  While nuclear magnetic resonance diffusion experiments are widely used to
resolve structures confining the diffusion process, it has been elusive whether
they can exactly reveal these structures. This question is closely related to
X-ray scattering and to Kac's "hear the drum" problem. Although the shape of
the drum is not "hearable", we show that the confining boundary of closed pores
can indeed be detected using modified Stejskal-Tanner magnetic field gradients
that preserve the phase information and enable imaging of the average pore in a
porous medium with a largely increased signal-to-noise ratio.
