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In a subsequent study fluorescent quantum dots were infused into the brain of rats and within 15 min could be observed in the esophagus using microscopy. These imaging studies clearly show that these large nanoparticle tracers (\u223c20 nm in diameter) leave the brain through the nasal cavity and end up in the gut. There are numerous studies going back decades reporting the clearance of tracers put directly into the brain. 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