• imaging systems;
  • magnet;
  • toxicity;
  • nanoparticles


Background:  Magnetic nanoparticles may be used for focal delivery for cells, plasmids or drugs, and other applications. Here we asked whether magnetic nanoparticles could be detected in vivo at different time points after intravitreal injection by magnetic resonance imaging.

Methods:  Adult Sprague-Dawley rats received intravitreal injections of 50-nm or 4-µm magnetic particles into the left eye, with an equal volume of phosphate-buffered saline into the right eye (as controls). Animals were examined by magnetic resonance imaging at 1 h, 1 day and 5 weeks after injection. Eyes, brain, liver, spleen and kidney were also imaged with high-resolution ex vivo magnetic resonance imaging scanning.

Results: In vivo magnetic resonance imaging at the 1 h and 1 day time points more clearly detected magnetic particles in the 4 µm group compared with the 50-nm group, although 50-nm magnetic nanoparticles were easily visualized with high-resolution magnetic resonance imaging ex vivo. Five weeks after intravitreal injection magnetic resonance imaging clearly detected 4-µm particles inside the eye, but by this time point the 50-nm magnetic nanoparticles could not be detected by either in vivo or ex vivo high-resolution magnetic resonance imaging. No magnetic particles were detected in any other organ.

Conclusions:  Magnetic resonance imaging could be used to track magnetic nanoparticles in the eye with the dosing selected for this study. Clearance varies by size, with 50-nm magnetic nanoparticles cleared more quickly than 4-µm particles. Thus, nanoparticles may provide advantages over micron-scale particles when considering risks associated with long-term persistence.