Se Jin Ahn

Seoul National University, Sŏul, Seoul, South Korea

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Publications (20)26.44 Total impact

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    ABSTRACT: To compare the image quality of shoulder CT arthrography performed using 120 kVp and 140 kVp protocols. Fifty-four CT examinations were prospectively included. CT scans were performed on each patient at 120 kVp and 140 kVp; other scanning parameters were kept constant. Image qualities were qualitatively and quantitatively compared with respect to noise, contrast, and diagnostic acceptability. Diagnostic acceptabilities were graded using a one to five scale as follows: 1, suboptimal; 2, below average; 3, acceptable; 4, above average; and 5, superior. Radiation doses were also compared. Contrast was better at 120 kVp, but noise was greater. No significant differences were observed between the 120 kVp and 140 kVp protocols in terms of diagnostic acceptability, signal-to-noise ratio, or contrast-to-noise ratio. Lowering tube voltage from 140 kVp to 120 kVp reduced the radiation dose by 33%. The use of 120 kVp during shoulder CT arthrography reduces radiation dose versus 140 kVp without significant loss of image quality.
    Preview · Article · Nov 2014 · Korean journal of radiology: official journal of the Korean Radiological Society
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    ABSTRACT: We report a new approach for fabricating pyrite (cubic FeS2) thin films via a non-vacuum direct solution coating route using iron-acetate dissolved in ethanol as a precursor solution. The precursor ink is deposited by spin coating and annealed in air at 300 oC to produce amorphous iron oxide films. Subsequent heat treating of the iron oxide films in a sulfur environment at 450 oC results in the formation of phase-pure, carbon-free, large-grained pyrite films. In particular, the phase evolution during the sulfurization is systematically investigated, focusing on the effects of the principal experimental variables of temperature and pressure. As the temperature increases, iron oxide first begins to transform into marcasite (orthorhombic FeS2) and then is completely converted to pyrite. Further increasing the temperature above 500 oC induces the decomposition of pyrite into pyrrhotite (Fe1-xS) and sulfur vapor. The decomposition is also strongly dependent on the working pressure: as the pressure decreases, the decomposition occurs at lower temperature. The synthesized single-phase pyrite films show an indirect band gap of 0.94 eV, a strong anodic photocurrent based on photo-electrochemical measurements, and an n-type semiconducting property based on Mott-Schottky analysis. Thus, the films demonstrate great potential for use as absorbing layers in solar cells
    No preview · Article · Sep 2014
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    Full-text · Dataset · Jun 2014
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    ABSTRACT: We developed a simple and effective method for the large scale formation of self-assembled Cu(In,Ga)Se2 (CIGS) nanocrystals by ion beam irradiation. The compositional changes and morphological evolution were observed as a function of the irradiation time. As the ion irradiation time increased, the nano-dots were transformed into a nano-ridge structure due to the competition between sputtering and diffusion processes during irradiation. In terms of the stoichiometry of the CIGS nano-dots, an increase in the Cu content was observed while the Se content decreased. The PL peak of the nano-dots formed CIGS thin film exhibited a blue-shift. Uniformly formed crystalline CIGS nano-dots can be adopted to increase the p–n junction area and the size confinement effect between the CdS and CIGS film in solar cell systems. This simple method can be exploited for band-gap engineering and enhancing photovoltaic properties.
