Analytical expressions for the NMR apparent diffusion coefficients in an anisotropic system and a simplified method for determining fiber orientation

Edward W. Hsu, Susumu Mori

Research output: Contribution to journalArticle

Abstract

NMR measurements of anisotropic diffusion were studied using a three‐dimensional random‐walk model. It was found that the apparent diffusion coefficient can be expressed in a canonical form as the product of a diagonal matrix, an orthonormal rotation matrix, and a vector representing the encoding magnetic field gradient. The diffusion coefficient can be interpreted as the sum of the corresponding coefficients measured along the principal diffusion axes, weighted by the squares of the directional cosines of the encoding direction with respect to the principal axes. The analysis revealed that determining the orientation of anisotropy, in a cylindrically symmetric system, requires a minimum of four diffusion measurements. A special pulse sequence which minimized gradient cross‐terms and possible restricted diffusion effects was used to characterize diffusion anisotropy in cut chicken gizzards. Diffusion coefficients parallel to the muscle fibers were found to be approximately two to three times larger than those in the transverse direction. Furthermore, the method was successful in detecting the angular change when the sample was rotated by 30°. Results indicate that the proposed approach to measure fiber orientation is valid and may be used to improve the time efficiency of diffusion anisotropy measurements.

Original languageEnglish (US)
Pages (from-to)194-200
Number of pages7
JournalMagnetic resonance in medicine
Volume34
Issue number2
DOIs
StatePublished - Aug 1995

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Keywords

  • anisotropy
  • diffusion
  • fiber orientation
  • random walk

ASJC Scopus subject areas

  • Radiology Nuclear Medicine and imaging

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