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Nam, Yoonho,Kim, Eung Yeop,Kim, Dong‐,Hyun Wiley Subscription Services, Inc., A Wiley Company 2011 Journal of magnetic resonance imaging Vol.33 No.5
<P><B>Abstract</B></P><P><B>Purpose:</B></P><P>To determine whether a spin‐echo‐based sequence, which are inherently insensitive to magnetic field inhomogeneity, can be used for brain cortical thickness measurement studies.</P><P><B>Materials and Methods:</B></P><P>By using a double inversion recovery (DIR) spin‐echo‐based sequence, cortical thickness estimates were performed from data acquired from seven healthy volunteers. The cortical thickness was also calculated from data acquired using an MPRAGE sequence and the Bland‐Altman analysis was performed for comparison of the two methods. The average signal and contrast to noise ratios (SNR, CNR) of the two methods were also calculated.</P><P><B>Results:</B></P><P>The bias over the entire brain between DIR and MPRAGE was 0.87 ± 0.08 mm. The bias calculated in the major regional lobes were temporal: 0.76 ± 0.09 mm, frontal: 0.89 ± 0.07 mm, parietal: 0.92 ± 0.10 mm, occipital: 0.75 ± 0.12 mm, and cingulate: 0.79 ± 0.10 mm. This thickness difference was due mainly to the boundary difference in the MPRAGE and DIR at the grey matter/cerebral spinal fluid (GM/CSF) regions. The mean SNR and CNR was CNR<SUB>MPRAGE</SUB> = 47.8 ± 8.4 and CNR<SUB>DIR</SUB> = 19.2 ± 2.9, SNR<SUB>MPRAGE</SUB> = 76.8 ± 10.5 and SNR<SUB>DIR</SUB> = 21.1 ± 2.8.</P><P><B>Conclusion:</B></P><P>The study suggests that cortical thickness measurements can be performed using a DIR spin‐echo sequence, which is inherently immune to main field inhomogeneity. Larger thickness measurements were consistently observed in DIR compared with MPRAGE. J. Magn. Reson. Imaging 2011;33:1218–1223. © 2011 Wiley‐Liss, Inc.</P>
Nam, Yoonho,Hwang, Eojin,Jung, Joon-Yong Korean Society of Magnetic Resonance in Medicine 2017 Investigative Magnetic Resonance Imaging Vol.21 No.1
Purpose: To investigate and compensate the effects of respiration-induced B0 variations on fat quantification of the bone marrow in the lumbar spine. Materials and Methods: Multi-echo gradient echo images with navigator echoes were obtained from eight healthy volunteers at 3T clinical scanner. Using navigator echo data, respiration-induced B0 variations were measured and compensated. Fat fraction maps were estimated using $T2^*$-IDEAL algorithm from the uncompensated and compensated images. For manually drawn bone marrow regions, the estimated B0 variations and the calculated fat fractions (before and after compensations) were analyzed. Results: An increase of temporal B0 variations from inferior level to superior levels was observed for all subjects. After compensation using navigator echo data, the effects of the B0 variations were reduced in gradient echo images. The calculated fat fractions show significant differences (P < 0.05) in L1 and L3 between the uncompensated and the compensated. Conclusion: The results of this study raise the need for considering respiration-induced B0 variations for accurate fat quantification using gradient echo images in the lumbar spine. The use of navigator echo data can be an effective way for the reduction of the effects of respiratory motion on the quantification.
프라이버시 보호를 위한 P2P 기반 분산형 소셜 네트워크 서비스
남윤호(Yoonho Nam),문종호(Jongho Mun),정재욱(Jaewook Jung),원동호(Dongho Won) 한국컴퓨터정보학회 2012 한국컴퓨터정보학회 학술발표논문집 Vol.20 No.2
최근 소셜 네트워크 서비스의 인기가 높아짐과 더불어 유저의 프라이버시에 대한 관심도 증가하고 있다. 기존의 소셜 네트워크 서비스는 중앙 집중형 구조를 가지고 있으므로 모든 유저의 프라이버시 정보와 행동들은 서비스 제공자에게 수집되어 진다. 본 논문에서는 중앙 집중식 구조의 무분별한 정보 수집을 제거하고자 오픈 소스를 이용한 P2P 기반 분산형 소셜 네트워크 서비스를 제안한다.
