Chi-Woong Mun

Brain morphology in juvenile myoclonic epilepsy and absence seizures

We evaluated the differences in brain morphology among patients with juvenile myoclonic epilepsy according to the occurrence of absence seizures.

Effect of Annealing on the X-ray Detection Properties of Nano-sized Polycrystalline Lead Oxide Films

Polycrystalline lead oxide (PbO) film is an excellent candidate material for a direct conversion X-ray detector. However, the thick-bulky film tends to significantly reduce the charge collection efficiency for recombination process, and the effective number of electron-hole pairs is lower than that of thin film, because it is difficult to fabricate high-dense and thick PbO films. In this paper, we first synthesized nano-sized PbO particles that could be used in a novel high-efficiency flat panel X-ray detector using a simple solution/combustion method. Energy dispersive X-ray spectrometry, X-ray diffraction, and field emission scanning electron microscopy were used to analyze the component ratio and morphology of the PbO particles as a function of annealing temperature. Then, 150-µm-thick PbO films were deposited on glass substrates using a particle-in-binder method at room temperature. The influences of annealing before deposition on the X-ray detection characteristics of the PbO films were investigated in detail. The key parameters–the dark current, X-ray sensitivity, signal-to-noise ratio, and signal decay–were measured. The annealing conditions strongly affected the electrical properties of the PbO films. The X-ray sensitivity of films annealed in oxygen gas increased dramatically with increasing annealing temperatures up to 500 °C.

Effective connectivity in temporal lobe epilepsy with hippocampal sclerosis

We hypothesized that temporal lobe epilepsy (TLE) patients with and without hippocampal sclerosis (HS) showed differences in their limbic networks. This study aimed to evaluate the role of the thalamus in TLE patients with HS.

Progressive topological disorganization of brain network in focal epilepsy

Increasing evidence has suggested that epilepsy is a network disease. Graph theory is a mathematical tool that allows for the analysis and quantification of the brain network. We aimed to evaluate the influences of duration of epilepsy on the topological organization of brain network in focal epilepsy patients with normal MRI using the graph theoretical analysis based on diffusion tenor imaging.

High-resolution phosphors as X-ray detectors for synchrotron radiation experiments

In this paper, the comparative performance of high-resolution phosphors using CCD(Charge Coupled Device) and CMOS(Complementary Metal Oxide Semiconductor) sensors for synchrotron radiation imaging is reported. Various phosphor films (ZnS [Cu, Al], ZnS [Ag, Al], Y/sub 2/O/sub 2/S [Tb], BiTiO/sub 3/) are deposited on glass substrates using printing techniques. The phosphor plates used were 5 /spl mu/m, 10 /spl mu/m, 15 /spl mu/m, and 20 /spl mu/m thick, respectively. To investigate the luminescent properties of phosphor plates, the excitation and photoluminescence spectra were measured. The 20 line-pair test pattern images were obtained and evaluated by calculating the modulation transfer function (MTF). From the experimental results, it was concluded that the thicker the phosphor plates, the better the luminescent efficiency of X-ray-induced light photons. The phosphor thickness, however, decreases the MTF due to light blurring. ZnS (Cu, Al) phosphor of 15 /spl mu/m thickness had a maximum value of 0.8 at 5 lp/mm.

Characterization of a-Se(As:Cl) Film for Phosphor Coupled X-Ray Light Modulators

Amorphous selenium (a-Se) film has the potential to fulfill the requirements of a novel x-ray image detector because of its good photo-to-dark impedance ratio, large area coverage, and low temperature deposition. In this work were studied the structural, optical and electrical properties of thermally- evaporated a-Se film for the phosphor-light modulator (PLM). From the x-ray diffraction and electron microscopy experimental results, the deposited film had an amorphous phase without any re-crystallization or defects. Also, the light absorption in widely visible range of 400 ~ 630 nm was over 95 %. From the electrical measurements, the low dark current density of 2.8 nA/cm2 was obtained at 10 V/㎛. The x-ray sensitivity of the 270㎛-Gd2O3:Eu phosphor coupled 20㎛-Se film was 7.31 nC/cm2-mR. From such experimental results, the novel x-ray detector which incorporates phosphor coupled x-ray light modulator, makes an operation at low x-ray exposure possible, therefore, the applications as a medical imaging detector are shown below.

