Pulsed magnetic field gradients generated by gradient coils are widely used in signal location in magnetic resonance imaging (MRI). However, gradient coils can also induce eddy currents in final magnetic field in the nearby conducting structures which lead to distortion and artifact in images, misguiding clinical diagnosis. We tried in our laboratory to measure the magnetic field of gradient-induced eddy current in 1.5 T superconducting magnetic resonance imaging device; and extracted key parameters including amplitude and time constant of exponential terms according to inductance-resistance series mathematical module. These parameters of both self-induced component and crossing component are useful to design digital filters to implement pulse pre-emphasize to reshape the waveform. A measure device that is a basement equipped with phantoms and receiving coils was designed and placed in the isocenter of the magnetic field. By applying testing sequence, contrast experiments were carried out in a superconducting magnet before and after eddy current compensation. Sets of one dimension signal were obtained as raw data to calculate gradient-induced eddy currents. Curve fitting by least squares method was also done to match inductance-resistance series module. The results also illustrated that pulse pre-emphasize measurement with digital filter was correct and effective in reducing eddy current effect. Pre-emphasize waveform was developed based on system function. The usefulness of pre-emphasize measurement in reducing eddy current was confirmed and the improvement was also presented. All these are valuable for reducing artifact in magnetic resonance imaging device.
Citation：
HE Wenjing, ZHU Yuanzhong, WANG Wenzhou, ZOU Kai, ZHANG Kai, HE Chao. Quantitative experiment and analysis of gradient-induced eddy currents on magnetic resonance imaging. Journal of Biomedical Engineering, 2017, 34(2): 220-226. doi: 10.7507/1001-5515.201506059
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Copyright © the editorial department of Journal of Biomedical Engineering of West China Medical Publisher. All rights reserved
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- 1. Bernstein M A, Kevin F K, Zhou Xiaohong J. Handbook of MRI pulse sequences. Burlington, MA: elsevier academic press, 2004: 1-1017.
- 2. Goodyear D J, Shea M, Beyea S D, et al. Single point measurements of magnetic field gradient waveform. J Magn Reson, 2003, 163(1): 1-7.
- 3. Alexander A L, Tsuruda J S, Parker D L. Elimination of eddy current artifacts in diffusion-weighted echo-planar images: the use of bipolar gradient. Magn. Reson, 1997, 38(6): 1016-1021.
- 4. Zheng Hai, Zhao Tiejun, Qian Yongxian, et al. Parallel transmission RF pulse design for eddy current correction at ultra high field. J Magn Reson, 2012, 221(8): 139-146.
- 5. Bartusek K, Kubasek R, Fiala P. Determination of pre-emphasis constants for eddy current reduction. Measurement Science and Technology, 2010, 21(10): 1-9.
- 6. Glover G H, Norbert J P, Bradshaw K M. Gradient and polarizing field compensation: United States Patent. Patent Number: 4950994. Aug. 21, 1990.
- 7. Jehenson P, Westphal M, Schuff N. Analytical method for the compensation of Eddy-Current effects induced by pulsed magnetic field gradients in NMR systems. Journal of Magnetic Resonance, 1990, 90(2): 264-278.
- 8. Liang Z, Lauterbur P. Principles of magnetic resonance imaging: a signal processing perspective. new york: wiley-ieee press, 2000: 1-416.
- 9. Johnson M L. Nonlinear least-squares fitting methods. Methods Cell Biol, 2008, 84: 781-805.