    Full-text · Article · Oct 2012 · Solar Energy Materials and Solar Cells
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    ABSTRACT: To evaluate the diagnostic performance of computed tomography (CT) colonography (CTC) reconstructed with different levels of adaptive statistical iterative reconstruction (ASiR, GE Healthcare) and Veo (model-based iterative reconstruction, GE Healthcare) at various tube currents in detection of polyps in porcine colon phantoms. Five porcine colon phantoms with 46 simulated polyps were scanned at different radiation doses (10, 30, and 50 mA s) and were reconstructed using filtered back projection (FBP), ASiR (20%, 40%, and 60%) and Veo. Eleven data sets for each phantom (10-mA s FBP, 10-mA s 20% ASiR, 10-mA s 40% ASiR, 10-mA s 60% ASiR, 10-mA s Veo, 30-mA s FBP, 30-mA s 20% ASiR, 30-mA s 40% ASiR, 30-mA s 60% ASiR, 30-mA s Veo, and 50-mA s FBP) yielded a total of 55 data sets. Polyp detection sensitivity and confidence level of 2 independent observers were evaluated with the McNemar test, the Fisher exact test, and receiver operating characteristic curve analysis. Comparative analyses of overall image quality score, measured image noise, and interpretation time were also performed. Per-polyp detection sensitivities and specificities were highest in 10-mA s Veo, 30-mA s FBP, 30-mA s 60% ASiR, and 50-mA s FBP (sensitivity, 100%; specificity, 100%). The area-under-the-curve values for the overall performance of each data set was also highest (1.000) at 50-mA s FBP, 30-mA s FBP, 30-mA s 60% ASiR, and 10-mA s Veo. Images reconstructed with ASiR showed statistically significant improvement in per-polyp detection sensitivity as the percent level of per-polyp sensitivity increased (10-mA s FBP vs 10-mA s 20% ASiR, P = 0.011; 10-mA s FBP vs 10-mA s 40% ASiR, P = 0.000; 10-mA s FBP vs 10-mA s 60% ASiR, P = 0.000; 10-mA s 20% ASiR vs 40% ASiR, P = 0.034). Overall image quality score was highest at 30-mA s Veo and 50-mA s FBP. The quantitative measurement of the image noise was lowest at 30-mA s Veo and second lowest at 10-mA s Veo. There was a trend of decrease in time required for image interpretation as the percent level of ASiR increased, and ASiR or Veo was used instead of FBP. However, differences from comparative analyses of overall image quality score, measured image noise, and interpretation time did not reach statistical significance. ASiR and Veo showed improved diagnostic performance with excellent sensitivity and specificity with less image noise and good image quality compared with FBP reconstruction of same radiation dose. Our study confirmed feasibility of low-dose CTC with iterative reconstruction as a promising screening tool with excellent diagnostic performance similar to that of the standard-dose CTC with FBP.
    No preview · Article · Sep 2012 · Journal of computer assisted tomography
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    Se Jin Ahn · In-One Kim
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    ABSTRACT: Radiation-induced spinal cord gliomas are extremely rare. Since the first case was reported in 1980, only six additional cases have been reported.; The radiation-induced gliomas were related to the treatment of Hodgkin's lymphoma, thyroid cancer, and medullomyoblastoma, and to multiple chest fluoroscopic examinations in pulmonary tuberculosis patient. We report a case of radiation-induced spinal cord glioblastoma developed in a 17-year-old girl after a 13-year latency period following radiotherapy for nasopharyngeal rhabdomyosarcoma. MRI findings of our case are described.
    Preview · Article · Sep 2012 · Korean journal of radiology: official journal of the Korean Radiological Society
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    ABSTRACT: A histologic grade in head and neck squamous cell carcinoma (HNSCC) is clinically important because of its association with prognosis. The purpose of this study was to investigate the efficacy of histographic analysis of apparent diffusion coefficient (ADC) maps on the basis of the entire tumor volume in differentiating histologic grades in HNSCC at standard (b = 1000 s/mm(2)) and high (b = 2000 s/mm(2)) b values. Fifty-four patients with HNSCC, including well-differentiated (WD; n = 35), moderately differentiated (MD; n = 13) and poorly differentiated (PD; n = 6) carcinomas, were retrospectively evaluated. ADC maps were obtained at two different b values (1000 and 2000 s/mm(2)) in each patient. Tumors were delineated on each slice of ADC maps, and data were collected to obtain a histogram for the entire tumor volume. Histographic parameters were calculated, including mean, standard deviation, kurtosis, skewness, and the ratio of the kurtosis measured at b values of 1000 and 2000 s/mm(2). These parameters were correlated with histologic grades. There was no significant correlation between tumor grades and histographic parameters obtained from ADC maps at b = 1000 s/mm(2). However, mean ADC at b = 2000 s/mm(2) was significantly higher in WD HNSCC (881 ± 131 × 10(-6) mm(2)/s) than in MD and PD HNSCC (770 ± 163 and 780 ± 158 × 10(-6) mm(2)/s, respectively) (P < .05). Kurtosis ratio was significantly higher in PD HNSCC (115 ± 10%) compared to WD and MD HNSCC (91 ± 21% and 86 ± 26%, respectively) (P < .05). Diagnostic accuracy was 100%, 76.9%, and 65.8% for PD, MD, and WD HNSCC, respectively. Histographic analysis of ADC maps on the basis of the entire tumor volume can be useful in differentiating histologic grades of HNSCC using mean ADC at b = 2000 s/mm(2) and kurtosis ratio.
