Effects of pore-size and shape distributions on diffusion pore imaging by nuclear magnetic resonance

In medical imaging and porous media research, NMR diffusion measurements are extensively used to investigate the structure of diffusion restrictions such as cell membranes. Recently, several methods have been proposed to unambiguously determine the shape of arbitrary closed pores or cells filled with an NMR-visible medium by diffusion experiments. The first approach uses a combination of a long and a short diffusion-weighting gradient pulse, while the other techniques employ short gradient pulses only. While the eventual aim of these methods is to determine pore-size and shape distributions, the focus has been so far on identical pores. Thus, the aim of this work is to investigate the ability of these different methods to resolve pore-size and orientation distributions. Simulations were performed comparing the various pore imaging techniques employing different distributions of pore size and orientation and varying timing parameters. The long-narrow gradient profile is most advantageous to investigate pore distributions, because average pore images can be directly obtained. The short-gradient methods suppress larger pores or induce a considerable blurring. Moreover, pore-shape-specific artifacts occur; for example, the central part of a distribution of cylinders may be largely underestimated. Depending on the actual pore distribution, short-gradient methods may nonetheless yield good approximations of the average pore shape. Furthermore, the application of short-gradient methods can be advantageous to differentiate whether pore-size distributions or intensity distributions, e.g., due to surface relaxation, are predominant.