오픈 아이디를 이용한 오픈 소스 기반 분산형 소셜 네트워크 서비스
남윤호 ( Yoonho Nam ),조승현 ( Seunghyun Cho ),문종호 ( Jongho Mun ),정재욱 ( Jaewook Jung ),전웅렬 ( Woongryul Jeon ),원동호 ( Dongho Won ) 한국정보처리학회 2013 한국정보처리학회 학술대회논문집 Vol.20 No.1
소셜 네트워크 서비스를 이용하는 사람들이 증가하면서, 프라이버시와 관련된 보안 문제들 또한 이슈가 되고 있다. 기존의 소셜 네트워크 서비스들은 일반적으로 중앙 집중형 구조를 가지고 있다. 서비스 사용자들의 기본적인 프로필 정보들은 서비스 제공자에게 수집되어 빅데이터를 이룬다. 이러한 빅데이터가 서비스 제공자 측면에서는 상업적인 용도로 사용되지만, 사용자 개인의 입장에서는 자신의 개인정보가 악의적인 목적으로 사용되는지 전혀 알 수가 없다. 따라서 서비스 제공자의 무분별한 정보 수집 문제를 해결하기 위해 원천적으로 중앙 집중형 구조를 제거하고, 기존의 포털사이트와의 연동을 통해 오픈 아이디로 이용 가능한 오픈 소스 기반 분산형 소셜 네트워크 서비스를 제안한다.
3D magnetotelluric modelling including surface topography
Nam, Myung Jin,Kim, Hee Joon,Song, Yoonho,Lee, Tae Jong,Son, Jeong-Sul,Suh, Jung Hee Blackwell Publishing Ltd 2007 Geophysical prospecting Vol.55 No.2
<P>ABSTRACT</P><P>An edge finite-element method has been applied to compute magnetotelluric (MT) responses to three-dimensional (3D) earth topography. The finite-element algorithm uses a single edge shape function at each edge of hexahedral elements, guaranteeing the continuity of the tangential electric field while conserving the continuity of magnetic flux at boundaries. We solve the resulting system of equations using the biconjugate gradient method with a Jacobian preconditioner. The solution gives electric fields parallel to the slope of a surface relief that is often encountered in MT surveys. The algorithm is successfully verified by comparison with other numerical solutions for a 3D-2 model for comparison of modelling methods for EM induction and a ridge model. We use a 3D trapezoidal-hill model to investigate 3D topographic effects, which are caused mainly by galvanic effects, not only in the Z<I>xy</I> mode but also in the Z<I>yx</I> mode. If a 3D topography were approximated by a two-dimensional topography therefore errors occurring in the transverse electric mode would be more serious than those in the transverse magnetic mode.</P>
Nam, Myung Jin,Kim, Hee Joon,Song, Yoonho,Lee, Tae Jong,Suh, Jung Hee Blackwell Publishing Ltd 2009 Geophysical journal international Vol.176 No.2
<P>SUMMARY</P><P>Magnetotelluric (MT) surveys have been conducted to investigate the volcanic structure in Jeju Island in the South Sea of Korea. The island is oval in shape, 31 km wide and 73 km long, with a peak elevation of 1950 m at Mt. Halla, located in the centre of the island. MT data were obtained in a wide range of frequencies in a mid-mountain region and may be affected by distortion of electric fields due to topographic and bathymetric effects. The purpose of this paper is to investigate the topographic and bathymetric effects on MT data for a four-layered model having the topographic and bathymetric undulation of Jeju Island, using an edge finite-element method. The model effectively reproduces a general feature of MT sounding curves observed on the island. The topographic effect appears only at high frequencies, whereas the sea effect causes a split of apparent resistivity curves in different polarizations at low frequencies. Since the island has a gentle slope of less than 10° except in the close vicinity of Mt. Halla, it has a similar effect of a change in thickness in the top layer. In contrast, the sea effect can be observed as quasi-2-D rather than 3-D along the major axis of the island and generates a parallel shift of the log-apparent resistivity curve at low frequencies.</P>
김동현,Yoonho Nam,Sung-Min Gho,Dongyeob Han,Eung Yeop Kim,Sheen-Woo Lee,Hongpyo Lee 대한자기공명의과학회 2018 Investigative Magnetic Resonance Imaging Vol.22 No.4
Purpose: The objective of this study was to obtain improved susceptibility weighted images (SWI) of the cervical spinal cord using respiratory-induced artifact compensation. Materials and Methods: The artifact from B0 fluctuations by respiration could be compensated using a double navigator echo approach. The two navigators were inserted in an SWI sequence before and after the image readouts. The B0 fluctuation was measured by each navigator echoes, and the inverse of the fluctuation was applied to eliminate the artifact from fluctuation. The degree of compensation was quantified using a quality index (QI) term for compensated imaging using each navigator. Also, the effect of compensation was analyzed according to the position of the spinal cord using QI values. Results: Compensation using navigator echo gave the improved visualization of SWI in cervical spinal cord compared to non-compensated images. Before compensation, images were influenced by artificial noise from motion in both the superior (QI = 0.031) and inferior (QI = 0.043) regions. In most parts of the superior regions, the second navigator resulted in better quality (QI = 0.024, P < 0.01) compared to the first navigator, but in the inferior regions the first navigator showed better quality (QI = 0.033, P < 0.01) after correction. Conclusion: Motion compensation using a double navigator method can increase the improvement of the SWI in the cervical spinal cord. The proposed method makes SWI a useful tool for the diagnosis of spinal cord injury by reducing respiratory-induced artifact.