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Characterizing inter‐compartmental water exchange in myelinated tissue using relaxation exchange spectroscopy
Author(s) -
Dortch Richard D.,
Harkins Kevin D.,
Juttukonda Meher R.,
Gore John C.,
Does Mark D.
Publication year - 2013
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.24571
Subject(s) - myelin , sciatic nerve , white matter , chemistry , optic nerve , relaxation (psychology) , myelin sheath , biophysics , spectroscopy , anatomy , nuclear magnetic resonance , central nervous system , neuroscience , biology , magnetic resonance imaging , physics , medicine , radiology , quantum mechanics
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 (τ myelin ). Results : In the rat optic nerve samples, τ myelin = 138 ± 15 ms (mean ± standard deviation) was estimated. In sciatic nerve, which possesses thicker myelin sheaths than optic nerve, a much longer τ myelin = 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 τ myelin < 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 τ myelin 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. Magn Reson Med 70:1450–1459, 2013. © 2012 Wiley Periodicals, Inc.