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 circuit built on glass contains the necessary switching components (thin‐film transistors or TFTs) to readout an image formed in either a phosphor or photoconductor layer. Extension of this concept to real time imaging—fluoroscopy—has had problems due to the very low noise required. A new design strategy for fluoroscopic active matrix flat panel detectors has therefore been investigated theoretically. In this approach, the active matrix has integrated thin‐film amplifiers and readout electronics at each pixel and is called 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 matrix was investigated for two semiconductors commonly used to construct active matrix arrays—hydrogenated amorphous silicon and polycrystalline silicon. 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 become the dominant sources of noise. The magnitude of these noise sources is strongly dependent on the TFT geometry and its fabrication process. Both of these could be optimized to make the APDA active matrix operate at lower noise levels than is possible with the conventional approach. However, the APDA cannot be made to operate ideally (i.e., have noise limited only by the amount of radiation used) at the lowest exposure rate required in medical fluoroscopy.