Magnetic dipole imaging of magnetite nanoparticles in brain tissue

The human brain contains magnetic iron oxide nanoparticles in the form of magnetite (Fe₃O₄); however, the origin and physiological implications of these crystals remain debated. Due to their low concentrations in brain tissue (∼1-20 ng g^-1), the identification and characterization of individual magnetic particles require nanometer-scale spatial resolution over large scan volumes. In contrast to conventional electron microscopy techniques that have field of views typically on micron scales, the Quantum Diamond Microscope (QDM), based on wide-field nitrogen-vacancy center imaging, can generate magnetic field maps over areas of several square millimeters while detecting nanoscale particles. Moreover, the QDM can directly quantify the strength and direction of the particles' magnetic moments. Operating the QDM in a high-sensitivity mode, coupled with long acquisition times, enabled the detection of magnetic moments as small as 3 × 10^-17 Am², corresponding to a magnetite particle diameter of approximately 50 nm, in maps covering 1.40 × 2.25 mm². This is the highest magnetic moment sensitivity of wide-field magnetic microscopy >1 mm² to date. In addition, collecting repeat, but slightly offset magnetic field maps resulted in the unique ability to distinguish sources within a sample from contamination and artifacts. By applying this technique to tissue, we demonstrate the detection of magnetic dipole-generating sources in human and rodent brain samples with the QDM. Detected particles span a size range of 60-135 nm, consistent with the larger end of magnetite particle sizes found by electron microscopy. These are the first direct magnetic observations of magnetite nanoparticles in brain tissue using quantum sensing techniques.