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

TLDR

The paper develops a theory of NMR signal behavior in biological tissues subject to static magnetic field inhomogeneities. The authors aim to analytically describe the NMR signal in the static dephasing regime, neglecting diffusion effects. They model the signal analytically and apply it to blood vessel networks, bone marrow trabecular structures, and ferrite contrast agents. All examined systems show a two‑regime decay: for echo times shorter than a characteristic time \(t_c\) the signal decays exponentially with a TE‑quadratic argument (R₂* linear in TE), whereas for TE longer than \(t_c\) the decay is simple exponential with R₂* independent of TE and linearly dependent on volume fraction, magnetic field, or susceptibility difference.

Abstract

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 tc 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 tc, 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 sigma occupied by the field-creating objects, b) magnetic field Bo or just the objects' magnetic moment for ferrite particles, and c) susceptibility difference delta chi between the objects and the medium.