Magnetic nanoparticles are widely used as a contrast agent in MRI. Presently, imaging with magnetic nanoparticles is mostly based on T2*/T2 contrast, which is not ideal because it results in a decrease of MR signal (negative contrast) and undesirable signal loss and exaggeration of spatial extent. A number of approaches have been developed to produce the so-called positive contrast but they are based on macroscopic magnetic field changes. We developed an alternative approach that is not related to T2*/T2 contrast and sensitive to the microscopic field disturbances induced by the magnetic nanoparticles. The technique is based on saturating spins near the particles and exploits an effect that can be controlled on the scale of T1 and is sensitive to diffusion. The technique was demonstrated to be sensitive to the magnetic nanoparticles. In addition, an empirical model was also developed to account for the dependence of the contrast on diffusion. Molecular imaging with magnetic nanoparticles are mostly done with cell labeling with exogenous particles or delivering of biologically conjugated particles; these approaches suffer from dilution in vivo and cannot be readily used to monitor in vivo gene expression. To avoid this problem, we have developed an MRI reporter gene using a gene native to magnetotactic bacteria. Expression of this gene in mammalian cells led to the production of magnetic nanoparticles and hence the MRI contrast. Gene expression in vivo also led to a significant MR contrast. This approach is expected to be used in MRI as GFP is used in optical imaging.

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