• 대한전기학회논문지:전기기기및에너지변환시스템부문B
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• 제50권12호
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• pp.587-591
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• 2001

This paper describes the general and efficient design method of an axial and a radial shim coils to correct field impurities of various harmonic orders in the imaging volume of Magnetic Resonance Imaging magnet. Shim coils are optimized by BCLSF subroutine of IMSL, which is the well-known commercial package for optimization, aiming at maximizing the magnitude of the desired field component as well as minimizing other field components. In order to evaluate their effect, the developed method was applied to the MRI magnet constructed in KERI.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2011년도 추계학술대회 논문집
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• pp.384-389
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• 2011

The vibration of a LP turbine casing may cause the problems of power generation and the life of its facility. In this study, we carried out on-site measurements of a LP Turbine in order to find the cause of the vibration and conducted experimental and numerical modal analysis of the turbine with its support frame. The measurement and the modal analysis show that the natural frequency of the turbine becomes close to 60 Hz due to the subsidence of the support. The elimination of the subsidence by shimming between the turbine and the support frame gave rise to the reduction of the vibration of the LP turbine case.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1999년도 하계학술대회 논문집 G
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• pp.2809-2811
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• 1999

In this paper, we develope an algorithm to calculate field inhomogeneity in MR imaging using a dual fast spin echo pulse sequence. Because phase modulation time can be easily modified with this pulse sequence, high resolution image can be obtained and acquisition time can be reduced compared to gradient echo technique. In the case of phase wrapping in field map, phase corrected using image processing technique. We assume the field pattern to be second order polynomial and apply Pseudo-Inverse equation to calculate second order polynomial coefficients. These coefficients can be used for the shimming of the magnetic field.

• 전자공학회논문지SC
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• 제48권6호
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• pp.57-62
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• 2011

본 논문에서는 초전도 자석의 자장 균일도를 개선하기 위한 방법으로 3가지 형태의 magnet 모델을 제안하고 각각의 자석 형태에 대하여 같은 세기의 자장을 (Magnet field strength) 가질 경우에 최소전력 방식으로 최적화된 전류 분포에 해당하는 coil wire의 길이, 그리고 해당 조건에서의 자장의 불균일도를 시뮬레이션을 통하여 비교, 분석하였다. 구성된 3가지 magnet type을 동일한 조건 (계산 점의 개수 18개, 20cm DSV)에 대해서 wire길이와 main field inhomogeneity를 비교하였으며, 이러한 시뮬레이션 결과를 통하여 얻을 수 있는 결론은 계산점의 수가 적을수록 wire의 길이는 짧아지나 field inhomogeneity는 높아진다는 것이다. 즉, Magnet shim을 수행할 경우 계산점을 줄이는 방법으로는 짧은 wire의 길이와 main field homogeneity를 동시에 만족하도록 최적화 하는 것이 거의 불가능함을 의미하는 것이다. 그러나 DSV를 줄였을 경우에 계산점을 줄였을 때에 비해 우수한 결과 값을 얻을 수 있었다. 결론적으로 공간적으로 개방되어 있는 magnet model의 경우 계산점을 줄여 shimming을 진행할 경우 동일한 imaging region의 크기에 대해 더 많은 전류(또는 wire 길이)가 필요하고 field 균일도도 떨어졌으나 작은 ROI를 대상으로 영상을 얻는 경우 유용하게 사용될 수 있다.

• 대한자기공명의과학회:학술대회논문집
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• 대한자기공명의과학회 2003년도 제8차 학술대회 초록집
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• pp.103-105
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• 2003

