• 한국의학물리학회:학술대회논문집
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• 한국의학물리학회 1999년도 Japanese Journal of Medical Physics
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• pp.414-415
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• 1999

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 1996년도 춘계학술대회 논문집
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• pp.198-201
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• 1996

In order to investigate control factors to critical current density, Ag sheathed Bi2223 superconducting tapes were fabricated using PIT process. Optimizing the reduction ratio of rolling, critical current density 7f rolled Bi2223 tape could be improved with the value of 15,000 A/$\textrm{cm}^2$(77 K, zero field). The correlation between J$\_$c/ and work inhomogeneity was revealed as a dependence of COV of measured oxide layer thickness.

• 한국표면공학회:학술대회논문집
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• 한국표면공학회 2015년도 추계학술대회 논문집
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• pp.137-137
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• 2015

The purposed of this work was to observe hydroxyapatite precipitation phenomena on micro-pore formed Ti-Nb alloy by PEO technique. The Ti-30Nb and Ti-30Ta alloys were remelted at least ten times in order to avoid inhomogeneity, and then cylindrical specimens (diameter 10 mm, thickness 4 mm) were cut by using laser from cast ingots of the Ti alloys. Heat treatment was carried out at $1050^{\circ}C$ for 2 h for homogenization in argon atmosphere. The morphologic change of the alloys were examined by X-ray diffractometer (XRD) and field emission scanning electron microscopy (FE-SEM).

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2003년도 한국우주과학회보 제12권2호
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• pp.40-40
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• 2003

When the magnetotail is disturbed by an impulsive input such as the substorm onset, compressional magnetohydrodynamic (MHD) waves play an important role in delivering perturbed energy and exciting various wave modes and currents. The plasmasheet, in which relatively hot plasmas exist, is surrounded by relatively cold plasmas at the plasma sheet boundary layer (PSBL) and the equatorial plasmasphere. Since the Alfven speed significantly varies near these regions, the compressional waves are expected to undergo mode conversion by inhomogeneity at the boundary between cold and hot plasma regions. We investigate how the initial compressional MHD wave energy is reflected, transmitted, and absorbed across that boundary by adopting the invariant imbedding method (IIM) which gives the exact reflection, transmission, and absorption coefficients without any theoretical approximations for given frequencies and wave numbers. The IIM method is very useful in quantifying the reflection and transmission of compressional waves in the sense that we can calculate how much fast mode wave energy is delievered into shear Alfven waves or field-aligned currents. Our results show that strongly localized absorption occurs at the boundary region. This feature suggests that localized field-aligned currents can be impulsively excited at such boundary regions by any compressional disturbances, which is highly associated with impulsive auroral brightening at the substorm onset. We compare our results with previous studies in cold inhomogeneous plasmas.

• Investigative Magnetic Resonance Imaging
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• 제20권3호
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• pp.181-184
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• 2016

Short inversion time inversion recovery (STIR) is widely used for spinal magnetic resonance imaging (MRI) because the pulse sequence of STIR is insensitive to magnetic field inhomogeneity and can be used to scan a large field of view. In this case report, we present a case of spinal epidural hematoma with unexpected signal decrease on a STIR image. The MRI showed an epidural mass that appeared with high signal intensity on both T1- and T2-weighted images. However, a signal decrease was encountered on the STIR image. This nonspecific decrease of signal in tissue with a short T1 relaxation time that is similar to that of fat (i.e., hemorrhage) could lead to a diagnostic pitfall; one could falsely diagnose this decrease of signal as fat instead of hemorrhage. Awareness of the nonselective signal suppression achieved with STIR pulse sequences may avert an erroneous diagnosis in image interpretation.

