EMPIRICAL-INVESTIGATION OF THE SIGNAL PERFORMANCE OF A HIGH-RESOLUTION, INDIRECT DETECTION, ACTIVE-MATRIX FLAT-PANEL IMAGER (AMFPI) FOR FLUOROSCOPIC AND RADIOGRAPHIC OPERATION

Signal properties of the first large-area, high resolution, active mat rix, flat-panel imager are reported. The imager is based on an array o f 1536 x 1920 pixels with a pixel-to-pixel pitch of 127 mu m. Each pix el consists of a discrete amorphous silicon n-i-p photodiode coupled t o an amorphous silicon thin-film transistor. The imager detects incide nt x rays indirectly by means of an intensifying screen placed over th e array. External acquisition electronics send control signals to the array and process analog imaging signals from the pixels. Consideratio ns for operation of the imager in both fluoroscopic and radiographic m odes are detailed and empirical signal performance data are presented with an emphasis on exploring similarities and differences between the two modes. Measurements which characterize the performance of the ima ger were performed as a function of operational parameters in the abse nce or presence of illumination from a light-emitting diode or x rays. These measurements include characterization of the drift and magnitud e of the pixel dark signal, the size of the pixel switching transient, the temporal behavior of pixel sampling and the implied maximum frame rate, the dependence of relative pixel efficiency and pixel response on photodiode reverse bias voltage and operational mode, the degree of linearity of pixel response, and the trapping and release of charge f rom metastable states in the photodiodes. In addition, x-ray sensitivi ty as a function of energy for a variety of phosphor screens for both fluoroscopic and radiographic operation is reported. Example images of a line-pair pattern and an anthropomorphic phantom in each mode are p resented along with a radiographic image of a human hand. General and specific improvements in imager design are described and anticipated d evelopments are discussed. This represents the first systematic invest igation of the operation and properties in both radiographic and fluor oscopic modes of an imager incorporating such an array. (C) 1997 Ameri can Association of Physicists in Medicine.