Sanghee Nam

Improvement in photoconductor film properties by changing dielectric layer structures

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 (HgI2 and PbI2) 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 PbI2 film with a bottom dielectric layer showed optimized electric properties. On the other hand, in the case of HgI2, 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.

Mercury iodide flat panel radiation detector for simultaneous acquisition of static and moving image

Mercuric iodide deposited on flat panel thin film transistor (TFT) array is one of the best alternate photoconductive materials for direct digital X-ray detectors for both static and moving image application in medical imaging. The mercuric iodide is coated onto the array by a Particle-In-Binder (PIB) process and scaled up to the 7inch 8.5inch size required in common medical imaging application. A TFT array with a pixel pitch of 139microns was used for detector. Mercuric iodide coating thickness around 200 microns was tested with beam energy between 40kVp and 100kVp utilizing exposure ranges typical for both static and dynamic imaging. Detector performances were evaluated by obtained images. Mercuric iodide deposited on flat panel thin film transistor (TFT) array is shown to exhibit high sensitivity to X-rays, excellent spatial resolution and high Detective Quantum Efficiency (DQE). Especially it is quite suitable for moving image because of low image lag. Resolution tests on resolution target phantoms showed that resolution is limited to the Nyquist frequency for the 139 microns (resolution ~3.6lp/mm) pixel detectors. The ability to operate at low voltages (~100V) gives adequate dark currents for most application and allows low voltage electronics designs. Also the detector can use exceptionally low dose-rate X-ray illumination because of the very high X-ray sensitivity, which exceeds any other known X-ray detector material. The fabricated detector represents the most advanced photoconductor material available today for flat panel, high resolution, x-ray, medical detector, which alternates conventional a-Se technology.

Application of charge-injection devices for digital X-ray imaging using a planar gas-type X-ray detector

We have developed a planar gas-type detector, based on a charge injection device; this device can be used for digital X-ray imaging. Previously, in order to obtain X-ray images, a planar gas-type detector utilized a line-scanning module based on a one-dimensional readout system; however, that technology suffered from a limitation such as a long readout time, not suitable for a fluoroscopy or a moving imaging acquisition. In this study, a readout module based on charge-injection devices was used in conjunction with the planar gas-type detector to acquire signals and two-dimensional digital images. In the original design, two orthogonally cross-shaped top electrodes, called X address and Y address, played important roles in transferring and collecting the generated charges using electrical potential. During the optimization process, the shape of these top electrodes was modified into a honeycomb shape to increase the efficacy of charge collection. A mixture of gas and dielectric layers were selected to make an efficient gas-type detector for digital X-ray imaging. From the result, the electrical properties of the detector were investigated and the effectiveness of its geometrical design was proved. Measurements demonstrated the linearity of X-ray detection, and the successful movement and collection of charge using electrical potential. Thus, this modified planar gas-type detector and charge readout module using a charge-injection device made it possible to obtain two-dimensional images without using a scanning mode.

A photoinduced discharge X-ray detector using photoconductors: influence of the material selection on the electronic properties.

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 of 495 nm. The mercury iodide-based PID sensitivity was measured at 0.4 nc/mR cm2 with a dark current at 0.5 nA/cm2 and signal to noise ratio (SNR) of 340. These results show a lower sensitivity compared to existing flat panel detectors, but show a much lower dark current. Consequently, SNR values approximately 260 times higher were observed in the PID detector as compared with a flat panel detector.

Synthesis and characterization of X-ray nanophosphors using solution-combustion

We investigated nanophosphor materials that exhibit high resolution and emission efficiency for use in X-ray medical imaging. Rare-earth phosphor material has long been used due to its high atomic number and emission efficiency, but these materials tend to exhibit lower resolution and emission efficiency when manufactured in bulk. In this study, we synthesized nanometer-scale phosphors of Gd2O3:Eu and Y2O3:Eu using the solution-combustion method, and we evaluated the dependence of the optical properties of these nanophosphors on europium concentration and synthesis atmosphere. The nanophosphors were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and photoluminescence (PL) spectroscopy. Characterization of the optical properties revealed that both Gd2O3:Eu and Y2O3:Eu exhibited peak emission intensity at 611 nm, which corresponded to that for commercial bulk phosphors. These results imply that manufacturing nanophosphors can achieve thin and compact displays that have enhanced performance, and that improvements in emission efficiency of nanophosphors could reduce the required patient dose for medical imaging.

The design of hybrid x-ray detector using quantum size effect

Group 2?6 compounds (e.g., CdTe, CdS, CdSe) are utilized as photoconductors at the bulk level but manufactured as phosphors at the nano-level. Each of these uses has strengths and weaknesses. Here we attempted to fuse the two uses to maximize the strengths of each by using only one compound. We invented an X-ray detector that could function at two different levels -as a photoconductor in the bulk state and as a phosphor at the nano-scale- by hybridizing two different kinds of layer from one compound. This system operates as follows. First, an X-ray is converted to light on the luminescence layer, after which the light is received on the photoconductor layer. This light has the exact wavelength range required on the photoconductor. The quantum size effect refers to the impact of changes in the electronic energy level density according to the size of the crystal in a nano-particle on its optical and electrical characteristics. On account of this effect, two different kinds of layer from one compound can be used by regulating its size. Thus, by controlling the particle size and changing the emission wavelength, the most appropriate absorption wavelength for a photoconductor in the bulk state can be emitted from the nano-phosphor. The conversion efficiency in the hybrid structure is apparently superior to that in the bulk-state single layer. In conclusion, the electrical and optical characteristics of the proposed hybrid structure are superior to those of a conventional structure. These findings confirm the feasibility of a hybrid structure based on the quantum size effect.

Characteristic study of multi-layer using Hybrid method for digital X-ray detector

Recently digital X-ray detector, often called Active-Matrix Flat Panel Detector (AMFPD), have been researched by using either the direct or the indirect conversion method in order to form the digital image. In this paper, we introduced the concept of Hybrid method using phosphor layer to complement a weak point of direct conversion method having low conversion efficiency about X-ray. The Hybrid receptor scheme consists of a photoconductor layer, phosphor layer and top/bottom electrodes to collect charges produced in the photoconductor layer. A light reflective layer was deposited under the phosphor as lower layer. Mercury Iodide (HgI2) is used as photoconductor layer to detect X-ray and light from the phosphor, and Cesium Iodide (CsI) is used as phosphor layer to convert X-ray to light. We fabricated a photoconductor (HgI2 200um) and a phosphor (CsI 50um) using PIB(Particle In Binder) method. As top and bottom electrodes, ITO (Indium Tin Oxide) layers were deposited to photoconductor layer and light reflective layer(Al 15 to 25um) in order to improve photon conversion efficiency. As results, higher X-ray sensitivity of Hybrid method exhibited comparing with that of direct conventional digital X-ray detector. The Hybrid method shows about 1.5 times higher sensitivity than the direct conversion method. This effect is caused by simultaneous detection of electric signal induced by the direct X-ray absorption in photoconductor layer and by the light absorption produced in phosphor layer. The Hybrid method can solve problems such as lower conversion efficiency of photoconductor, breakdown because of high voltage application in the direct conversion method as well as low fill fator of TFT, low electric image signals and low SNR in indirect conversion method.