In this study, an experimental method based on theory is used to develop photoconductor that can replace the a-Se currently used as X-ray conversion layer in digital mammography. This is necessary because a-Se produced by the commercial fabrication method, of physical vapor deposition, has exhibited several problems when applied to digital mammography: instability due to crystallization and defect expansion due to high operating voltages, which is called the aging effect. Therefore, our work focused on developing a method of fabricating X-ray conversion films that do not suffer from crystallization and X-ray damage and optimizing the factors affecting the properties of the candidate photoconductors in order to acquire sufficient electrical signals to detect minute calcifications. The photoconductors were initially selected after the requirements for X-ray conversion materials, such as high atomic absorption, density, band-gap energy, work function, and resistivity, were examined. We selected HgI 2 , PbI 2 , and PbO because of their basic properties. Next, we experimentally investigated the performance of film samples fabricated by sedimentation and screen printing instead of physical vapor deposition. The structure of the X-ray conversion films (e.g., the thickness, electrodes, and blocking layer) were optimized for the application of a relatively low voltage to the X-ray conversion layer. The performance of the films were morphologically and electrically evaluated under mammography X-ray exposure conditions, and compared with those of a-Se films produced by physical vapor deposition. PbO appeared to be the most suitable alternative material because its electrical properties, such as the dark current, sensitivity, and signal-to-noise ratio (SNR), did not reveal the X-ray damage problem, and thus were maintained after repeated exposure to X-rays. Although PbO showed low sensitivity to X-ray exposure, its SNR was superior to that of the other materials, which is expected to improve its detective quantum efficiency, one of the factors used in evaluating images acquired by digital mammography.

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The clinical use of radiation image detectors is influenced by the degree to which patients are exposed to radiation. Phosphors are being used as the radiation receptor materials in a number of radiation imaging systems for the detection of radiation. Rare earth phosphors such as those of Gd, Y, Lu, and La are attracting attention in particular as they exhibit improved properties. However, there has not been any research on the conditions for the synthesis of these phosphors, including the optimal concentrations in which the sensitizer should be added to them. Therefore, in this study, the optimal conditions for the phosphor synthesis were determined by analyzing the characteristics of the phosphors fabricated using various sensitizer concentrations. The deposition method used to form films of the synthesized phosphors was screen printing. This technique is suitable for large-area deposition and allowed for imaging to be performed in conjunction with a complementary metal-oxide semiconductor (CMOS) image detector. The phosphors synthesized were Gd 2 O 3 :Eu and Lu 2 O 3 :Eu, and the sensitizer used was citric acid, which was added in varying concentrations (0.00-0.05 g) to the phosphors during synthesis. Films of the phosphors 5 × 5 cm in size, which was the size of the active area of the CMOS image sensor, and 100-250 µm in thickness were formed. The structural characteristics of the phosphors were determined through X-ray diffraction analyses and scanning electron microscopy, and the optical characteristics through photoluminescence (PL) measurements. A CMOS-based X-ray detector was manufactured by attaching the phosphor films to the CMOS image sensor and evaluating the modulation transfer functions of the images obtained. The results showed that of all the phosphor samples synthesized, the Gd 2 O 3 :Eu and Lu 2 O 3 :Eu samples synthesized using 0.02 g of citric acid exhibited the best luminescence characteristics.

In the Photo-Induced Discharge (PID) method of X-ray detection, image information generated by X-rays is derived using a readout laser. The structure of the PID detector consists of multiple layers with different functionalities. A high voltage is applied between the top and bottom layers, generating an electric field for separating the electron-hole pairs (EHPs) generated by X-rays in a photoconductor layer. A separate dielectric layer is used to trap EHPs. Improvements to detection efficiency are accomplished by selecting new photoconductor materials as a substitute for amorphous selenium (a-Se), commonly used in existing PID methods. The photoconductor material also determines the optimal wavelength of an appropriate readout laser. Lead oxide, lead iodide, mercury iodide, and mercury iodide (with additives) were tested as alternative photoconductor materials with low dark current. The readout laser used here operates at wavelengths of 459 nm and 620 nm. The reaction characteristic on the readout laser was measured in all four types of photoconductors. Detection properties were compared to a flat panel detector. No reaction characteristics could be measured for lead oxide and lead iodide for either wavelength. The mercury iodide photoconductor (including additives) reacted to the 495 nm wavelength, but showed a weak, inconsistent signal for different measurements. The mercury iodide photoconductor showed excellent characteristics for both wavelengths, but was superior for a wavelength

In recent times, digital X-ray detectors have been actively applied to the medical field; for example, digital radiography offers the potential of improved image quality and provides opportunities for advances in medical image management, computer-aided diagnosis and teleradiology. In this study, two candidate materials (HgI 2 and PbI 2 ) have been employed to study the influence of the dielectric structure on the performance of fabricated X-ray photoconducting films. Parylene C with high permittivity was deposited as a dielectric layer using a parylene deposition system (PDS 2060). The structural and morphological properties of the samples were evaluated field emission scanning electron microscopy and X-ray diffraction. Further, to investigate improvements in the electrical characteristics, a dark current in the dark room and sensitivity to X-ray exposure in the energy range of general radiography diagnosis were measured across the range of the operating voltage. The electric signals varied with the dielectric layer structure of the X-ray films. The PbI 2 film with a bottom dielectric layer showed optimized electric properties. On the other hand, in the case of HgI 2 , the film with a top dielectric layer showed superior electric characteristics. Further, although the sensitivity of the film decreased, the total electrical efficiency of the film improved as a result of the decrease in dark current. When a dielectric layer is deposited on a photoconductor, the properties of the photoconductor, such as hole-electron mobility, should be considered to improve the image quality in digital medical imaging application. In this study, we have thus demonstrated that the use of dielectric layer structures improves the performance of photoconductors.