Local D-T₂ correlation maps of porous media acquired by surface coil

Spatially resolved NMR measurements can be obtained in three different ways by NMR imaging, the gradient method and the surface-coil techniques [1]. Compared to the two other methods, the surface coil method can obtain localized information without using gradients of the static magnetic field, B₀ [2]. Instead, a sensitive volume is defined which contains localized information. The advantage of the surface-coil is that the sample does not need to fit inside the coil. This means that a sample with any shape can be measured without having to cut it. One example of such device is NMR-MOUSE. NMR-MOUSE is a portable equipment and can be carried out to implement on-site measurements [3].
Here we used NMR-MOUSE (PM 25, Magritek) to obtain spatially resolved D-T₂ correlation maps for porous media (glass beads and rock cores). By changing the relative position between the sample and the coil (either lift the magnet or move the sample) D-T₂ correlation maps from different position of the sample were acquired. The experiments were done with different number of scans to evaluate the influence of noise on correlation maps and optimize the parameter setting. Considering the inhomogeneous magnetic field generated by NMR-MOUSE, the T₂ distributions obtained in these experiments does not reflect the true information about pore structure. In addition, diffusion information was encoded by varying the echo time with constant gradient, during which the relaxation process was also encoded. In order to evaluate the two effects mentioned above, D-T₂ correlation maps of the same samples were acquired by a 2 MHz Rock core analyzer (Magritek) with pulsed field gradient. The results acquired by NMR-MOUSE and Rock core analyzer were compared to interpret the data better.