The present invention can provide a method of determining the communication of substances between a first region and a second region of a patients body. An example method according to the present invention can comprise: (a) introducing into the first region a plurality of superparamagnetic nanoparticles, having properties such that they undergo Brownian motion that randomizes the orientation of the nanoparticles according to a predetermined characteristic time (b) after a time sufficient to allow transport of nanoparticles from the first region to the second region, subjecting the second region to an applied magnetic field of sufficient strength to induce magnetization of individual nanoparticles, and having a substantially uniform direction throughout the second region (c) measuring the magnetic field of the second region at a plurality of times after ceasing application of the magnetic field (d) analyzing the measured magnetic field to detect signals that correspond to decay of the magnetic field due to randomization of the nanoparticles orientation by Brownian motion (e) determining the presence of nanoparticles in the second region from the signals detected in step (d).
