Theory of NMR signal behavior in magnetically inhomogeneous tissues: The static dephasing regime

AbstractThis 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 timeTEis less than a known characteristic timetcfor a given system, then the signal decays exponentially with an argument which depends quadratically onTE. This is equivalent to an R2* relaxation rate which is a linear function ofTE. In the opposite case, whenTEis greater thantc, 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 B0or just the objects' magnetic moment for ferrite particles, and c) susceptibility difference Δχ between the objects and the medium.

Theory of NMR signal behavior in magnetically inhomogeneous tissues: The static dephasing regime Journal Articles