Modulated Fluorophore Signal Recovery Buried within Tissue Mimicking Phantoms

Abstract

Optically modulated fluorescence from ∼140 nM Cy5 is visualized when embedded up to 6 mm within skin tissue mimicking phantoms, even in the presence of overwhelming background fluorescence and scatter. Experimental and finite element analysis (FEA)-based computational models yield excellent agreement in signal levels and predict biocompatible temperature changes. Using synchronously amplified fluorescence image recovery (SAFIRe), dual-laser excitation (primary laser: λ = 594 nm, 0.29 kW/cm(2); secondary laser: λ = 710 nm, 5.9 kW/cm(2), intensity-modulated at 100 Hz) simultaneously excites fluorescence and dynamically optically reverses the dark state buildup of primary laser-excited Cy5 molecules. As the modulated secondary laser both directly modulates Cy5 emission and is of lower energy than the collected Cy5 fluorescence, modulated Cy5 fluorescence in phantoms is free of obscuring background emission. The modulated fluorescence emission due to the secondary laser was recovered by Fourier transformation, yielding a specific and unique signature of the introduced fluorophores, with largely background-free detection, at excitation intensities close to the maximum permissible exposure (MPE) for skin. Experimental and computational models agree to within 8%, validating the computational model. As modulated fluorescence depends on the presence of both lasers, depth information as a function of focal position is also readily obtained from recovered modulated signal strength.