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Theory of NMR signal behavior in magnetically inhomogeneous tissues: The static dephasing regime
Author(s) -
Yablonskiy Dmitriy A.,
Haacke E. Mark
Publication year - 1994
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.1910320610
Subject(s) - dephasing , nuclear magnetic resonance , exponential decay , magnetostatics , relaxation (psychology) , ferrite (magnet) , magnetic field , signal (programming language) , spin echo , exponential function , free induction decay , exponential growth , physics , chemistry , condensed matter physics , mathematical analysis , magnetic resonance imaging , mathematics , quantum mechanics , computer science , medicine , psychology , social psychology , radiology , programming language
This paper is devoted to a theory of the NMR signal behavior in biological tissues in the presence of static magnetic field inhomogeneities. We have developed an approach that analytically describes the NMR signal in the static dephasing regime where diffusion phenomena may be ignored. This approach has been applied to evaluate the NMR signal in the presence of a blood vessel network (with an application to functional imaging), bone marrow (for two specific trabecular structures, asymmetrical and columnar) and a ferrite contrast agent. All investigated systems have some common behavior. If the echo time TE is less than a known characteristic time t c for a given system, then the signal decays exponentially with an argument which depends quadratically on TE . This is equivalent to an R2* relaxation rate which is a linear function of TE . In the opposite case, when TE is greater than t c , the NMR signal follows a simple exponential decay and the relaxation rate does not depend on the echo time. For this time interval, R2* is a linear function of a) volume fraction ζ occupied by the field‐creating objects, b) magnetic field B 0 or just the objects' magnetic moment for ferrite particles, and c) susceptibility difference Δχ between the objects and the medium.