Direct-conversion 50 μm pixel-pitch detector for digital mammography using amorphous selenium as a photoconductive switching layer for signal charge readout

A novel mammography detector with dual amorphous-Selenium (α-Se) layer has been developed that employs photoinduced discharge in its readout procedure. The detector consists of a bias electrode, a thick α-Se layer for x-ray-to-electron conversion, an electron-trapping layer (ETL), a thin α-Se layer for the photoinduced discharge, alternately arranged transparent and opaque stripe electrodes, and a linear optical source for readout. The detector directly converts x-rays into electrons to accumulate the electrons in ETL. When photoinduced discharge arises on the transparent electrodes through readout light irradiation, an accumulated electron image is transferred to the transparent electrodes and detected as signals at charge amplifiers, which we call 'Photoconductive Switching' readout. Readout efficiency and image readout speed have been improved enough to be used as a practical level by the adoption of the alternatelyarranged stripe electrodes. Furthermore, such a simple stripe-electrode structure accomplishes low electrical-noise readout and easy fabrication of fine-pitch pixels. A prototype 50 ìm pixel-pitch detector with the 18×24 cm imaging area has been prepared, which shows high DQE performances more than 64, 48, and 28% at 1, 4, and 7 lp/mm, respectively, not only in the usual dose range of around 100 μGy but at the low dose of 32 μGy. The 'Photoconductive Switching' readout mechanism makes it possible to realize the high DQE and the finest resolution of 50 μm in the direct-conversion detectors for full-field digital mammography.