Gyuseong Cho

Korea Advanced Institute of Science and Technology , Sŏul, Seoul, South Korea

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Publications (151)105.9 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: For 2-d X-ray imaging, such as mammography and non-destructive test, a sensor should have a large-area because the sensor for typical X-ray beams cannot use optical lens system. To make a large-area 2-d X-ray image sensor using crystal Si, a technique of tiling unit CMOS image sensors into 2 × 2 or 2 × 3 array can be used. In a unit CMOS image sensor made of most common 8-inch Si wafers, the signal line can be up to ~ 180 mm long. Then its parasitic capacitance is up to ~ 25 pF and its resistance is up to ~ 51 kΩ (0.18 μm, 1P3M process). This long signal line may enlarge the row time up to ~ 50 μsec in case of the signal from the top row pixels to the readout amplifiers located at the bottom of the sensor chip. The output signal pulse is typically characterized by three components in sequence; a charging time (a rising part), a reading time and a discharging time (a falling part). Among these, the discharging time is the longest, and it limits the speed or the frame rate of the X-ray imager. We proposed a forced discharging method which uses a bypass transistor in parallel with the current source of the column signal line. A chip for testing the idea was fabricated by a 0.18 μm process. A active pixel sensor with three transistors and a 3-π RC model of the long line were simulated together. The test results showed that the turning on-and-off of the proposed bypass transistor only during the discharging time could dramatically reduce the discharging time from ~ 50 μsec to ~ 2 μsec, which is the physically minimum time determined by the long metal line capacitance.
    Journal of Instrumentation 08/2014; 9(08):P08011. · 1.66 Impact Factor
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    ABSTRACT: The single-slope analog-to-digital converter (SS-ADC) is the most commonly used column-level ADC for high-speed industrial, complementary metal-oxide semiconductor (CMOS)-based X-ray image sensors because of its small chip area (the width of a pixel), its simple circuit structure, and its low power consumption. However, it generally has a long conversion time, so we propose an innovative design: a complimentary dual-slope ADC (CDS-ADC) that uses two opposite ramp signals instead of a single ramp to double the conversion speed. This CDS-ADC occupies only 15% more area than the original SS-ADC. A prototype 12-bit CDS-ADC and a 12-bit SS-ADC were fabricated using a 0.35-µm 1P 4M CMOS process. During comparison of the two, the measured maximum differential non-linearity (DNL) of the CDS-ADC was a 0.49 least significant bit (LSB), the maximum integral non-linearity (INL) was a 0.43 LSB, the effective number of bits (ENOB) was 9.18 bits, and the figure of merit (FOM) was 0.03 pJ/conversion. The total power consumption was 0.031 uW. The conversion time of the new CDS-ADC was half that of the SS-ADC. The proposed dual-slope concept can be extended to further multiply the conversion speed by using multiple pairs of dual-slope ramps.
    01/2014; 64(4).
  • Jongyul Kim, Seung Wook Lee, Gyuseong Cho
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    ABSTRACT: Visibility evaluation of neutron gratings for a polychromatic thermal neutron beam was performed for a neutron grating interferometer. Four sets of neutron gratings designed for neutron wavelengths of 2.0 Å, 2.7 Å, 3.5 Å, and 4.4 Å were fabricated and tested to find the neutron grating interferometer setup with maximum visibility. The measurements were carried out at the thermal neutron beamline of the Ex-core Neutron irradiation Facility (ENF) of the High-flux Advanced Neutron Application Reactor. The maximum visibility was obtained at the neutron grating set designed for a neutron wavelength of 2.7 Å among the four sets, and the visibility was 9.7%. The experimental data can be the basis for an optimization of the neutron grating interferometer at the thermal neutron beamline, and can be further optimized for neutron dark-field imaging with high spatial resolution and a shorter data acquisition time.
    01/2014; 746:26–32.
