Continuous wave MRI diffusion study of water in bentonite clay

Abstract

A novel NMR technique for imaging materials exhibiting extremely short T2 relaxation times is described and some examples of its application to materials of scientific and industrial interest are presented. The technique employs continuous wave (CW) radiofrequency irradiation of the sample together with continual detection, and thus the experimental deadtime inherent in pulsed techniques is effectively eliminated. The mechanisms which broaden the NMR linewidths of materials in the solid state and liquids in confined geometries are described, and the resulting difficulties which these broad linewidths pose to NMR imaging are outlined. A brief review of existing NMR imaging techniques is then presented. The CW-NMR approach to imaging is expounded, and the CW-NMRI system is described in detail. 1-D and 2-D images of the ingress of water (1H imaging) and brine (23Na imaging) into various cementitious materials are presented, together with images of the solid 27Al phase of a high-temperature refractory cement. The inter- and self-diffusion coefficients of a clay mineral, bentonite, were successfully measured, and the 2-D and 3-D imaging performance of the system was demonstrated on phantoms of poly(methyl methacrylate) (PMMA, Perspex / Plexiglas).