Drug delivery monitoring by photoacoustic tomography with an ICG encapsulated double emulsion

The absorption spectrum of indocyanine green (ICG), a nontoxic dye used for medical diagnostics, depends upon its concentration as well as the nature of its environment, i.e., the solvent medium into which it is dissolved. In blood, ICG binds with plasma proteins, thus causing changes in its photoacoustic spectrum. We successfully encapsulated ICG in an ultrasound-triggerable perfluorocarbon double emulsion that prevents ICG from binding with plasma proteins. Photoacoustic spectral measurements on point target as well as 2-D photoacoustic images of blood vessels revealed that the photoacoustic spectrum changes significantly in blood when the ICG-loaded emulsion undergoes acoustic droplet vaporization (ADV), which is the conversion of liquid droplets into gas bubbles using ultrasound. We propose that these changes in the photoacoustic spectrum of the ICG emulsion in blood, coupled with photoacoustic tomography, could be used to spatially and quantitatively monitor ultrasound initiated drug delivery. In addition, we suggest that the photoacoustic spectral change induced by ultrasound exposure could also be used as contrast in photoacoustic imaging to obtain a background free image.