A method for improving the performance of gradient systems for diffusion-weighted MRI

  • The MR signal is sensitive to diffusion. This effect can be increased by the use of large, balanced bipolar gradients. The gradient systems of MR scanners are calibrated at installation and during regular servicing visits. Because the measured apparent diffusion constant (ADC) depends on the square of the amplitude of the diffusion sensitizing gradients, errors in the gradient calibration are exaggerated. If the error is varying among the different gradient axes, it will affect the estimated degree of anisotropy. To assess the gradient calibration accuracy in a whole-body MRI scanner, ADC values were calculated for a uniform water phantom along each gradient direction while monitoring the temperature. Knowledge of the temperature allows the expected diffusion constant of water to be calculated independent of the MRI measurement. It was found that the gradient axes (±x, ±y, ±z) were calibrated differently, resulting in offset ADC values. A method is presented to rescale the amplitude of each of the six principal gradient axes within the MR pulse sequence. The scaling factor is the square root of the ratio of the expected and observed diffusion constants. In addition, fiber tracking results in the human brain were noticeably affected by improving the gradient system calibration. Magn Reson Med 58:763–768, 2007. © 2007 Wiley-Liss, Inc. Keywords: diffusion tensor imaging, apparent diffusion constant, magnetic field gradient, fibre tracking, anisotropy

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Author:Zoltan Nagy, Nikolaus WeiskopfORCiDGND, Daniel C. Alexander, Ralf DeichmannORCiD
Parent Title (German):Magnetic Resonance in Medicine
Document Type:Article
Year of Completion:2007
Year of first Publication:2007
Publishing Institution:Universitätsbibliothek Johann Christian Senckenberg
Release Date:2010/01/27
Tag:anisotropy; apparent diffusion constant; diffusion tensor imaging; fibre tracking; magnetic field gradient
First Page:763
Last Page:768
Copyright © 2007 Wiley-Liss, Inc., A Wiley Company. Re-use of this article is permitted in accordance with the Creative Commons Deed, Attribution 2.5, which does not permit commercial exploitation.
Source:Magnetic Resonance in Medicine, 58(4): 763–768. doi: 10.1002/mrm.21379 ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2683063/
Institutes:Medizin / Medizin
Dewey Decimal Classification:6 Technik, Medizin, angewandte Wissenschaften / 61 Medizin und Gesundheit / 610 Medizin und Gesundheit
Licence (German):License LogoCreative Commons - Namensnennung-Nicht kommerziell 2.0