On the nature of the NAA diffusion attenuated MR signal in the central nervous system

Christopher D. Kroenke, Joseph J.H. Ackerman, Dmitriy A. Yablonskiy

Research output: Contribution to journalArticlepeer-review

135 Scopus citations


In the brain, on a macroscopic scale, diffusion of the intraneuronal constituent N-acetyl-L-aspartate (NAA) appears to be isotropic. In contrast, on a microscopic scale, NAA diffusion is likely highly anisotropic, with displacements perpendicular to neuronal fibers being markedly hindered, and parallel displacements less so. In this report we first substantiate that local anisotropy influences NAA diffusion in vivo by observing differing diffusivities parallel and perpendicular to human corpus callosum axonal fibers. We then extend our measurements to large voxels within rat brains. As expected, the macroscopic apparent diffusion coefficient (ADC) of NAA is practically isotropic due to averaging of the numerous and diverse fiber orientations. We demonstrate that the substantially non-monoexponential diffusion-mediated MR signal decay vs. b value can be quantitatively explained by a theoretical model of NAA confined to an ensemble of differently oriented neuronal fibers. On the microscopic scale, NAA diffusion is found to be strongly anisotropic, with displacements occurring almost exclusively parallel to the local fiber axis. This parallel diffusivity, ADC, is 0.36 ± 0.01 μm 2/ms, and ADC is essentially zero. From ADC the apparent viscosity of the neuron cytoplasm is estimated to be twice as large as that of a temperature-matched dilute aqueous solution.

Original languageEnglish (US)
Pages (from-to)1052-1059
Number of pages8
JournalMagnetic Resonance in Medicine
Issue number5
StatePublished - Nov 2004
Externally publishedYes


  • CWS
  • Cytoplasm
  • Diffusion
  • MRI
  • Viscosity

ASJC Scopus subject areas

  • Radiology Nuclear Medicine and imaging


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