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    ABSTRACT: The fabrication of gratings including metal deposition processes for highly neutron absorbing lines is a critical issue to achieve a good visibility of the grating-based phase imaging system. The source grating for a neutron Talbot-Lau interferometer is an array of Gadolinium (Gd) structures that are generally made by sputtering, photo-lithography, and chemical wet etching. However, it is very challenging to fabricate a Gd structure with sufficient neutron attenuation of approximately more than 20 μm using a conventional metal deposition method because of the slow Gd deposition rate, film stress, high material cost, and so on. In this article, we fabricated the source gratings for neutron Talbot-Lau interferometers by filling the silicon structure with Gadox particles. The new fabrication method allowed us a very stable and efficient way to achieve a much higher Gadox filled structure than a Gd film structure, and is even more suitable for thermal polychromatic neutrons, which are more difficult to stop than cold neutrons. The newly fabricated source gratings were tested at the polychromatic thermal neutron grating interferometer system of HANARO at the Korea Atomic Energy Research Institute, and the visibilities and images from the neutron phase imaging system with the new source gratings were compared with those fabricated by a Gd deposition method.
    The Review of scientific instruments 06/2013; 84(6):063705. · 1.52 Impact Factor
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    ABSTRACT: This paper introduces a gamma-ray tomographic system which is transportable and can be used for on-line systems such as a pipeline operation. In a previous study, a feasibility study on a gamma-ray tomographic system with a scanning geometry of Electron Beam CT was carried out by Monte Carlo simulation. This paper contains a successive work on a previous study by developing and evaluating a real system. To construct a gamma-ray CT, 137Cs was used as a gamma-ray source and radiation measurement system with 72 channel CsI detectors whose crystal is a 12 mm×12 mm×20 mm rectangular parallelepiped was developed to operate jointly with a motion control system. ML-EM algorithm was used for image reconstruction of experimental data. Using the developed transportable gamma-ray system, laboratory and field experiments were carried out successfully. The field experiment results show that a gamma-ray CT with an Electron Beam CT scanning geometry can be a transportable gantry for objects which are parts of processes.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 11/2012; 693:203–208. · 1.14 Impact Factor
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    ABSTRACT: 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. Recently, 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 mm2 active area and a 48 μm 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).
    Journal of Instrumentation 04/2012; 7(04):C04020. · 1.66 Impact Factor
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    ABSTRACT: 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.
    Journal of Instrumentation 01/2012; 7(03). · 1.66 Impact Factor
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    ABSTRACT: In order to develop a high-resolution and high-sensitivity digital mamographic detector, to use a commercially-available and well-developed CMOS image sensor (CIS) process can be a cost-effective way. However, in any commercial CIS process, several different types of n- or p-layers can be used so that various pn-junction structures could be formed depending on the choice of n- and p-layer combination. We performed a comparative analysis on the characteristics of three types of photodiodes formed on a high-resistivity p-type epitaxial wafer by applying three available n-layer processes in order to develop the high-sensitivity photodiode for a scintillator-based X-ray imaging detector. As a preliminar study, a small test-version CIS chip with an 80 × 80 pixel array of a 3-transistor active pixel sensor structure, 50 μm pitch and 80{%} fill factor was fabricated. The pixel area is subdivided into four 40 × 40 sub-arrays and 3 different types of photodides are designed for each sub-array by using n+, n− and n-well layers. All other components are designed to be identical for impartial comparison of the photodiodes only. Among 3 types, the n−/p-epi photodiode exhibited high charge-to-voltage gain (0.86 μV/e−), high quantum efficiency (49% at 532 nm wavelength) and low dark current (294 pA/cm2). The test CIS chip was coupled to a phosphor screen, Lanex Fine or Lanex Regular, both composed of Gd2O2S:Tb, and was tested using X-rays in a mammography setting. Among 6 cases, n−/p-epi photodiode coupled with the Lanex Regular also showed the highest sensitivity of 30.5 mV/mR.
    Journal of Instrumentation 12/2011; 6(12):C12046. · 1.66 Impact Factor
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    ABSTRACT: Retroelements play important roles in primate evolution. Specifically, human endogenous retroviruses (HERVs) and Alu elements are primate-specific retroelements. In addition, SVA elements belong to the youngest family of hominid non-long terminal repeat (LTR) retrotransposons. Retroelements can af-fect adjacent gene expression, supplying cis-regulatory ele-ments, splice sites, and poly-A signals. We developed a data-base, GEnome-wide Browser for RETroelement (GEBRET,, for comparing the dis-tribution of primate-specific retroelements and adjacent genes. GEBRET database components include 47,381 HERVs, 53,924 Alus and 4639 SVAs in five primate genomes of hu-man, chimpanzee, orangutan, rhesus macaque, and marmoset. Host genes located upstream of a retroelement were also vi-sualized and classified as five categories (0.0, 0.5, 1.0, 2.0, and 3.0Kb). Our results suggest that retroelements preferen-tially integrate into the distal promoter region relative to the core promoter region. GEBRET database is designed to inves-tigate the distribution of retroelements (HERVs, Alus and SVAs) in the primate genomes that have been sequenced. Our software will be useful in the field to study the impact of H.-S. Ha and W.-K. Chung contributed equally to this work. retroelements on primate genome evolution.
