Fluorescent microsphere imaging: a particle-tracking approach to the hemodynamic assessment of the retina and choroid

Quantitative assessment of choriocapillaris circulation has proven difficult. Although the fluorescent vesicle system provides a means of quantifying the retinal circulation, the attempts at imaging fluorescent liposomes in the choroidal microcirculation have been largely unsuccessful. The authors introduce a new tool, fluorescent microsphere imaging, and examine its utility for evaluating the hemodynamics of the retina and choroidal microcirculation. The usefulness of fluorescent microsphere imaging is demonstrated through the examination of the retinal and choroidal circulations of three rhesus monkeys.

Fluorescent microsphere imaging uses polystyrene latex microspheres that incorporate one or more dyes. These microspheres are injected intravenously into an animal and are excited in the eye through the resident lasers of a scanning laser ophthalmoscope. The excited particles are detected by the ophthalmoscope, and its output is then digitized directly or recorded on a videocassette recorder for subsequent image analysis. Multiple-dye microspheres use the principle of resonance energy transfer for the activation of the final dye in a non-radiative cascade. These microspheres enable the investigator to tailor the excitation and emission spectra of the particles for the investigation of different ocular tissues.

Using 488/515 microspheres (excitation and emission peaks at 488 nm and 515 nm, respectively), the authors captured images of particles circulating in the perimacular retinal circulation. Shifting excitation and emission spectra toward the red and infrared enabled the imaging of blood flow in progressively deeper tissue. Using 633/825 microspheres, the authors recorded and tracked particles in the microcirculation of the choroid.

The authors' findings suggest that fluorescent microsphere imaging provides images useful for studying the retinal circulation and for evaluating previously inaccessible choroidal hemodynamics.