Restricted Diffusion

Abstract: The pulsed‐gradient, spin‐echo technique has been used to study self‐diffusion of protons in several colloidal systems in order to examine the usefulness of that technique in determining the extent to which the free movement of molecules in these systems is restricted by the colloidal structures present. The pulsed‐gradient experiment is preferred to the steady‐gradient experiment because it affords better definition and control over the time during which diffusion is observed. Diffusion times between 1 sec and 10−3 sec have been used. One artificial system of thin liquid layers, three different kinds of plant cells, and one emulsion have been studied. Clear indications of restricted diffusion are found in all the systems. When fitted to theoretical expressions derived for such behavior, the data yielded a description of each system, as seen by the diffusing molecules, adequately in agreement with the known structure and properties. Critiera for recognizing and analyzing restricted diffusion are discussed. Necessary conditions for the successful study of restricted diffusion are also discussed.

Abstract: An analytical model of restricted diffusion in bovine optic nerve is presented. The nerve tissue model is composed of two different objects: prolate ellipsoids (axons) and spheres (glial cells) surrounded by partially permeable membranes. The free diffusion coefficients of intracellular and extracellular water may differ. Analytical formulas for signal loss due to diffusion in the pulsed gradient spin echo (PGSE) experiment for this tissue model are derived. The model is fitted to experimental data for bovine optic nerve. The obtained model parameters are shown to be reasonable. The model describes all of the characteristics of the PGSE data: anisotropy, upward curvature of decay curves, and diffusion time dependence. The validity and sensitivity of the model are also discussed.

Abstract: This review summarizes the work performed during the last 40 years in the field of diffusion measurement by nuclear magnetic resonance (NMR), with emphasis on biomedical diffusion imaging. Measuring molecular displacements in biological tissues in vivo has an enormous potential, but remains technically challenging. After a review of the nature of the diffusion process, the basic principles of diffusion measurements with NMR are introduced, followed by a presentation of various diffusion imaging methods. The paper covers many previously resolved theoretical and technical issues and new problems that are more specific to clinical diffusion imaging, such as the calculation of diffusion effects in the presence of multiple magnetic field gradient pulses, the elimination of motion artifacts, and the meaning of anisotropic or restricted diffusion in relation to tissue microdynamics and microstructure. The concept of diffusion imaging is then extended to blood microcirculation imaging. Finally, the current and potential clinical applications of these techniques are described.