Molecular Diffusion in NMR Microscopy

Abstract

The signal-to-noise ratio and the T 2 contrast in 1H NMR microscopy are strongly affected by self-diffusion effects. Here, we investigate the free diffusion of water within imaging gradients. As a result we obtain an apparent relaxation time T 2 which in NMR microscopy is at least one order of magnitude smaller than the true T 2 value of water in the object. This apparent T 2 relaxation is considerably reduced by improving spatial resolution. We conclude that quantitative true T 2 values cannot be calculated from series of images with increasing echo time. Furthermore, from the knowledge of the apparent T 2, an optimum short echo time can be found in order to maximize signal-to-noise ratio. Our theoretical findings are confirmed by phantom experiments at 11.75 T field strength.