    No preview · Article · Jul 2012 · Academic radiology
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    ABSTRACT: To assess the technical success and complication rates of the radiologic placement of central venous ports via the internal jugular vein. We retrospectively reviewed 1254 central venous ports implanted at our institution between August 2002 and October 2009. All procedures were guided by using ultrasound and fluoroscopy. Catheter maintenance days, technical success rates, peri-procedural, as well as early and late complication rates were evaluated based on the interventional radiologic reports and patient medical records. A total of 433386 catheter maintenance days (mean, 350 days; range 0-1165 days) were recorded. The technical success rate was 99.9% and a total of 61 complications occurred (5%), resulting in a post-procedural complication rate of 0.129 of 1000 catheter days. Among them, peri-procedural complications within 24 hours occurred in five patients (0.4%). There were 56 post-procedural complications including 24 (1.9%, 0.055 of 1000 catheter days) early and 32 (2.6%, 0.074 of 1000 catheter days) late complications including, infection (0.6%, 0.018 of 10000 catheter days), thrombotic malfunction (1.4%, 0.040 of 1000 catheter days), nonthrombotic malfunction (0.9%, 0.025 of 1000 catheter days), venous thrombosis (0.5%, 0.014 of 1000 catheter days), as well as wound problems (1.1%, 0.032 of 1000 catheter days). Thirty six CVPs (3%) were removed due to complications. Bloodstream infections and venous thrombosis were the two main adverse events prolonging hospitalization (mean 13 days and 5 days, respectively). Radiologic placement of a central venous port via the internal jugular vein is safe and efficient as evidenced by its high technical success rate and a very low complication rate.
    Full-text · Article · May 2012 · Korean journal of radiology: official journal of the Korean Radiological Society
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    ABSTRACT: In this paper, we demonstrate, for the first time, the manufacture of a CuInSe2 thin film whose absorber layer is coated using an electrostatic spray deposition (ESD) technique; its complete transformation into a working device with measured conversion efficiency is presented. ESD is superior to pneumatic spraying because it produces nano-scaled, self-dispersive (non-agglomerating), highly wettable (electrowetting) and adhesive droplets to yield a uniform coating on a substrate. Furthermore, ESD’s extremely low material consumption rate holds promises for practical use in the solar cell industry. Copper and indium salts are added to various solvents, which are electrostatically sprayed onto a molybdenum-coated soda-lime glass substrate. The effect of substrate temperature on the thin film characteristics is examined. Our cell is completed by adding CdS and ZnO layers onto the CuInSe2 absorber layer. Light illuminated current-density voltage (J-V) characteristics demonstrate a power conversion efficiency of η = 1.75% ± 0.09 with an open-circuit voltage of VOC = 0.23 V, a short-circuit current density of JSC = 21.72 mA/cm2, and fill factor of FF = 0.34.
    Full-text · Article · Feb 2012 · Journal of The Electrochemical Society
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    ABSTRACT: CuInxGa1−xSeyS2−y (CIGS) thin films were synthesized on glass substrates by a paste coating of Cu, In, and Ga precursor solution with a three-step heat treatment process: oxidation, sulfurization, and selenization. In particular, morphological changes of CIGS films for each heat treatment step were investigated with respect to the kinds of glass substrates: bare, Mo-coated, and F-doped SnO2 (FTO) soda-lime glasses. Very high quality CIGS film with large grains and low degree of porosity was obtained on the bare glass substrate. Similar morphology of CIGS film was also acquired on the Mo-coated glass except the formation of an undesired Mo oxide interfacial layer due to the partial oxidation of Mo layer during the first heat treatment under ambient conditions. On the other hand, CIGS film with much smaller grains and higher degree of porosity was gained when FTO glass was used as a substrate, resulting in slight solar to electricity conversion behavior (0.20%). Higher power conversion efficiency (1.32%) was attained by the device with the CIGS film grown on Mo-coated glass in spite of the presence of a Mo oxide impurity layer.
