  Nuclear magnetic resonance (NMR) diffusion measurements are widely used to
derive parameters indirectly related to the microstructure of biological
tissues and porous media. However, a direct imaging of cell or pore shapes and
sizes would be of high interest. For a long time, determining pore shapes by
NMR diffusion acquisitions seemed impossible, because the necessary phase
information could not be preserved. Here we demonstrate experimentally using
the measurement technique which we have recently proposed theoretically that
the shape of arbitrary closed pores can be imaged by diffusion acquisitions,
which yield the phase information. For this purpose, we use hyperpolarized
xenon gas in well-defined geometries. The signal can be collected from the
whole sample which mainly eliminates the problem of vanishing signal at
increasing resolution of conventional NMR imaging. This could be used to
non-invasively gain structural information inaccessible so far such as pore or
cell shapes, cell density or axon integrity.