After fifteen years of development, Magnetic Resonance (MR) technology for human imaging and spectroscopy is reaching a refined state with FDA approved 3T clinical products from Siemens, GE, and Philips. Broker has cleared CE approval with a 4T system. Varian supports a 4T system platform as well. Shielded magnets are standard at 3T from GE, Oxford, Magnex, and IGC. A shielded 4T whole body magnet is available from Oxford. Stronger switched gradients and dynamic shim coils, desired at any field, areespecially useful at higher static magnetic fields B0. In addition to the higher currents required for higher resolution slice or volume selection afforded by higher SNR, whole body gradient coils will be driven at increasing slew rates to meet the needs of new cardiac applications and other requirements. For example 3T and 4T systems are now being equipped with 2kV, 500A gradient coils and amplifiers capable of generating 4G/cm in 200msec, over a 67+/-cm bore diameter. High field EPI applications require oscillation rates at 1 kHz and higher. To achieve a benchmark 0.2 ppm shim over a 30cm sphere in a high field magnet, at least four stages of shimming need to be considered. 1) A good high field magnet will be built to a homogeneity spec. falling in the range of 100 to 150 ppm over this 30cm spherical "sweet spot" 2) Most modern high field magnets will also have superconducting shim coils capable of finding 1.5 ppm by their adjustment during system installation. 3) Passive ferro-magnetic shimming combined with 4) active, high order room temperature shim coils (as many as five orders are now being recommended) will accomplish 0.2 ppm over the 30cm sphere, and 0.1 ppm over a human brain in even the highest field magnets for human studies. Safety concerns for strong, fast gradients at any B0 field include acoustic noise and peripheral nerve stimulation. One or more of the mechanical decoupling methods may lead to quieter gradients. Patient positioning relative to asymmetric or short gradient coils may limit peripheral nerve stimulation at higher slew rates. Gradient designs combining a short coil for local speed and strength with a longer coil for coverage are being developed for 3T systems. Local gradients give another approach to maximizing performance over a limited region while keeping within the physiologically imposed dB0/dt performance limits.

• Agricultural and Biosystems Engineering
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• 제1권1호
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• pp.49-53
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• 2000

A magnet assembly is a critical element of a nuclear magnetic resonance(NMR) based sensor. Magnetic flux density and homogeneity are essential to its optimum performance. Geometry and magnet material properties determine the magnetic flux density and homogeneity of the assembly. This study was carried out to develop the design for a magnet assembly. A 2-D finite element model for the magnetic assembly was developed using ANSYS and evaluated the effects of adding shimming frames and steel bars in the corners of the rectangular steel cover which surrounded the magnet. The assembly was manufactured and evaluated. According to the ANSYS model, modified pole frames increased magnetic flux density by 8.3% and increased homogeneity by 83%. Addition of steel bars in the corners increased the magnetic flux density by 1%, and improved homogeneity up to three times. The difference between simulated and measured magnetic flux densities at the center point of the air gap was within 2.4%.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2008년도 학술대회 논문집 정보 및 제어부문
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• pp.486-488
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• 2008

300 MHz가 넘는 초고자장 MRI에서는 송신 또는 수신 RF Magnetic Field 의 불균일도가 심해져서 이를 개선하기 위한 많은 방법들이 제안되고 있다. 그 중 가장 대표적인 방법은 $4{\sim}32$ 채널의 Transmit Array의 각 채널에 인가되는 전압과 위상을 변화시켜 RF Magnetic Field의 불균일도를 개선하는 방법이다. 본 논문에서는 Transmit Array 내부에서 머리위치의 변화에 따라 RF Magnetic Field ($B_1$ Field) 의 불균일도가 많이 변화하며 이에 따라 RF 송신용 전압과 위상의 Pattern을 새로 최적화 해야 함을 확인하였다. 또한 RF field Mapping을 하기 위해서 Composite RF Sequence를 사용한 Rapid Sequence의 사용과 채널 전압과 위상을 최적화하기 위해서 일반적인 Iterative 방식보다 간편하고 빠른 Target Method를 제안하였다. Driving 패턴의 최적화는 Complex 행렬식을 사용했으며 RF Magnetic Field ($B_1$ Field) 분포는 FDTD 방식으로 계산하였다.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2001년도 춘계학술대회 논문집 전기기기 및 에너지변환시스템부문
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• pp.22-25
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• 2001

The research results on the superconducting magnet for whole body MRI are presented. The magnet consists of main coil with 6 solenoid coils, shielding coil with 2 solenoid coils and 6 sets of cryogenic shim coil. The ferromagnetic shim assembly is installed on the inside wall of the room temperature bore for shimming inhomogeneous field components generated due to manufacturing tolerances, installation misalignments and external ferromagnetic materials near the magnet. Also, the magnet is enclosed with the horizontal type cryostat with 80cm room temperature bore to keep the magnet under the operating temperature. The magnetic field distributions within the imaging volume were measured by the NMR field mapping system. Through the test, the central field of magnet was 1.5 Tesla and the field homogeneity of 9.3 ppm has been obtained on 40cm DSV(the diameter of spherical volume) and using this magnet, comparatively good images for human body, fruits and water phantoms have been achieved.