    Genes & genomics 12/2011; · 0.50 Impact Factor
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    ABSTRACT: The electron beam X-ray tomographic scanner has been used in industrial and medical field since it was developed two decades ago. However, X-ray electron beam tomography has remained as indoor equipment because of its bulky hardware of X-ray generation devices. By replacing X-ray devices of electron beam CT with a gamma-ray source, a tomographic system can be a portable device. This paper introduces analysis and simulation results on industrial gamma-ray tomographic system with scanning geometry similar to electron beam CT. The gamma-ray tomographic system is introduced through the geometrical layout and analysis on non-uniformly distributed problem. The proposed system adopts clamp-on type device to actualize portable industrial system. MCNPx is used to generate virtual experimental data. Pulse height spectra from F8 tally of MCNPx are obtained for single channel counting data of photo-peak and gross counting. Photo-peak and gross counting data are reconstructed for the cross-sectional image of simulation phantoms by ART, Total Variation algorithm and ML-EM. Image reconstruction results from Monte Carlo simulation show that the proposed tomographic system can provide the image solution for industrial objects. Those results provide the preliminary data for the tomographic scanner, which will be developed in future work.
    Applied radiation and isotopes: including data, instrumentation and methods for use in agriculture, industry and medicine 10/2011; 70(2):404-14. · 1.09 Impact Factor
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    ABSTRACT: A SiPM is an array of several thousand micro-Geiger-mode APDs (GAPD), and a GAPD generally consists of a PN diode (Cd), a quenching resistor (Rq) and any parasitic capacitor (Cq) in parallel to the quenching resistor. The single-photon pulse shape is determined by a RC decay time, a product of Ctotal (Cd+Cq) and Rq. A large fraction of current in the long tailed single-photon pulse shape causes Scintillation detectors with SiPM to exhibit a poor timing resolution compared to PMTs, which have very narrow Gaussian single-photon pulse shape. In this paper, a ∼40fF Metal–Insulator–Metal (MIM) quenching capacitor (Cq-MIM) parallel to a quenching resistor in a Geiger mode APD is proposed to modify the single-photon pulse shape of a SiPM, in order to improve the timing performance of PET detectors. A single-photon pulse shape with Cq-MIM has a very fast pulse with a low slow tail, consequently the increasing initial current.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 09/2011; 650(1):125–128. · 1.14 Impact Factor
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    ABSTRACT: We have synthesized europium-doped gadolinium oxide (Gd2O3:Eu) scintillators by a precipitation method with advantages of simple process and low-temperature calcinations for high resolution X-ray imaging detectors. The powders with about 15–30 nm particle sizes were obtained by subsequent calcinations of the precursor at a different temperature of 600–800 °C for 10 h. The wavelength of the main emission peak was about 610 nm and as temperature increased, light intensity of the scintillator increased in the X-ray luminescence case. Imaging performance such as X-ray linearity and spatial resolution of both commercial bulk and nanocrystalline-Gd2O3:Eu scintillator screens was measured and compared with the X-ray imaging system after coupling these to a CCD image sensor.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 05/2011; 633:S294–S296. · 1.14 Impact Factor
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    ABSTRACT: 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.
