Nuclear magnetic resonance microscopy in liquids using the dipolar field

Abstract

We demonstrate theoretically and experimentally how the dipolar field can be used in nuclear magnetic resonance (NMR) to investigate the structure of heterogeneous liquid systems. Using the Fourier transformed dipolar field and magnetization distribution, a simple relation between the NMR signal generated by the dipolar field and the sample structure can be established. On the basis of this relation, theoretical models for periodic structures have been derived and used to analyze the variation of the NMR signal as a function of the spatial modulation imposed on the magnetization. If the spatial modulations imposed on the transverse and longitudinal magnetizations have the same wavelength, the signal generated by the dipolar field is a continuous function of the modulation wavelength and is sensitive to the structure of the sample. When this condition is not met, diffraction phenomena may be possible in periodic structures. To test the theoretical work, experimental data have been obtained from water surrounding randomly packed microspheres. These data are in agreement with the theoretical predictions and show that a resolution of the order of 10 μm can be achieved for highly mobile systems. For spin bearing molecules, whose self-diffusion coefficient is two orders of magnitude less than that of free water, submicrometer resolution is expected.

In VivoMultiple Spin Echoes

“…Thus the second echo amplitude strongly depends on the apparent diffusion coefficient of the spins, and multiple spinecho imaging might represent an alternative to pulsed gradient spin echo (PGSE) studies. Multiple spin echo experiments can also be used to probe the structure of heterogeneous samples (11). In fact, the echo amplitude is sensitive to sample structure on a length scale equal to the pitch of the spatially modulated longitudinal magnetization (9,11).…”

Imaging the Long-Range Dipolar Field in Structured Liquid State Samples

“…By varying the modulation through manipulation of gradient pulse areas, structures can be probed at different length scales. In previous work, measurements were made of the average structure, in simple phantoms consisting of water surrounding polystyrene beads of known diameter (9). This was accomplished by analyzing the signal amplitude recorded from the whole sample as a function of the degree of modulation.…”

Multiple Nonlinear Stimulated Echoes

“…Thus, the structure can be probed on different length scales by varying this period of spatial modulation. A Fourier-space relationship between the structure of heterogeneous samples and the amplitude of multiple spin echoes was established on this basis (20,21). NOSE experiments of an analogous type might be of interest.…”