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A Better Characterization of Spinal Cord Damage in Multiple Sclerosis: A Diffusional Kurtosis Imaging Study - AJNR News Digest
May 2014
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A Better Characterization of Spinal Cord Damage in Multiple Sclerosis: A Diffusional Kurtosis Imaging Study

Eytan Raz

Eytan Raz

Maxim Bester

Maxim Bester

Histopathological studies have shown that spinal cord WM and GM are affected by demyelination and neuronal loss in patients with MS. Advanced MRI techniques such as diffusion-weighted and diffusion tensor imaging (DWI and DTI) allow the detection and quantification of lesional and diffuse tissue injury. Although spinal cord DWI/DTI is technically challenging, a few DTI studies in patients with MS have demonstrated a significant decrease in fractional anisotropy (FA) that reflects the degree of integrity and alignment of fiber tracts. While the anisotropic sensitivity of DTI is useful for the study of highly organized anisotropic tissue such as WM tracts, it provides less useful information in highly isotropic organized tissue such as GM.

Diffusional kurtosis imaging (DKI) is an MR imaging technique developed by our coauthors Jens Jensen, Joseph Helpern, and Eric Sigmund to assess the degree of non-Gaussian diffusion.1 The mean kurtosis (MK) is a DKI-derived metric that provides a measure of the deviation of the diffusion probability distribution from a Gaussian form. MK has been shown to be sensitive to structural changes in both anisotropic tissue, such as WM, and isotropic tissue, such as GM, and therefore may provide information on tissue microarchitecture complementary to that given by FA and mean diffusivity (MD). DKI has already been used to study traumatic brain injury2 (featured in the March 2014 issue of the AJNR News Digest), brain tumors, and the spinal cord in patients with cervical myelopathy.

We hypothesized that studying the spinal cord using a non-Gaussian diffusion modality may be more sensitive to subtle pathologic changes because diffusional non-Gaussianity is a direct consequence of diffusion barriers and compartments and, therefore, a measurement of tissue complexity, regardless of the presence of microstructural anisotropy.3

Our DKI study in patients with MS showed extensive cervical spinal cord damage, more severe in cross-sectional areas with lesions than in normal-appearing white and gray matter.3 This is in agreement with previous DTI studies that found decreased FA and increased MD in the cervical spinal cord of patients with MS in comparison with healthy controls. The pathologic substrate underlying the changes in DTI metrics is represented by demyelination and axonal loss occurring in the spinal cord in T2-visible lesions and in normal-appearing tissue. Both demyelination and axonal loss lead to a decrease of diffusion barriers and tissue anisotropy, consequently leading towards an increase of the extracellular volume, which is reflected by reduced FA and increased MD. Decreased MK points toward reduced tissue complexity and allows the characterization of tissue damage, especially in isotropic organized GM, that is not fully assessable by FA measurements.

The feasibility of the spinal cord damage quantification of GM and WM in MS might allow the translation of a novel technology into a beneficial clinical tool. Future perspective includes the improvement of spatial resolution by using ultra-high-field MRI4 and the application of DKI to patients with different forms of MS over time, to test the potential of MK as a diagnostic and prognostic marker.

We thank our mentor Dr. Matilde Inglese for her invaluable support, and all the coauthors for their precious help.

References

  1. Jensen JH, Helpern JA, Ramani A, et al. Diffusional kurtosis imaging: the quantification of non-Gaussian water diffusion by means of magnetic resonance imaging. Magn Reson Med 2005;53:1432–40, 10.1002/mrm.20508
  2. Grossman EJ, Jensen JH, Babb JS, et al. Cognitive impairment in mild traumatic brain injury: a longitudinal diffusional kurtosis and perfusion imaging study. AJNR Am J Neuroradiol 2013;34:951–57, 10.3174/ajnr.A3358
  3. Raz E, Bester M, Sigmund EE, et al. A better characterization of spinal cord damage in multiple sclerosis: a diffusional kurtosis imaging study. AJNR Am J Neuroradiol 2013;34:1846–52, 10.3174/ajnr.A3512
  4. Sigmund EE, Suero GA, Hu C, et al. High-resolution human cervical spinal cord imaging at 7T. NMR Biomed 2012;25:891–99, 10.1002/nbm.1809

 

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