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Abstract
The use of contrast agents and tracers in medical imaging has a long history. They
provide important information for diagnosis and therapy, but for some desired applications,
a higher resolution is required than can be obtained using the currently available
medical imaging techniques. Consider, for example, the use of magnetic tracers in
magnetic resonance imaging: detection thresholds for in vitro and in vivo imaging
are such that the background signal from the host tissue is a crucial limiting factor.
A sensitive method for detecting the magnetic particles directly is to measure their
magnetic fields using relaxometry; but this approach has the drawback that the inverse
problem (associated with transforming the data into a spatial image) is ill posed
and therefore yields low spatial resolution. Here we present a method for obtaining
a high-resolution image of such tracers that takes advantage of the nonlinear magnetization
curve of small magnetic particles. Initial 'phantom' experiments are reported that
demonstrate the feasibility of the imaging method. The resolution that we achieve
is already well below 1 mm. We evaluate the prospects for further improvement, and
show that the method has the potential to be developed into an imaging method characterized
by both high spatial resolution as well as high sensitivity.