Ultrasonic encoding of diffused light: From optical imaging to light focusing in turbid media

In optical scattering media such as biological tissue, light propagation is randomized by multiple scattering. Beyond one transport mean free path, where photon propagation is in the diffusive regime, direct light focusing becomes infeasible. The resulting loss of light localization poses serious challenge to optical imaging in thick scattering media. Ultrasound modulated optical tomography (UOT) combines high optical contrast and good ultrasonic resolution, and is therefore an ideal imaging modality for soft biological tissue. A variety of detection techniques have been developed in UOT in an effort to discriminate the ultrasonically encoded diffused light as the imaging signal. We developed a photorefractive crystal based detection system, which has the ability to image both the optical and acoustic properties of biological tissues. With the improved photorefractive crystal based detection, tissue-mimicking phantom samples as thick as 9.4 cm can be imaged. We further exploit the virtual source concept in UOT and combine it with optical time reversal to achieve diffusive light focusing into scattering media. Experimental implementation of this new technology is presented.