Deeply penetrating photoacoustic tomography in biological tissues enhanced with an optical contrast agent

Photoacoustic tomography (PAT) in a circular scanning configuration was developed to image deeply embedded optical heterogeneity in biological tissues. While the optical penetration was maximized with near-infrared laser pulses of 800-nm wavelength, the optical contrast was enhanced by Indocyanine Green (ICG) dye whose absorption peak matched the laser wavelength. This optimized PAT was able to image objects embedded at depths of as much as 5.2 cm, 6.2 times the $1/e$ optical penetration depth, in chicken breast muscle at a resolution of $<780 \mathit{\mu }\mathrm{m}$ and a sensitivity of $<7 \mathrm{pmol}$ of ICG in blood. The resolution was found to deteriorate slowly with increasing imaging depth. The effects of detection bandwidth on the quality of images acquired simultaneously by four different ultrasonic transducers are described.