Fluorescently-tagged magnetic protein nanoparticles for high-resolution optical and ultra-high field magnetic resonance dual-modal cerebral angiography

Extremely small paramagnetic iron oxide nanoparticles (FeMNPs) (<5 nm) can enhance positive magnetic resonance imaging (MRI) contrast by shortening the longitudinal relaxation time of water (T₁), but these nanoparticles experience rapid renal clearance. Here, magnetic protein nanoparticles (MPNPs) are synthesized from protein-conjugated citric acid coated FeMNPs (c-FeMNPs) without loss of the T₁ MRI properties and tagged with fluorescent dye (f-MPNPs) for optical cerebrovascular imaging. The c-FeMNPs shows average size 3.8 ± 0.7 nm with T₁ relaxivity (r₁) of 1.86 mM^-1 s^-1 and transverse/longitudinal relaxivity ratio (r₂/r₁) of 2.53 at 11.7 T. The f-MPNPs show a higher r₁ value of 2.18 mM^-1 s^-1 and r₂/r₁ ratio of 2.88 at 11.7 T, which generates excellent positive MRI contrast. In vivo cerebral angiography with f-MPNPs enables detailed microvascular contrast enhancement for differentiation of major blood vessels of murine brain, which corresponds well with whole brain three-dimensional time-of-flight MRI angiograms (17 min imaging time with 60 ms repetition time and 40 μm isotropic voxels). The real-time fluorescence angiography enables unambiguous detection of brain capillaries with diameter < 40 μm. Biodistribution examination revealed that f-MPNPs were safely cleared by the organs like the liver, spleen, and kidneys within a day after injection. Blood biochemical assays demonstrated no risk of iron overload in both rats and mice. With hybrid neuroimaging technologies (e.g., MRI-optical) on the rise, f-MPNPs built on this platform can generate exciting neuroscience applications.