THEORY OF NMR SIGNAL BEHAVIOR IN MAGNETICALLY INHOMOGENEOUS TISSUES -THE STATIC DEPHASING REGIME

This paper is devoted to a theory of the NMR signal behavior in biolog ical tissues in the presence of static magnetic field inhomogeneities. We have developed an approach that analytically describes the NMR sig nal in the static dephasing regime where diffusion phenomena may be ig nored. This approach has been applied to evaluate the NMR signal in th e presence of a blood vessel network (with an application to functiona l imaging), bone marrow (for two specific trabecular structures, asymm etrical and columnar) and a ferrite contrast agent. All investigated s ystems have some common behavior. If the echo time TE is less than a k nown characteristic time t(c) for a given system, then the signal deca ys exponentially with an argument which depends quadratically on TE. T his 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 si gnal follows a simple exponential decay and the relaxation rate does n ot depend on the echo time. For this time interval, R2 is a linear fu nction of a) volume fraction sigma occupied by the field-creating obje cts, b) magnetic field B-o or just the objects' magnetic moment for fe rrite particles, and c) susceptibility difference Delta chi between th e objects and the medium.