• 대한의용생체공학회:학술대회논문집
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• 대한의용생체공학회 1997년도 추계학술대회
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• pp.517-527
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• 1997

Magnetic Resonance Spectroscopic Imaging is a methodology combining the imaging and spectroscopy. It can provide the spectrum of each areas of image so that one can easily compare the spectrum of one position to another position of the image. In this study, we developed pulse sequence or the spectroscopic imaging method, RF wave forms or the saturation of water signal, computer simulations to validate our method, and confirmed the methodology with phantom experiment. Then we applied the spectroscopic method to human subject and identified a few important metabolites in in vivo. To develope a water saturating RF waveform, we used Shinnar-Le-Roux algorithm and obtained maximum phase RF waveform. With this RF pulse, it could suppress the water signal to 1:1000. The magnet is shimmed to under 1.0ppm with auto-shimming technique. The saturation bandwidth is 80Hz(2ppm). The water and fat seperation is 3.3ppm(about 140Hz at 1 Tesla magnet), the bandwidth is enough to resolve the difference. But we are more concerned about the narrow window in between the two peaks, in which the small quantity of metabolites reside. We performed the computer simulation and phantom experiments in 8*8 matrix form and showed good agreement in the image and spectrum. Finally we applied spectroscopic imaging to the brain of human subject. Only the lipid signal was shown in the periphery region which agrees with the at distribution in human head surface area. The spectrum inside the brain shows the important metabolites such as NAA, Cr/PCr, Choline. We here have shown the spectroscopic imaging which is normally done above 1.5 Tesla machine can be performed in the 1 Tesla Magnetic Resonance Imaging Unit.

• 한국유통학회:학술대회논문집
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• 한국유통학회 2003년도 동계학술대회 발표논문집
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• pp.123-166
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• 2003

마케팅 경로는 여러 개의 유통 기관이 상호 관련을 맺으면서 분업과 협업을 하는 기능적 조직의 성격을 가진 사회 시스템의 하나로 인식되고 있다. 유통경로 내에서의 경로구성원들은 서로 상호작용을 하면서 유리한 위치에서 거래를 성사시키기 위해 노력을 한다. 따라서 경로구성원들은 유리한 위치를 차지하기 위해 파워를 행사하게 되고 행사된 파워는 경로구성원들간의 만족과 갈등에 영향을 미친다. 최근 대형소매업체들의 증가로 구매파워를 내세운 바이어들이 생산업자보다 더 막강한 파워를 행사하는 현상이 유통경로 내에서 일반적으로 일어나고 있다. 그러나 공급자의 입장에서 다양한 공급경로가 있고 또한 제품의 특성상 계절성이 강하고 공급의 불안정성이 상대적으로 큰 농산물인 경우에는 공급자와 바이어의 파워형태가 공산품인 경우와는 다른 모습을 보일 것이라 예상된다. 따라서 유통업체와 공급업자에게는 지속적인 관계 유지를 위해 효과적인 방안들을 제시하고 또한 정책입안자들에게도 각각 맞는 시사점을 제시하여 유통경로 전체의 성과를 높일 수 있도록 발전적인 방향을 제시하고자 한다. 서로간의 믿음이나 신뢰성이 있는 거래관계를 유지하기 위해서는 유통경로구성원들간의 인식의 변화가 필요하다고 본다. 그러기 위해서는 공급자와 바이어의 관계를 종속관계가 아닌 협력관계로의 사고전환이 경로구성원 모두에게 필요하다고 본다. 상대적으로 파워가 약한 공급자는 스스로 경쟁력 요소를 강화하여 전문적 파워를 갖추고 유통업체의 강력한 제품 공급처를 유지하기 위해 품질유지에 대한 노하우로 기술적인 전문성을 강화시켜야 할 것이다. 또한 농산물의 브랜드화도 유통거래 상에서 경쟁력을 높일 수 있는 좋은 방안이 될 것이다.example 3T and 4T systems are now being equipped with 2kV, 500A gradient coils and amplifiers capable of generating 4G/cm in 200msec, over a 67+/-cm bore diameter. High field EPI applications require oscillation rates at 1 kHz and higher. To achieve a benchmark 0.2 ppm shim over a 30cm sphere in a high field magnet, at least four stages of shimming need to be considered. 1) A good high field magnet will be built to a homogeneity spec. falling in the range of 100 to 150 ppm over this 30cm spherical "sweet spot" 2) Most modern high field magnets will also have superconducting shim coils capable of finding 1.5 ppm by their adjustment during system installation. 3) Passive ferro-magnetic shimming combined with 4) active, high order room temperature shim coils (as many as five orders are now being recommended) will accomp