The multi-resolution technology was developed for dynamic flat panel detectors for X-ray imaging. This multi-resolution
technology allows us to switch between the 1 x 1 mode (150 μm square) and the 2 x 2 mode (300 μm square)
instantaneously, using an external control. We developed a 17" x 17" dynamic detector using this multi-resolution
technology. This novel dynamic detector has a high sensitivity and a high-speed readout and it can reduce the radiation
exposure dose and deliver a smooth image. The key feature of our multi-resolution technology is capable of reading 4
pixel signals simultaneously. The sensitivity and the readout speed in the 2 x 2 mode were 4 times higher than those in
the 1 x 1 mode. The multi-resolution technology was implemented using a unique thin film transistor structure; that is,
one pixel has two switches, each of which are turned on/off depending on the readout mode. As a result, the dynamic
detector with a large active area of 17" x 17" realized a high detective quantum efficiency value of 40% under the low
radiation of RQA5 20 nGy and a high-speed readout of 30 frames/sec. This multi-resolution technology made it possible
to reduce the radiation exposure dose in a variety of applications.
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.
We have developed a novel direct conversion detector for digital radiography by using a fullerene (C60)-doped polymer
layer added on a thick amorphous selenium (a-Se) layer coupled to an amorphous silicon thin-film transistor (a-Si TFT)
array. This detector exhibits considerable improvement in the lag characteristics and durability in high ambient
temperatures. The C60-doped polymer layer, which is directly and uniformly solution cast on the a-Se layer and
followed by an inorganic electron-transporting layer, smoothly changes the electronic junction between the a-Se layer
and the inorganic layer. It lubricates the emission of photocurrents from the a-Se photo-conversion layer and leads to the
improved lag characteristics. Another merit of using a C60-doped polymer is that it is stabile in high-temperature
ambient conditions and is not degraded by humidity or a large amount of X-ray exposure. The polymer layer prevents the
crystallization of a-Se, which otherwise occurs on exposure of a-Se to high temperature not only during the deposition of
the inorganic layer or the metal electrode layer in the manufacturing process but also in actual use.
A prototype detector, with a size of 17 in × 17 in and a pixel pitch of 150 μm, exhibited a good resolution; its DQE is
approximately 48% at 1 cy/mm in 258 μC/kg (RQA5). This new development can simplify cooling apparatus and
detector modules and also make a wide range of operational environments available. In addition, the improved lag
characteristics make it possible to reduce the exposure intervals for static imaging, tomosynthesis, and other various
exposure techniques.
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