    No preview · Article · Oct 2011 · Applied Surface Science
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    ABSTRACT: CuGaSe 2 layers, which are wide-gap chalcopyrite compounds, were prepared by evaporating elemental Cu, Ga, and Se on molybdenum-coated soda lime glass. To control the compositional ratios of each element of the resultant CuGaSe 2 layers and to optimize the grain sizes and the surface roughness of the CuGaSe 2 layers, we utilized an in-situ substrate temperature monitoring system. By increasing the Cu supplement, we obtained a large and smooth grain by means of the Cu-Se secondary phase, which led to a temperature drop of the substrate. However, many pores along the grain boundaries were observed on the surface. The pores on the surface were then removed by an additional Ga supplement while a small grain Ga-rich layer was formed on the surface, increasing the series resistance of the solar cell. Thus, with appropriate ratios of Cu and Ga composition, a conversion efficiency of 6.3 % was achieved.
    Preview · Article · Jun 2007 · Journal- Korean Physical Society
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    ABSTRACT: To make a dense CIGS absorber layer, spray deposited CIGS films were annealed in the two-zone RTP furnace in Se atmosphere. More Se supply by increasing Se evaporation temperature or by increasing the flow rate of carrier gas resulted in the larger CIGS grains. However, a thick MoSe2 layer was formed between CIGS and Mo, as the Se supply increased, results in partial detachment of CIGS/MoSe2/Mo layers from the glass substrate. From the result, it was found that the short heat- treatment with high Se vapor pressure is better than the long heat-treatment with low Se vapor pressure. The large CIGS grains without peeling off, can be obtained from the following conditions; Se evaporation temperature of 450°C, substrate temperature of 550°C, annealing time of 5 min, and flow rate of carrier gas of 30 sccm.
    Full-text · Article · Jun 2007 · Solid State Phenomena
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    ABSTRACT: The photovoltaic properties of CIGS cells on an alumina substrate were improved through the use of Na-doped Mo as the bottom layer of a Mo back contact. Na was supplied to the CIGS bulk region from an alumina/Na-doped Mo/Mo structure, similar to the Na diffusion from soda-lime glass. The diffusion of Na from the Na-doped Mo was controlled effectively compared to that from Soda-lime glass (SLG). The present results indicate that Na-doped Mo acts as a Na source material and that the Na amount can be controlled by adjustment of thickness of Na-doped Mo layer, without the use of an alkali barrier layer. The highest conversion efficiency of 13.34% (Jsc=34.62 mA/cm2, Voc=0.58 V and FF=66%) for an active area of 0.45 cm2 on an alumina substrate was obtained for 100 nm Na-doped Mo/1000 nm Mo.
    Full-text · Article · May 2007 · Thin Solid Films
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    ABSTRACT: The dynamic behavior of the cylindrical shell with uniform flow is formulated by the finite element method. The dynamics of the shell is based on Donnell’s theory and the fluid in cylindrical shell is considered satisfying the Helmholtz equation. The effective thickness of fluid is calculated according to the circumferential modes and the frequencies. An estimation of the FRF (frequency response function) of the shell with taking into consideration of the coupled effects of the internal fluid is presented. These results are compared with the results considering fluid satisfying Laplace equation. The influence of fluid velocity on the FRF is also discussed.
