Digital radiology using active matrix readout: Amplified pixel detector array for fluoroscopy

Active matrix array technology has made possible the concept of flat panel imaging systems for radiography. In the conventional approach a thin-film c ircuit built on glass contains the necessary switching components (thin-fil m transistors or TFTs) to readout an image formed in either a phosphor or p hotoconductor layer. Extension of this concept to real time imaging-fluoros copy-has had problems due to the very low noise required. A new design stra tegy for fluoroscopic active matrix flat panel detectors has therefore been investigated theoretically. In this approach, the active matrix has integr ated thin-film amplifiers and readout electronics at each pixel and is call ed the amplified pixel detector array (APDA). Each amplified pixel consists of three thin-film transistors: an amplifier, a readout, and a reset TFT. The performance of the APDA approach compared to the conventional active ma trix was investigated for two semiconductors commonly used to construct act ive matrix arrays-hydrogenated amorphous silicon and polycrystalline silico n. The results showed that with amplification close to the pixel, the noise from the external charge preamplifiers becomes insignificant. The thermal and flicker noise of the readout and the amplifying TFTs at the pixel becom e the dominant sources of noise. The magnitude of these noise sources is st rongly dependent on the TFT geometry and its fabrication process. Both of t hese could be optimized to make the APDA active matrix operate at lower noi se levels than is possible with the conventional approach. However, the APD A cannot be made to operate ideally (i.e., have noise limited only by the a mount of radiation used) at the lowest exposure rate required in medical fl uoroscopy. (C) 1999 American Association of Physicists in Medicine. [S0094- 2405(99)01205-5].