Diffusive diffraction observed with volume-selective STEAM MRS in 100 μm water-filled capillaries

Diffusive diffraction patterns may be useful for probing the local environment of diffusing molecules in materials with geometrically ordered microstructure. A model system consisting in a bundle of water-filled 100 μm glass capillaries was probed with volume-selective stimulated echo (STEAM) MRS on a 7 T Bruker PharmaScan tomograph with variable diffusion times for both, parallel and perpendicular diffusion-weighting with respect to the capillaries' axes. The precise orientation of the capillaries was determined with image processing methods. Echo attenuation curves were numerically evaluated with respect to the inner radius of the capillaries R and the diffusion coefficient D using equations given in the literature. Good agreement was found between simulation and experiment. For perpendicular diffusion-weighting and diffusion times in the order of R2/D two diffraction minima were observed which were not present for shorter diffusion times and for parallel diffusion-weighting. In conclusion, volume-selective diffusive diffraction was observed with a standard small-animal tomograph.