Diffusion Encoding with General Gradient Waveforms
Molecular translation from diffusion and microscopic flow probed with NMR/MRI provides non-invasive insight into the microscopic structures of liquid-filled voids of a material. This opens a complementary viewing window into porous materials and biological tissues between the chemical structure and the macroscale perspective provided by the spatial resolution of MRI. While the diffusion of particles is limited by geometry, the direct relation to the structure is lost through the fuzzy lens of a stochastic process. Particular features of such processes can be resolved by using magnetic field gradients, which are varying on a millisecond time scale. Different gradient waveforms, which can be applied in multiple directions, thus provide sophisticated and highly adjustable filters to examine diffusion and the associated microstructural features. While the specificity of these filters can vary significantly, we generally strive for highly specific ones that can unambiguously resolve different characteristics of the observed phenomena. We will in this chapter contrast different diffusion encoding waveforms and discuss how they can be adjusted to benefit various experimental observations. We will show how the waveforms can be designed to separate and correlate specific types of translational motion: local diffusivity, restriction, anisotropy, flow, and diffusional exchange.