    Journal of Instrumentation 01/2011; 6(01):C01064. · 1.66 Impact Factor
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    ABSTRACT: In this paper, a tomographic scan method with fixed installed detectors and a rotating gamma-ray source system is presented to diagnose industrial plants, which were impossible to examine by conventional tomographic systems. Monte Carlo simulations had been performed for two kinds of phantoms. Lab-scale experiments with the same condition as one of the phantoms, have been carried out. Algebraic and statistical reconstruction methods were applied for the reconstruction of simulation and experimental data. The reconstruction results from different algorithms were compared. Simulation results showed that reconstruction from the photopeak counting measurement gave better results than the gross counting measurement, although the photopeak counting measurement had large statistical errors. The statistical algorithm gives better results for tomographic scan methods than the algebraic method for the simulation data. Simulation and experimental data showed that this work demonstrates the feasibility of the fixed detection and rotating source system for scanning of industrial plants. Those results appear to be promising for industrial tomographic applications, especially for petrochemical industries.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2011; 640(1):139-150. · 1.14 Impact Factor
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    ABSTRACT: Due to the uncertainty of the annihilation event’s position along a given line-of-response (LOR), there is a large amount of statistical noise that is inherent to conventional PET image reconstruction. To overcome the poor spatial resolution of PET, time-of-flight positron emission tomography (TOF-PET) has been suggested. The use of the time-of-flight PET provides significant improvements to the image quality and 3 dimensional imaging. In this study, 2×2 mm2 SiPMs, operated in the Geiger avalanche mode, were designed and fabricated on epitaxial silicon wafers. Fabricated SiPMs whose doping structure is N+/P/π/P+, have a high geometric fill-factor with quenching resistors of the high resistive poly-silicon layer and a high sensitivity to short-wavelength light with the shallow-junction. Fundamental characteristics of SiPM were measured and analyzed. Achievable timing resolution was investigated and quantified with a LYSO crystal and SiPMs.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2011; 633. · 1.14 Impact Factor
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    ABSTRACT: Thallium doped cesium iodide (CsI:Tl) scintillator films for the use as a converter for X-ray imaging detectors were fabricated by the thermal deposition method. The microstructures of these scintillating layers were affected by various deposition conditions such as vapor pressure, substrate temperature and post-heat treatment or rapid thermal annealing (RTA). CsI(Tl) scintillator films with various polycrystalline structures were manufactured under different process conditions and prepared for experiments. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate the crystal structure and morphology properties. Light output and spatial resolution of the samples were strongly affected by the microstructures, which are determined by the deposition conditions and post-heat treatment. Imaging characteristics of the various CsI:Tl films were also measured under X-ray exposure conditions by coupling them to a CCD image sensor.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2011; 633. · 1.14 Impact Factor
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    ABSTRACT: X-ray imaging detectors in combination with scintillator screens have been widely used in digital X-ray imaging applications. Gd2O2S:Tb was used as scintillation material for pixelated scintillator screens based on silicon substrates (wafer) with a micropore array of various dimensions fabricated using the photolithography and deep reactive ion etching (DRIE) process. The relative light output and the modulation transfer function (MTF) of each fabricated scintillator screen were measured by a cooled CCD and compared with those of Lanex screens. The spatial resolution of our scintillator screens was higher but their light outputs were lower than those of Lanex screen probably due to the loss of light at the wall surfaces. Therefore further treatment of the wall surface, such as reflective coating, seems necessary to compensate the light loss.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2011; 633. · 1.14 Impact Factor
