• Title/Summary/Keyword: ferromagnetic shimming

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A Ferromagnetic Shimming Method for NMR Magnet Using Linear Programming (리니어 프로그래밍을 이용한 NMR 마그넷의 수동 자장보정 방법)

  • Lee, Sang-Jin;Hahn, Seung-Yong;Sim, Ki-Deok
    • The Transactions of The Korean Institute of Electrical Engineers
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    • v.59 no.6
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    • pp.1059-1063
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    • 2010
  • Shimming is an important technique in development of nuclear magnetic resonance (NMR) magnets where image resolution is highly dependent on magnetic field homogeneity. Classically, shimming may be categorized into two types: 1) active shimming that incorporates with extra coils and precise tuning of their currents; and 2)passive shimming that incorporates with pieces of steel placed in a bore of a main magnet and their uniform magnetization under homogeneous external fields. Additional magnetic fields, produced by the coils and/or the steel sheets, compensate original magnetic field from the main magnet in such a way that the total field becomes more homogeneous. In this paper, we developed a passive shimming method based on linear programming optimization. Linear programming is well known to be highly efficient to find a global minimum in various linear problems. We firstly confirmed the linearity of magnetization of ferromagnetic pieces under a presence of external magnetic fields. Then, we adopted the linear programming to find optimized allocation of the steel pieces in the inner bore of a main magnet to improve field homogeneity.

Passive shimming design with commercially available rectangular shim sheets on a cylinder for HTS NMR magnets

  • Ahn, Min Cheol
    • Progress in Superconductivity and Cryogenics
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    • v.20 no.2
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    • pp.29-33
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    • 2018
  • Although high-temperature superconducting (HTS) magnets have the potential merit of producing ultra-high field (>25 T), they have been not easy to apply to Nuclear Magnetic Resonance (NMR) because of the difficulty of field homogeneity improvement. This paper presents a design technique of passive shimming for HTS magnets. Ferromagnetic shimming design code was developed though MALAB, which includes the optimization algorithm. The proper shim element size was determined by a simulation. This design technique was verified by a case study design of a 3-T HTS magnet. We succeed to improve field homogeneity of the magnet from 634 ppm to 6.39 ppm at 10-mm diameter sphere volume. Feasibility of passive shimming for all-HTS NMR magnet was confirmed by this result.

Design of ferromagnetic shims for an HTS NMR magnet using sequential search method

  • Yang, Hongmin;Lee, SangGap;Ahn, Minchul
    • Progress in Superconductivity and Cryogenics
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    • v.23 no.4
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    • pp.39-43
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    • 2021
  • This study deals with the ferromagnetic shims design based on the spherical harmonic coefficient reduction method. The design method using the sequential search method is an intuitive method and has the advantage of quickly reaching the optimal result. The study was conducted for a 400 MHz all-REBCO magnet, which had difficulty in shimming due to the problem of SCF (screening current induced field). The initial field homogeneity of the magnet was measured to be 233.76 ppm at 20 mm DSV (Diameter Spherical Volume). In order to improve the field homogeneity of the magnet, the ferromagnetic shim with a thickness of 1 mil to 11 mil was constructed by a design method in which sequential search algorithm was applied. As a result, the field homogeneity of the magnet could be significantly improved to 0.24 ppm at 20 mm DSV and 0.05 ppm at 10 mm DSV.

Development of Ferro-magnetic Shimming System for 1.5T MRI Magnet (1.5T MRI용 Ferro-magnetic shimming 장치 개발)

  • 심기덕;배준한;고락길;진홍범;권영길
    • Proceedings of the Korea Institute of Applied Superconductivity and Cryogenics Conference
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    • 2001.02a
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    • pp.178-181
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    • 2001
  • Two methods can be used to correct the undesirable magnetic field of MRI. One is active shimming method and the other is ferro-magnetic shimming. Ferro-magnetic shimming method is more inexpensive, more convenient in operation and more effective on correcting magnetic field. So, nowadays, it is the general method for shimming the commercialized MRI magnet. We have developed a 1.5T MRI magnet and its ferromagnetic shimming system. Using the ferro-magnetic shimming system, we have improved the field homogeneity of the 1.5T MRI magnet.

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The Domestic Development of a Superconducting MRI Magnet (초전도 MRI 마그네트 국산화 개발)

  • 배준한;심기덕;고락길;진홍범;조전욱;하동우;오상수;권영길;류강식
    • Proceedings of the Korea Institute of Applied Superconductivity and Cryogenics Conference
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    • 2001.02a
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    • pp.121-124
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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.

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The Development of Popular type Domestic Superconducting MRI Magnet with Middle Magnetic Field Range (중자장급 보급형 국산 초전도 MRI 마그네트 개발)

  • Bae, J.H.;Ko, R.K.;Sim, K.D.;Jin, H.B.;Cho, J.W.;Lee, E.Y.;Kwon, Y.K.;Ryu, K.S.
    • Proceedings of the KIEE Conference
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    • 2001.04a
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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.

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