Internal gradient mapping with distant dipolar field contrast.

E.V. SILLETTA; M.B. FRANZONI; R.H. ACOSTA

Congreso; 13th Nuclear Magnetic Resonance Users Meeting; 2011

A particle diffusing in a confining medium is a general model for a number of physical, chemical, biological, and industrial processes. It may describe organic molecules or metabolites in biological cells or brain tissue, reactive species near porous catalysts, ions near rough electrodes or cellular membranes, oxygen in human lungs, water molecules in cements or rocks, etc. When such a particle encounters an interface, they may interact in different ways depending on their physical and chemical properties. The interaction at the microscopic level can often be represented in terms of “reflection” and “absorption” at the interface. In the former case, the particle does not change its state and continues to diffuse in the bulk. In the latter case, the motion of the particle is terminated, either by absorption on or transfer through the interface, by chemical transformation into another particle, or by surface relaxation in a NMR experiment. NMR is of particular interest as being a method to “label” or “encode” Brownian trajectories of spin-bearing particles by using magnetic fields. The heterogeneous nature of the samples broadens the spectrum of the NMR measurement due to the existence of magnetic susceptibility differences between materials in the samples, for example, between a porous matrix with saturating fluid local magnetic field gradients develop at the interfaces. These local magnetic fields with pronounced spatial variations are commonly referred to as “internal gradients” and the main facts governing their strength depend on the susceptibility difference between the materials, the applied magnetic field and the pore size, shape and the geometry of the pore network. The internal gradients scale roughly with the applied magnetic field as: g B0 where  is the magnetic susceptibility difference experienced between the pore surface and the detected fluid, and B0 is the applied field. The presence of the internal gradient often interferes with NMR relaxation and diffusion measurements and great effort has been made in the design of pulse sequences that can cope with this effect [1,2]. On the other hand, decay rates due to susceptibility differences can readily be exploited to obtain characteristics of porous media by using the information provided by the internal gradients [3]. An alternative method to probe the influence of internal gradients relies on the use of the Distant Dipolar Field (DDF). In liquids at high magnetic fields the dipolar interaction that is normally averaged out can be reintroduced by the application of sequences like the Cosy Revamped by Asymmetric Z-Gradient Echo Detection (CRAZED) [4]. If the symmetry on the sample is broken, as for instance by the application of a magnetic field gradient, intermolecular Multiple Quantum Coherences (iMQC) are converted into observable signal by intermolecular dipolar couplings. The area of the gradients (GT) defines a characteristic length scale, referred to as the correlation distance dc = GT that is typically between 10 m – 10 mm, over which the magnetization is highly modulated. In macroscopic homogeneous samples the correlation distance plays an unimportant role, unless it is comparable to the diffusion occurring during the time of the sequence [5]. However, in heterogeneous samples, the contrast in an imaging experiment can be greatly influenced, since the iMQC signal provides a direct measure of the dipolar field at a selected correlation length [6].