Bo Kyung Cha

A Pixelated CsI (Tl) Scintillator for CMOS-based X-ray Image Sensor

Direct and indirect imaging detectors for digital radiography have been developed during the past a few years. Among scintillators, columnar CsI(Tl) screens are used for indirect digital X-ray imaging for industrial and medical radiography. By the help of recent developments CMOS (Complementary Metal Oxide Semiconductor) is replacing the CCD (Charge Coupled Devices) in X-ray image sensor because of low operating power, standard CMOS manufacturing process, and low cost. An X-ray imaging detector consisting of pixelated CsI(Tl) scintillator and CMOS imaging sensor for application in dental radiography was developed. The manufactured CMOS imaging sensor has 128 by 128 pixels with the pitch size of 50 mum and gap width of 5 mum. A 0.5 mum CMOS process was used for sensor fabrication. Pixel structured CsI(Tl) scintillator was directly deposited on the patterned CMOS imaging sensor by thermal evaporation and photo-lithography process. The fabricated monolithic X-ray imaging sensor showed dramatic improvement in spatial resolution due to enhancement of the scintillation light guiding effect and reduction of the light cross-talk between scintillator pixels.

Investigation of the Performance of Scintillator-Based CMOS Flat Panel Detectors for X-Ray and Thermal Neutron Imaging

Recent advances in of silicon-based CMOS (complementary metal-oxide-semiconductor) flat panel detectors have resulted in an attractive use of cost-effective radiation imaging devices for X-ray and neutron radiography/tomography system. Indirect detection methods consisted of an X-ray converter (or a scintillator screen) and photodiode arrays are more widely used in high resolution micro-CT (computed tomography), dental and industrial NDT applications. In this study, The terbium-doped gadolinium oxysulfide (Gd2O2 S:Tb, Gadox) scintillator screens with different thickness (several 30-140 m thickness) were directly coupled with a fiber-optic plate (FOP) of a commercially available CMOS imaging device for high resolution X-ray and thermal neutron radiography. The RadEye1 CMOS APS (active pixel sensor) imager having a large active area of 25 × 50 mm2 and 48 m pixel pitch was selected for X-ray and thermal neutron imaging with high resolution. The scintillation properties and imaging performance such as relative light output, linearity and spatial resolution were measured and evaluated under X-ray and thermal neutron beam exposure. The good linearity and high spatial resolution characteristics in X-ray and thermal neutron imaging experiments were achieved by using compact, cost-effective imaging detector with exchangeable Gadox scintillator screens.

Design and image-quality performance of high resolution CMOS-based X-ray imaging detectors for digital mammography

In digital X-ray imaging systems, X-ray imaging detectors based on scintillating screens with electronic devices such as charge-coupled devices (CCDs), thin-film transistors (TFT), complementary metal oxide semiconductor (CMOS) flat panel imagers have been introduced for general radiography, dental, mammography and non-destructive testing (NDT) applications. Re- cently, a large-area CMOS active-pixel sensor (APS) in combination with scintillation films has been widely used in a variety of digital X-ray imaging applications. We employed a scintillator- based CMOS APS image sensor for high-resolution mammography. In this work, both powder-type Gd2O2S:Tb and a columnar structured CsI:Tl scintillation screens with various thicknesses were fabricated and used as materials to convert X-ray into visible light. These scintillating screens were directly coupled to a CMOS flat panel imager with a 25 50 mm 2 active area and a 48 mm pixel pitch for high spatial resolution acquisition. We used a W/Al mammographic X-ray source with a 30 kVp energy condition. The imaging characterization of the X-ray detector was measured and analyzed in terms of linearity in incident X-ray dose, modulation transfer function (MTF), noise-power spectrum (NPS) and detective quantum efficiency (DQE).

