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Dispersion of relaxation rates in the rotating frame under the action of spin‐locking pulses and diffusion in inhomogeneous magnetic fields
Author(s) -
Spear John T.,
Zu Zhongliang,
Gore John C.
Publication year - 2014
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.24837
Subject(s) - diffusion , dispersion (optics) , condensed matter physics , relaxation (psychology) , magnetic field , parametric statistics , nuclear magnetic resonance , spin diffusion , magnetic susceptibility , physics , materials science , computational physics , optics , mathematics , quantum mechanics , psychology , social psychology , statistics
Purpose A method is described for characterizing magnetically inhomogeneous media and the spatial scales of intrinsic susceptibility variations within samples. The rate of spin‐lattice relaxation in the rotating frame, R 1ρ , is affected by diffusion effects to a degree that depends on the magnitude of an applied spin‐locking field. Appropriate analysis of the dispersion of R 1ρ with locking field may be used to characterize susceptibility variations in inhomogeneous tissues. Theory and Methods The contribution of diffusion to R 1ρ is quantified by an analytic expression derived by analyzing of the effects of diffusion through periodic variations of magnetic susceptibility and is used to predict the effects of inhomogeneities in simple phantoms. The theory is further applied to imaging to derive parametric images that portray the dimensions of susceptibility inhomogeneities independent of their magnitude. Results Significant dispersion of R 1ρ with locking field was predicted and measured experimentally for suspensions of microspheres ranging from 1 to 90 μm in diameter. For scales of practical interest, these dispersion effects occur at much lower locking fields than the range in which chemical exchange effects cause similar dispersion. Conclusion There is good agreement between theory and experiment, and the method has potential for quantitative tissue characterization and functional imaging. Magn Reson Med 71:1906–1911, 2014. © 2013 Wiley Periodicals, Inc .