New CsI:Na-selenium x-ray detector

Digital x-ray imager known to flat-panel detector has been studied for the application of a various medical modalities. Currently, two types of detection methods have been realized in digital radiography. One is an indirect conversion method and the other is a direct conversion method. we have been developing a new x-ray detector that combines a columnar CsI:Na scintillation layer with a photosensitive a-Se with dielectric thin film. In this structure, an x-ray is converted to visible light in a CsI:Na scintillation layer and visible light is then converted to electric charges in a-Se layer. The electron-hole pairs can be also generated from x-ray interaction in the a-Se photoconductor, which can improve the detection efficiency of electric charge. We designed the thickness of CsI:Na scintillator by using computer simulation. MCNP is a general-purpose, continuous-energy, generalized-geometry, time-dependent, coupled neutron / photon / electron Monte Carlo transport code. The spectra of x-ray absorption was simulated by using MCNP 4C code. The morphology of the vacuum deposited CsI:Na scintillator and the parylene film were analyzed. Photoluminescence characterization of CsI:Na showed a light emission peak centered at 420nm as expected, which matched the absorption spectrum of amorphous selenium(a-Se). For an electric field of 10V/μm, the dark currents of our detector were below 370 pA/cm2 and the SNR of CsI:Na coupled a-Se detector with a dielectric layer was 1.8 times greater than that without CsI:Na layer.

Synthesis and luminescence characterization of fine gadolinium oxide phosphors for high resolution X-ray imaging

In this work, fine Gd/sub 2/O/sub 3/:Eu powder was synthesized using a solution-combustion method for high-resolution digital X-ray imaging detectors. The structure of the phosphors was characterized by X-ray diffraction (XRD), high-resolution scanning electron microscopy (HR-SEM). It was possible to obtain fine Gd/sub 2/O/sub 3/:Eu crystallized particles with an average particle size of only 20 nm. The optical properties investigated were photoluminescence emission and excitation spectra, luminescence decay time, and X-ray conversion efficiency. The emission corresponding to the /sup 5/D/sub 0//spl rarr//sup 7/F/sub 2/ transition at 610 nm was dominant, and the Eu/sup 3+/ ions substituted well for the Gd/sup 3+/ sites. Quenching starts at an europium concentration of 8% wt, and the strongest X-ray conversion efficiency was achieved at the phosphor with 5% wt. The mean lifetime of synthesized phosphors is 2.3-2.6 ms.

X-ray detection properties of polycrystalline Cd1-xZnxTe detectors for digital radiography

There has been considerable recent progress in II-IV semiconductor materials and in methods for improving performance of the associated digital x-ray detectors. Cd1-xZnxTe is known as promising medical x-ray detector material. The CdTe and Cd1-xZnxTe (x=0.15,0.25,0.3) detectors were fabricated by vacuum thermal evaporator for the large area deposition. First, the stoichimetric ratio and the x-ray diffraction of the deposited (Cd,Zn)Te films were analyzed by EPMA and XRD. Secondly, leakage current, x-ray sensitivity, SNR, and linearity were measured to analyze the x-ray detection effect of Zn in (Cd,Zn)Te film. Experimental results showed that the increase of Zn concentration rates in Cd1-xZnxTe detectors reduced the leakage current and improved the x-ray detection performance.

Zinc Sulfide-selenium X-ray Detector for Digital Radiography

The high bias voltage associated with the thick layer (typically 500-1000 ㎛) of selenium required to have an acceptable x-ray absorption in radiography and fluoroscopy applications may have some practical inconvenience. A hybrid x-ray detector with zinc sulfide-amorphous selenium structure has been developed to improve the x-ray sensitivity of a a-Se based flat-panel digital imaging detector. Photoluminescence(PL) characteristic of a ZnS:Ag phosphor layer showed a light emission peak centered at about 450 nm, which matches the sensitivity spectrum of selenium. The dark current of the hybrid detector showed similar characteristics with that of a a-Se detector. The x-ray sensitivity of hybrid and a-Se x-ray detector was 345 pC/㎠/mR and 295 pC/㎠/mR at an applied voltage of 10 V/㎛, respectively. The purpose of this study was to evaluate the pertinence of a solution using a thin selenium layer, as a photosensitive converter, with a thick coating of silver doped zinc sulfide phosphor.