Microbubble enhancement of ultrasound-modulated optical sensing with incoherent light

Diffuse optical techniques in tissue are insensitive to oxygenation changes inside large blood vessels, due to the high optical absorption relative to the surrounding tissue. To overcome this a hybrid technique of diffuse light modulated by focused ultrasound (US) was used to detect an acousto-optic (AO) signal from a large (1 cm diameter) blood-filled tube surrounded by a turbid medium. An injection of microbubbles, a contrast agent used in clinical diagnostic US, amplified this AO signal to an experimentally detectable level. The blood was diluted to vary its optical absorption, and a resulting change in the magnitude of the AO signal was observed. A mechanism by which microbubbles can enhance USmodulation of light is proposed by deriving a 2^nd order approximation to the Rayleigh-Plesset equation of motion for a bubble in an US field. A Monte Carlo (MC) model of a deep blood vessel geometry has also been developed: this takes into account the optical scattering from oscillating microbubbles in the blood, which is expected to vary spatially and temporally. Results of the MC model show that the US-modulated light signal is more sensitive to oxygenation changes within the blood vessel than a diffuse optical signal. Experimental results show a significant enhancement of the USmodulated optical signal when microbubbles were introduced.