read out one pixel line at a time by manipulating the source and gate lines of the TFT matric. Data are digitized to 14 bits. This paper will discuss the statistical photon counting analysis performed on an early prototype device. Measurements will include modulation transfer function, detector quantum efficiency, linearity, and noise analysis. Image analysis will include small contrast object visibility studies using a Faxil X-Ray Test Object T016. Advantages of this flat-panel electronic sensor over conventional systems are discussed.