A novel amorphous selenium (a-Se) detector with the hexagonal pixel has been developed for full-field digital
mammography. The pixel area of the detector was designed to be same as that of the 68 μm square pixel detector, while
the pixel pitch between neighboring pixels was set to be 73-75 μm in six directions. By applying the hexagonal pixels,
the sensitivity of the detector has improved by 18% compared with a conventional square pixel. A simulation showed
that the hexagonal pixel provided a more uniform electric field in the a-Se layer than the square pixel, which has lead to
higher sensitivity. The modulation transfer function of the detector was 92 % at 2 mm-1 and 62 % at 5 mm-1. These
values were as high as that of a conventional a-Se detector with 50 μm square pixels. As a result, the detective quantum
efficiency of this detector achieved 75 % with 5 mR and 72 % with 2.5mR at 2 mm-1. The exposure conditions were 28
kV W/Rh with a 2 mm aluminum filter. Therefore, the new detector can reduce the exposure dose while maintaining a
high image quality.
In this study, we characterized the image quality of two types of indirect-conversion flat-panel detectors: an X-ray
incident-side photo-detection system (IS) and an X-ray penetration-side photo-detection system (PS). These detectors
consist of a Gd2O2S:Tb (GOS) scintillator coupled with a photodiode thin film transistor (PD-TFT) array on a glass
substrate. The detectors have different X-ray incident directions, glass substrates, and scintillators. We also characterized
the effects of layered scintillator structures on the image quality by using a single-layered scintillator containing large
phosphor grains and a double-layered scintillator consisting of a layer of large phosphor grains and a layer of small
phosphor grains. The IS system consistently demonstrated a higher MTF than the PS system for a scintillator of the same
thickness. Moreover, the IS system showed a higher DQE than the PS system when a thick scintillator was used. While
the double-layered scintillators were useful for improving the MTF in both systems, a thick single-layered scintillator
was preferable for obtaining a high DQE when the IS system was applied. These results indicate that an IS system can
efficiently utilize the light emitted from the phosphor at the far side of the PD without the occurrence of blurring. The
use of IS systems makes it possible to increase the thickness of the scintillator layer for improving the sensitivity without
reducing the MTF, which increases the DQE. The DQE of the IS system was 1.2 times that of the PS system, despite the
absorption of X-rays at the glass substrate before entering the phosphor.
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