Fractional differential and fractional integral modified-Bloch equations for PFG anomalous diffusion and their general solutions

TL;DR

This paper develops and compares two new fractional modified-Bloch equations for PFG anomalous diffusion, providing general solutions that incorporate FGPW effects and match CTRW simulations, advancing NMR and MRI analysis.

Contribution

Introduces two distinct fractional modified-Bloch equations for anomalous diffusion, offering comprehensive solutions including FGPW effects and addressing inconsistencies in prior models.

Findings

The integral type equation preserves linear and nonlinear process contributions.

Derived solutions align with CTRW simulations and literature.

Relaxation behavior differs from normal diffusion under fractional dynamics.

Abstract

The studying of anomalous diffusion by pulsed field gradient (PFG) diffusion technique still faces challenges. Two different research groups have proposed modified Bloch equation for anomalous diffusion. However, these equations have different forms and, therefore, yield inconsistent results. The discrepancy in these reported modified Bloch equations may arise from different ways of combining the fractional diffusion equation with the precession equation where the time derivatives have different derivative orders and forms. Moreover, to the best of my knowledge, the general PFG signal attenuation expression including finite gradient pulse width (FGPW) effect for time-space fractional diffusion based on the fractional derivative has yet to be reported by other methods. Here, based on different combination strategy, two new modified Bloch equations are proposed, which belong to two significantly different types: a differential type based on the fractal derivative and an integral type based on the fractional derivative. The merit of the integral type modified Bloch equation is that the original properties of the contributions from linear or nonlinear processes remain unchanged at the instant of the combination. The general solutions including the FGPW effect were derived from these two equations as well as from two other methods: a method observing the signal intensity at the origin and the recently reported effective phase shift diffusion equation method. The relaxation effect was also considered. It is found that the relaxation behavior influenced by fractional diffusion based on the fractional derivative deviates from that of normal diffusion. The general solution agrees perfectly with continuous-time random walk (CTRW) simulations as well as reported literature results. The new modified Bloch equations is a valuable tool to describe PFG anomalous diffusion in NMR and MRI.