    No preview · Article · Jun 2006 · JSME International Journal Series C
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    ABSTRACT: The wide band-gap CuIn1-xAlxSe2(CIAS) films were obtained by selenization process of metallic precursors. The metallic precursors were deposited sequentially by using sputtering system. As the ratio of Al/(Al+ln) in the precursors increased, the chalcopyrite (112) peak shifted to high value and the band-gap of CIAS layer increased to 1.38 eV. However the bilayer morphology with well crystallized large grain on the surface and small grain thin bottom layer was observed. Although the sequences of precursors were changed in order to get uniform layer, the distinguishable difference was not observed. Moreover the films were peeled off completely during the deposition of buffer CdS. The other process conditions such as selenizing at high temperatures, Se containing precursors and post annealing will be tried to remove the bilayer and to get homogeneous films
    No preview · Article · Jan 2006
  • Se Jin Ahn · Weui Bong Jeong · Wan Suk Yoo
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    ABSTRACT: Since the impulse response of a system is a transient and non-stationary signal, difference of record length makes the unit impulse response spectrum of the system to be not unique. It means that the magnitudes and phases of the frequency response function obtained from impact hammer testing depend on the record length. Whenever the record length is not longer than the duration time during which the impulse response completely diminishes, ‘finite record length error’ takes place in the impulse response spectrum. In this paper, the finite record length error is theoretically verified and a new method reducing the error is introduced. The system parameters to induce the finite record length error are investigated, and an optimization method is also proposed to reduce this error. Numerical and experimental examples are carried out to show the characteristics of the finite record length error and the validity of the new method proposed in this paper.
    No preview · Article · Dec 2005 · Journal of Sound and Vibration

  • No preview · Article · Jan 2005 · JSME International Journal Series C
  • Se Jin Ahn · Weui Bong Jeong · Wan Suk Yoo
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    ABSTRACT: The exponential window function is widely used in impact hammer testing to reduce leakage errors as well as to get a nice S/N ratio. The larger the exponential decay rate of the window is, the more effectively the leakage errors are reduced. But if the decay rate of the exponential window function is too large, the frequency response function (FRF) is distorted by its side effects. The modal parameters of the system cannot be exactly identified from the distorted FRF even though modal analysis technique is used. Therefore, it is a difficult problem to determine a proper exponential decay rate in an impact hammer testing.In this paper, the amount of the FRF distortion caused by the exponential window function is theoretically uncovered, and an unbiased expression of the exponential-windowed FRF is represented. A new circle fitting method is also proposed so that the modal parameters are directly extracted from the impulse response spectrum distorted by the exponential window function. The results identified by the conventional and proposed circle fitting method are compared through numerical examples.
    No preview · Article · Nov 2004 · Journal of Sound and Vibration
  • Se Jin AHN · Weui Bong JEONG · Wan Suk YOO
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    ABSTRACT: The spectrum of impulse response signal from the impact hammer testing is widely used to obtain the frequency response function (FRF). However the FRFs obtained from the impact hammer testing have not only leakage errors but also finite record length errors when the record length for the signal processing is not sufficiently long. The errors cannot be removed by the conventional signal analyzer which treats the signals as if they are always steady and periodic. Since the response signals generated by the impact hammer testing are transient and damped, they are undoubtedly non-periodic. It is inevitable that the signals acquired for limited recording time should cause several errors. This paper makes clear the relation between the errors in the FRF and the length of the recording time. A new method is suggested to reduce the errors in the FRF in this paper. Several numerical examples involving 1-dof models are considered to elucidate the properties of the errors and the validity of the proposed method.
    No preview · Article · Sep 2004 · JSME International Journal Series C
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    ABSTRACT: Acoustic intensity is usually estimated from the cross-spectrum of acoustic pressures measured by two adjacent microphones. The cross-spectrum, calculated by the digital Fourier transform technique, has unavoidable leakage error because the signal period does not usually coincide with the record length. Therefore, acoustic intensity, estimated by a conventional FFT analyzer, has distorted values. In our research, the expression of Fourier-transformed data of a harmonic signal with a single frequency was formulated when there was leakage error. A method to eliminate the effect of leakage error from contaminated data was also investigated. Some numerical examples show the validation of the proposed method.
    Preview · Article · Jan 2004 · JSME International Journal Series A

Publication Stats

160 Citations
26.44 Total Impact Points

Institutions

  • 2012-2014
    • Seoul National University
      • College of Medicine
      Sŏul, Seoul, South Korea
    • Seoul National University Hospital
      Sŏul, Seoul, South Korea
  • 2007-2014
    • Korea Institute of Energy Research
      • Solar Energy Department
      Sŏul, Seoul, South Korea
  • 2004-2006
    • Pusan National University
      • Department of Mechanical Engineering
      Busan, Busan, South Korea