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Characterizing inter-compartmental water exchange in myelinated tissue using relaxation exchange spectroscopy.

Dortch RD, Harkins KD, Juttukonda MR, Gore JC, Does MD
Magn Reson Med. 2013 70 (5): 1450-9

PMID: 23233414 · PMCID: PMC3596465 · DOI:10.1002/mrm.24571

PURPOSE - To investigate inter-compartmental water exchange in two model myelinated tissues ex vivo using relaxation exchange spectroscopy.

METHODS - Building upon a previously developed theoretical framework, a three-compartment (myelin, intra-axonal, and extra-axonal water) model of the inversion-recovery prepared relaxation exchange spectroscopy signal was applied in excised rat optic nerve and frog sciatic nerve samples to estimate the water residence time constants in myelin (τmyelin ).

RESULTS - In the rat optic nerve samples, τmyelin = 138 ± 15 ms (mean ± standard deviation) was estimated. In sciatic nerve, which possesses thicker myelin sheaths than optic nerve, a much longer τmyelin = 2046 ± 140 ms was observed.

CONCLUSION - Consistent with previous studies in rat spinal cord, the extrapolation of exchange rates in optic nerve to in vivo conditions indicates that τmyelin < 100 ms. This suggests that there is a significant effect of inter-compartmental water exchange on the transverse relaxation of water protons in white matter. The much longer τmyelin values in sciatic nerve supports the postulate that the inter-compartmental water exchange rate is mediated by myelin thickness. Together, these findings point to the potential for MRI methods to probe variations in myelin thickness in white matter.

Copyright © 2012 Wiley Periodicals, Inc.

MeSH Terms (14)

Algorithms Animals Body Water Magnetic Resonance Spectroscopy Male Myelin Sheath Nerve Fibers, Myelinated Optic Nerve Rats Rats, Sprague-Dawley Reproducibility of Results Sensitivity and Specificity Spinal Cord Xenopus laevis

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