• 한국의학물리학회지:의학물리
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• 제31권4호
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• pp.189-193
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• 2020

An MR-based attenuation correction (MRAC) map plays an important role in quantitative positron emission tomography (PET) image evaluation in PET/magnetic resonance imaging (MRI) systems. However, the MRAC map is affected by the magnetic field inhomogeneity of MRIs. This study aims to evaluate the characteristics of MRAC maps of physical phantoms on PET/MRI images. Phantom measurements were performed using the Siemens Biograph mMR. The modular type physical phantoms that provide assembly versatility for phantom construction were scanned in a four-channel Body Matrix coil. The MRAC map was generated using the two-point Dixon-based segmentation method for whole-body imaging. The modular phantoms were scanned in compact and non-compact assembly configurations. In addition, the phantoms were scanned repeatedly to generate MRAC maps. The acquired MRAC maps show differently assigned values for void areas. An incorrect assignment of a void area was shown on a locally compact space between phantoms. The assigned MRAC values were distorted using a wide field-of-view (FOV). The MRAC values also differed after repeated scans. However, the erroneous MRAC values appeared outside of phantom, except for a large FOV. The MRAC map of the phantom was affected by phantom configuration and the number of scans. A quantitative study using a phantom in a PET/MRI system should be performed after evaluation of the MRAC map characteristics.

• Radiation Oncology Journal
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• 제22권2호
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• pp.155-163
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• 2004

목적 : 두경부에 방사선 치료를 받는 환자에서 투과선량측정시스템을 적용하여 시스템의 재현성과 선량 계산 알고리즘의 정확도를 확인하기 위하여 본 연구를 시행하였다. 대상 및방법 : 2001년 9월 5일부터 9월 18일까지 괴 MV선형가속기를 이용하여 두경부에 방사선치료를 시행한 환자를 대상으로 하였으며, 3회 이상의 측정을 시행하고 조사야의 중심이 차폐되지 않은 35명의 환자를 대상으로 분석을 시행하였다. 매일 환자의 치료 시작 전과 치료 중 한 시간 간격으로 10$\times$10 cm$^{2}$개방 조사야에 100 MU을 조사하고 측정을 시행하여 시스템의 안정성 및 재현성을 확인하였다. 투과선량 계산 알고리즘의 정확도를 평가하기 위하여 두경부 방사선치료 환자에서 투과선량의 예상치를 구하고 측정치와 비교하였다. 투과선량 예상치는 환자의 CT 또는 MR 영상과 모의치료 시의 X-선 필름을 사용하여 방사선 투과 부위의 골조직, 공기, 지방조직의 두께를 측정하고 이에 대한 불균질 조직의 영향을 보정하였다. 결과 : 10$\times$10 cm$^{2}$개방 조사야에 100 MU을 조사하여 측정을 시행하였을 때 측정치의 일 중의 오차가 $\pm$0.5$\%$ 이내이고 일간의 오차가 $\pm$1.0$\%$ 이내로 시스템의 안정성 및 재현성을 확인할 수 있었다. 투과선량 예상치와 실제 측정치 간의 평균 오차는 뇌 부위 치료 환자에서는 $\pm$5$\%$ 이하, 두부 치료 환자에서는 $\pm$2.5$\%$ 이하, 경부 치료 환자에서는 $\pm$5$\%$ 이하이었다. 결론 : 투과선량측정시스템의 안정성과 재현성을 확인할 수 있었고, 외부 형태가 불규칙하고 불균질 조직이 포함되는 부위에 대하여 CT, MR 등의 영상을 이용하여 불균질 조직에 대한 보정을 함으로써 정확한 예상치를 구할 수 있음을 확인하였다.

• 대한방사선기술학회지:방사선기술과학
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• 제25권1호
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• pp.63-71
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• 2002

