Analysis of transient eddy currents in MRI using a cylindrical FDTD method

Trakic, Adnan, Wang, Hua, Liu, Feng, Sanchez Lopez, Hector and Crozier, Stuart (2006) Analysis of transient eddy currents in MRI using a cylindrical FDTD method. IEEE Transactions On Applied Superconductivity, 16 3: 1924-1936. doi:10.1109/TASC.2006.874000

Author Trakic, Adnan
Wang, Hua
Liu, Feng
Sanchez Lopez, Hector
Crozier, Stuart
Title Analysis of transient eddy currents in MRI using a cylindrical FDTD method
Journal name IEEE Transactions On Applied Superconductivity   Check publisher's open access policy
ISSN 1051-8223
Publication date 2006
Sub-type Article (original research)
DOI 10.1109/TASC.2006.874000
Volume 16
Issue 3
Start page 1924
End page 1936
Total pages 13
Editor J. Spargo
Place of publication Piscataway
Publisher IEEE-Inst Electrical Electronics Engineers Inc
Collection year 2006
Language eng
Subject C1
090399 Biomedical Engineering not elsewhere classified
Abstract Most magnetic resonance imaging (MRI) spatial encoding techniques employ low-frequency pulsed magnetic field gradients that undesirably induce multiexponentially decaying eddy currents in nearby conducting structures of the MRI system. The eddy currents degrade the switching performance of the gradient system, distort the MRI image, and introduce thermal loads in the cryostat vessel and superconducting MRI components. Heating of superconducting magnets due to induced eddy currents is particularly problematic as it offsets the superconducting operating point, which can cause a system quench. A numerical characterization of transient eddy current effects is vital for their compensation/control and further advancement of the MRI technology as a whole. However, transient eddy current calculations are particularly computationally intensive. In large-scale problems, such as gradient switching in MRI, conventional finite-element method (FEM)-based routines impose very large computational loads during generation/solving of the system equations. Therefore, other computational alternatives need to be explored. This paper outlines a three-dimensional finite-difference time-domain (FDTD) method in cylindrical coordinates for the modeling of low-frequency transient eddy currents in MRI, as an extension to the recently proposed time-harmonic scheme. The weakly coupled Maxwell's equations are adapted to the low-frequency regime by downscaling the speed of light constant, which permits the use of larger FDTD time steps while maintaining the validity of the Courant-Friedrich-Levy stability condition. The principal hypothesis of this work is that the modified FDTD routine can be employed to analyze pulsed-gradient-induced, transient eddy currents in superconducting MRI system models. The hypothesis is supported through a verification of the numerical scheme on a canonical problem and by analyzing undesired temporal eddy current effects such as the B-0-shift caused by actively shielded symmetric/asymmetric transverse x-gradient head and unshielded z-gradient whole-body coils operating in proximity to a superconducting MRI magnet.
Keyword Engineering, Electrical & Electronic
Physics, Applied
Downscaling Speed Of Light
Finite-difference Time-domain (fdtd)
Magnetic Resonance Imaging (mri)
Symmetric And Asymmetric Gradient Coils
Transient Eddy Currents
Nonlinear Magnetic Diffusion
Q-Index Code C1

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