Direct-Conversion X-Ray Detector with 50-µm High-Gain Pixel Amplifiers for Low-X-Ray-Dose Digital Mammography

• Published in: Journal of medical and biological engineering Vol. 35; no. 2; pp. 249 - 257
• Main Authors: Kuo, Tsung-Ter, Wu, Chien-Ming, Chan, Isaac, Lu, Hui-Hsin, Pao, Sun-Hua
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.04.2015
• Subjects: Biomedical Engineering and Bioengineering; Cell Biology; Engineering; Imaging; Originla Article; Radiology; X-ray detector; Thin-film transistor (TFT); Amorphous selenium (a-Se); Direct conversion; Active pixel sensor (APS)
• ISSN: 1609-0985; 2199-4757
• DOI: 10.1007/s40846-015-0024-9

This study proposes an active pixel sensor (APS) design for readout signal amplification to replace the traditional passive pixel sensor (PPS) design. The APS design adds only one extra thin-film transistor (TFT) amplifier, giving it the high signal sensitivity of the APS architecture and the high spatial resolution of the PPS architecture. The mobility of the amorphous silicon (a-Si) TFT is 0.56 cm 2 /Vs, the signal amplification of the APS circuit is more than 10×, the on/off current ratio is more than 10 7 , and the leakage current is on the order of 1 fA. A 200-µm-thick layer of amorphous selenium (a-Se) as the X-ray photoconductor was deposited on the APS array with 160 × 160 pixels of various sizes, down to 50 µm. The 40-kVp X-ray spectra obtained with tube currents of 40 and 50 mA, respectively, were used for measuring the gray-level contrast induced by the X-rays. The experimental results show that the gray-level contrasts for the 50-µm pixels were 5.57 ± 0.89 and 13.19 ± 1.94 and those for the 70-µm pixels were 15.94 ± 2.98 and 24.19 ± 2.76 under 40-kVp X-ray exposure at 40 and 50 mA, respectively.