The aim of this study Is to develop a simple and fast method which computes in-vivo doses from transmission doses measured doting patient treatment using an ionization chamber. Energy fluence and the dose that reach the chamber positioned behind the patient is modified by three factors: patient attenuation, inverse square attenuation. and scattering. We adopted a straightforward empirical approach using a phantom transmission factor (PTF) which accounts for the contribution from all three factors. It was done as follows. First of all, the phantom transmission factor was measured as a simple ratio of the chamber reading measured with and without a homogeneous phantom in the radiation beam according to various field sizes($r_p$), phantom to chamber distance($d_g$) and phantom thickness($T_p$). Secondly, we used the concept of effective field to the cases with inhomogeneous phantom (patients) and irregular fields. The effective field size is calculated by finding the field size that produces the same value of PTF to that for the irregular field and/or inhomogeneous phantom. The hypothesis is that the presence of inhomogeneity and irregular field can be accommodated to a certain extent by altering the field size. Thirdly, the center dose at the prescription depth can be computed using the new TMR($r_{p,eff}$) and Sp($r_{p,eff}$) from the effective field size. After that, when TMR(d, $r_{p,eff}$) and SP($r_{p,eff}$) are acquired. the tumor dose is as follows. $$D_{center}=D_t/PTF(d_g,\;T_p){\times}(\frac{SCD}{SAD})^2{\times}BSF(r_o){\times}S_p(r_{p,eff}){\times}TMR(d,\;r_{p,eff})$$ To make certain the accuracy of this method, we checked the accuracy for the following four cases; in cases of regular or irregular field size, inhomogeneous material included, any errors made and clinical situation. The errors were within 2.3% for regular field size, 3.0% irregular field size, 2.4% when inhomogeneous material was included in the phantom, 3.8% for 6 MV when the error was made purposely, 4.7% for 10 MV and 1.8% for the measurement of a patient in clinic. It is considered that this methode can make the quality control for dose at the time of radiation therapy because it is non-invasive that makes possible to measure the doses whenever a patient is given a therapy as well as eliminates the problem for entrance or exit dose measurement.

• 대한의용생체공학회:의공학회지
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• 제29권2호
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• pp.99-106
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• 2008

To improve $B_1$ homogeneity in high field MRI, the RF power is applied to the transmit array coil elements sequentially in the time-multiplexed way. Since only a single coil element is activated in a time-multiplexing slot, the global standing wave formation in the human body is greatly suppressed. The time-multiplexing slot width is on the order of micro seconds, hence, high-order-harmonic slices can be placed far from the transmit coil and simultaneous multiple slice selection can be avoided. The $B_1$ homogeneities of a birdcage coil and an eight-channel transmit array coil have been compared through finite difference time domain simulations. The simulation results indicate that the proposed technique can reduce the peak-to-peak $B_1$ inhomogeneity down to one fourth of the transmission with a birdcage coil on the central plane of the human head model at 3 T. The mimicking experiments at 3 T, eight separate experiments with a single coil element activated and image reconstruction by combining the eight images, also show promising results. It is expected that the proposed technique has some advantages over other $B_1$ improving methods in real practice since simple RF switching circuitries are only necessary and electromagnetic coupling between the coil elements is out of concern in its realization.

• 한국전자파학회논문지
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• 제23권1호
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• pp.96-100
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• 2012

정 자장($B_0$)의 세기가 7 T(Tesla) 또는 9.4 T 고자기장 MRI(Magnetic Resonance Imaging) 시스템은 정 자장의 세기가 1.5 T 또는 3 T MRI(Magnetic Resonance Imaging) 시스템에 비하여 인가된 RF(Radio Frequency) 필드의 높은 불균질성을 보여준다. 다채널(multi-channel) RF 코일에서는 인가된 RF 자장($B_1^+$)의 불균질성을 개선시키기 위해서 각각의 코일 소자(element)에 인가되는 전류의 크기와 위상을 독립적으로 조절할 수 있다. 선택된 관심 영역에서의 RF 자장이 균일하도록 RF 코일의 각 요소로 들어가는 최적화된 전류의 크기와 위상 값을 얻기 위해서 iterative 방법과 함께 convex 최적화 방법이 사용된다. 이러한 방법을 입증하기 위하여 9.4 T MRI 시스템에 RF 코일의 공진 주파수가 400 MHz을 가지는 다채널 전송 선로 코일이 인체 두상 모형과 함께 모델링되었으며, 이 코일에 의하여 자장이 얻어진다. 9.4 T MRI 시스템을 위한 시뮬레이션 결과가 자세히 논의된다.