Synthesis and scintillation characterization of nanocrystalline Lu2O3(Eu) powder for high-resolution X-ray imaging detectors

Lu2O3:Eu(CEu:5mol%) powder scintillators with nanocrystalline structures were successfully synthesized via a precipitation method and subsequent calcination treatment as a conversion material for X-ray imaging detectors. In this work, a homogeneous precipitation process was carried out using DEA(diethanolamine) as a precipitant to prepare nanocrystalline Eu-doped Gd2O3 powders. The microstructures, crystal structure and scintillation properties such as luminescent spectra, decay time and light intensity were measured as a function of calcination temperature in heat-treatment of the synthesized powder. The sample prepared at 1200?C calcination temperature showed the highest light intensity. And the scintillator showed a strong red emission light at near 611nm under photo- and X-ray luminescence for its potential X-ray imaging detector applications.

Development and characterization of CMOS-based monolithic X-ray imager sensor

We proposed a new design of CMOS-based X-ray image sensor with monolithically grown pixelated CsI(Tl) on photosensor area for securing the maximally achievable spatial resolution for a given sensitivity determined by the CsI(Tl) thickness at a certain X-ray energy. The test version of a CMOS image sensor (CIS) was designed and fabricated using AMIS 0.5 mum standard CMOS process. The chip includes an 128times128 CMOS active pixel sensor(APS) array with 50 mum pitch, a row/column decoder, a sample & hold circuit with buffers, an analog signal processor(ASP) and a 10 bit pipe-lined analog-to-digital converter(ADC). A data acquisition system (DAS) for operating the image sensor and for processing digital signals was also developed. Then the X-ray image sensor was fabricated by depositing thermally evaporated CsI(Tl) blocks of 50 mum thick on each pixel sensor of CIS using a photo-resist pattern as a basis of CsI(Tl) deposition. Linearity of the response, dark current noise, dynamic range and spatial resolution etc. were measured with bare and scintillator-coupled CIS image sensors by green LED light and X-ray beam respectively. The mean charge generation rate per pixel measured at room temperature in the dark was about 45,600 electrons/sec which results in the electronic noise of 210 electrons for 1 sec exposure time. Therefore the dynamic range is ~67 dB if we consider the statistical noise together. The spatial resolution of scintillator-coupled CMOS image sensor was measured to be ~7 lp/mm by a lead line pattern and 80 kVp X-ray. This works was supported by Nuclear R&D Program of MOST, Korea through KOSEF.

Linear scanning sensors with gas-based detector modules for X-ray imaging

Parallel-plate-type scanning detectors with metallic and resistive cathodes and strip anodes have been successfully used for medical imaging. In this study, we manufactured an X-ray image sensor that can be used to obtain scanning images, by using the plasma display panel (PDP) fabrication process. And we investigated the characteristics of linear scanning sensors that use gas-based detector modules. We evaluated the electric signal generated by the ionization of electrons and positive ions in the sensor for various gas mixtures — Xe (100), Xe/CO2 (90/10), and Xe/CO2 (80/20) — at atmospheric pressure. We also fabricated a data acquisition system (DAS) and acquired X-ray scanning images with our imaging system developed in this study.

Fabrication and comparison Gd 2 O 2 S(Tb) and CsI(Tl) films for X-ray imaging detector application

During the last decade, digital X-ray imaging systems have been replacing analog X-ray imaging systems of conventional X-ray film-screen combination for radiography applications. Indirect detection methods consisted of an X-ray converter (or a scintillator film) and photodiode arrays are more widely used in medical diagnoses and industrial fields. Two major scintillation materials such as terbium doped gadolinium oxysulfide (Gd 2 O 2 S:Tb, GOS) and thallium doped cesium iodide (CsI:Tl) are commonly used. In this work, GOS scintillator films were manufactured by mixing and thermal hardening of Gd 2 O 2 S:Tb powder, dispersion agent, hardening agent, and other organic additives. And CsI:Tl scintillator films with columnar structure were also fabricated by the thermal evaporation method. The scintillation properties, such as emission spectrum and light yield etc., of the GOS and CsI:Tl films were measured by X-ray luminescence and photo-luminescence (PL) methods. The maximum luminescent intensity of both scintillators was observed at 540–560nm wavelength. In order to investigate the imaging performances of both GOS and CsI:Tl films as converters of X-ray imaging detectors, both scintillator films were coupled with an CCD sensor. The light response to X-ray dose, signal-to-noise ratio (SNR), spatial resolution were measured and analyzed under the same X-ray conditions. As X-ray dose increases, the SNR curves showed linear relationship. And the spatial resolution of two scintillator films was resolved at 7∼8lp/mm.

