Internal Gradient Mapping with Distant Dipolar Field

MARÍA BELÉN FRANZONI; RODOLFO H. ACOSTA; EMILIA VICTORIA SILLETTA

Alta Gracia - Córdoba

Congreso; Magnetic Resonance in a Cordubensis Perspective: NEw developments in NMR; 2011

LaNAIS - FaMAF - UNC

INTERNAL GRADIENT MAPPING WITH DISTANT DIPOLAR FIELD CONTRAST E.V. Silletta, M.B. Franzoni, R.H. Acosta* FaMAF-Universidad Nacional de Córdoba & IFEG-CONICET, Córdoba, Argentina racosta@famaf.unc.edu.ar Keywords: porous media, internal gradients, distant dipolar field 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 in biological cells or brain tissue, oxygen in human lungs, water molecules in cements, etc. When such a particle encounters an interface, they may interact in different ways depending on their physical and chemical properties. 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. 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 [1]. An alternative method to probe the influence of such 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) [2]. 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. In this work we explore the use of double quantum intermolecular coherences (iDQC) as a contrast agent to probe the internal gradients of a model sample. Here we use a set of capillaries of outer diameter OD=1.4 mm contained in a 10 mm NMR tube which is filled with distilled water [3]. Figure 1a shows a spin-warp spin echo (TE = 3 ms) 2D image with a 1 mm slice selection along the long axis of the sample. In order to generate the iDQC contrast, a CRAZED sequence is applied prior to the imaging sequence, in this way only signals arising from distant dipolar couplings contribute to the image formation. Figure 1b shows the difference between a reference image (Fig. 1a) and an iDQC image. The iDQC signal generation in the presence of the internal gradients can be observed to be greatly influenced. REFERENCES: 1. Y.-Q. Song; S. Tyu; P.N. Sen; Nature 2000 406, 178. 2. W.S. Warren; W. Richter; A.H. Andreotti; B.T. Farmer, II; Science 1993, 262, 2005. 3. H. Cho; S. Ryu; J.L. Ackerman; Y.-Q. Song; J. Magn. Reson. 2009 198, 88. CONICET, FONCYT, SECYT-UNC, MPIP-MPG