Visualization of inhomogeneous local magnetic field gradient due to susceptibility contrast

We visualized inhomogeneous local magnetic field (internal magnetic field) gradients arising from susceptibility contrast between an array of cylindrical glass tubes (solid matrix) and surrounding water (pore fluids) in a uniform applied magnetic field. MRI was performed to determine the spatially resolved decay rates due to diffusion in the internal magnetic field which were proportional to the strength of local gradient. We also spatially resolved the interference pattern of the cross-terms between the internal and the applied field gradient in order to extract the orientation of the internal field gradient. These experimental results were found to be consistent with the theoretical calculations. This work demonstrates a simple yet representative case for visualizing the strength and orientation of the local susceptibility induced magnetic field gradients in porous media.