Development of a novel direct X-ray detector using photoinduced discharge (PID) readout for digital radiography

We developed a novel direct X-ray detector using photoinduced discharge (PID) readout for digital radiography. The pixel resolution is 512???512 with 200??m pixel and the overall active dimensions of the X-ray imaging panel is 10.24?cm???10.24?cm. The detector consists of an X-ray absorption layer of amorphous selenium, a charge accumulation layer of metal, and a PID readout layer of amorphous silicon. In particular, the charge accumulation is pixelated because image charges generated by X-ray should be stored pixel by pixel. Here the image charges, or holes, are recombined with electrons generated by the PID method. We used a 405 nm laser diode and cylindrical lens to make a line beam source with a width of 50??m for PID readout, which generates charges for each pixel lines during the scan. We obtained spatial frequencies of about 1.0 lp/mm for the X-direction (lateral direction) and 0.9?lp/mm for the Y-direction (scanning direction) at 50% modulation transfer function.

Use and imaging performance of CMOS flat panel imager with LiF/ZnS(Ag) and Gadox scintillation screens for neutron radiography

In digital neutron radiography system, a thermal neutron imaging detector based on neutron-sensitive scintillating screens with CMOS(complementary metal oxide semiconductor) flat panel imager is introduced for non-destructive testing (NDT) application. Recently, large area CMOS APS (active-pixel sensor) in conjunction with scintillation films has been widely used in many digital X-ray imaging applications. Instead of typical imaging detectors such as image plates, cooled-CCD cameras and amorphous silicon flat panel detectors in combination with scintillation screens, we tried to apply a scintillator-based CMOS APS to neutron imaging detection systems for high resolution neutron radiography. In this work, two major Gd2O2S:Tb and 6LiF/ZnS:Ag scintillation screens with various thickness were fabricated by a screen printing method. These neutron converter screens consist of a dispersion of Gd2O2S:Tb and 6LiF/ZnS:Ag scintillating particles in acrylic binder. These scintillating screens coupled-CMOS flat panel imager with 25x50mm2 active area and 48μm pixel pitch was used for neutron radiography. Thermal neutron flux with 6x106n/cm2/s was utilized at the NRF facility of HANARO in KAERI. The neutron imaging characterization of the used detector was investigated in terms of relative light output, linearity and spatial resolution in detail. The experimental results of scintillating screen-based CMOS flat panel detectors demonstrate possibility of high sensitive and high spatial resolution imaging in neutron radiography system.

Hydrothermal synthesis and characterization of nano Gd 2 O 3 (Eu) scintillator for high resolution X-ray imaging application

Gd2O3:Eu scintillators with nano-crystalline structures were successfully synthesized through a hydrothermal process and subsequent calcination treatment as a converter for X-ray imaging detectors. In this work, Eu-doped Gd2O3 nano-crystalline powders as a function of calcination temperatures and times were prepared by a hydrothermal process and subsequent calcination in the electrical furnace. Scintillation properties such as luminescent spectra, light intensity and decay time were measured by changing the temperatures and times in heat-treatment of as-synthesized nanocrystalline powders. The synthesized samples with different morphology such as nano rods and particles due to calcination temperature were observed. The sample prepared at 1100°C calcination temperature and 5 hour showed the highest light intensity by X-ray excitation. And the scintillator emitted a strong red light at near 611nm under photo-and X-ray luminescence for its potential X-ray imaging detector applications.