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    ABSTRACT: A novel quasi-pixel structured scintillation screen with nanocrystalline Gd2O3:Eu particle sizes was introduced for indirect X-ray imaging sensors with high sensitivity and high spatial resolution. A nanocrystalline Gd2O3:Eu scintillating phosphor with average 100nm sizes was used as a conversion material for incident X-rays into optical photons. In this work, silicon-based pixel structures with different 100 and 50μm pixel sizes, 10μm wall width and 120μm thickness were fabricated by a standard photolithography and deep reactive ion etching (DRIE) process. The pixelated scintillation screen was fabricated by filling the synthesized nanocrystalline Gd2O3:Eu scintillating phosphor into pixel-structured silicon arrays, and X-ray imaging performance such as relative light intensity, X-ray to light response and spatial resolution in terms of modulation transfer function (MTF) of the fabricated samples were measured. Although high spatial resolution imaging was largely achieved by pixel-structured nanocrystalline Gd2O3:Eu scintillation screens, X-ray sensitivity was still low for medical imaging applications. As a result, novel quasi-pixel structured screens with additional thin Gd2O2S:Tb scintillating layer were proposed for X-ray imaging detector with suitable sensitivity and spatial resolution in comparison with pixel-structured screens, and X-ray imaging performance of quasi-pixel structured nanocrystalline Gd2O3:Eu scintillating screens was investigated.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2011; 648. · 1.14 Impact Factor
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    ABSTRACT: We developed a novel pixel-structured scintillation screen with nanocrystalline Gd2O3:Eu particle sizes for high spatial resolution X-ray imaging detectors. Nanocrystalline Gd2O3:Eu scintillators were successfully synthesized with a hydrothermal method and a subsequent calcination treatment, which were used as a material for converting incident X-rays into visible light. In this work, silicon-based pixel structures with different 100, 50 and 30μm pixel sizes, a 10μm wall width and a 120μm thickness were prepared with the standard photolithography and the deep reactive ion etching (DRIE) process. Subsequently, a micro-pixel-structured scintillation screen was fabricated by adding the synthesized nanocrystalline Gd2O3:Eu scintillating phosphor to pixel-structured silicon arrays. Additionally, X-ray imaging performance such as relative light intensity, X-ray to light response and the spatial resolution in terms of modulation transfer function (MTF) were measured by using an X-ray source and a lens-coupled charge coupled device (CCD) camera system. The light intensity of the pixel-structured nanocrystalline Gd2O3:Eu screen was much higher than that of a pixel-structured sample made with a commercial microcrystalline Gd2O3:Eu product due to the density of the nanocrystalline Gd2O3:Eu scintillating powder-filled silicon structure. As the pixel size of the pixel-structured silicon decreased, the light intensity decreased. However, as the pixel size decreased, the spatial resolution significantly improved with no evident crosstalk from the emitted optical photons between adjacent scintillating pixels. The MTF of pixel-structured nanocrystalline Gd2O3:Eu screens with a 100 and a 50μm pixel size was 20% and 30% at 6lp/mm, respectively. As a result, this new technology showed that a microchannel structure based on a nanocrystalline Gd2O3:Eu scintillator could provide higher light intensity and high spatial resolution imaging compared to conventional microcrystalline scintillating phosphor.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2011; 652(1):717-720. · 1.14 Impact Factor
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    ABSTRACT: Retroelements play important roles in primate evolution. Specifically, human endogenous retroviruses (HERVs) and Alu elements are primate-specific retroelements. In addition, SVA elements belong to the youngest family of hominid non-long terminal repeat (LTR) retrotransposons. Retroelements can af-fect adjacent gene expression, supplying cis-regulatory ele-ments, splice sites, and poly-A signals. We developed a data-base, GEnome-wide Browser for RETroelement (GEBRET,, for comparing the dis-tribution of primate-specific retroelements and adjacent genes. GEBRET database components include 47,381 HERVs, 53,924 Alus and 4639 SVAs in five primate genomes of hu-man, chimpanzee, orangutan, rhesus macaque, and marmoset. Host genes located upstream of a retroelement were also vi-sualized and classified as five categories (0.0, 0.5, 1.0, 2.0, and 3.0Kb). Our results suggest that retroelements preferen-tially integrate into the distal promoter region relative to the core promoter region. GEBRET database is designed to inves-tigate the distribution of retroelements (HERVs, Alus and SVAs) in the primate genomes that have been sequenced. Our software will be useful in the field to study the impact of H.-S. Ha and W.-K. Chung contributed equally to this work. retroelements on primate genome evolution.
    genes and genomics. 01/2011;

Publication Stats

376 Citations
105.90 Total Impact Points

Institutions

  • 1997–2014
    • Korea Advanced Institute of Science and Technology
      • Department of Nuclear and Quantum Engineering
      Sŏul, Seoul, South Korea
  • 2011
    • Korea Atomic Energy Research Institute (KAERI)
      Daiden, Daejeon, South Korea
  • 2005
    • Pusan National University
      • Department of Mechanical Engineering
      Pusan, Busan, South Korea
  • 2004–2005
    • Sungkyunkwan University
      • Department of Nuclear Medicine
      Sŏul, Seoul, South Korea
  • 2003
    • Kyung Hee University
      Sŏul, Seoul, South Korea
  • 1989–2002
    • Lawrence Berkeley National Laboratory
      • Physics Division
      Berkeley, CA, United States
  • 1988–1993
    • University of California, Berkeley
      • Lawrence Berkeley Laboratory
      Berkeley, MO, United States
  • 1989–1991
    • CSU Mentor
      Long Beach, California, United States
  • 1990
    • Palo Alto Research Center
      Palo Alto, California, United States