Exploring ultrasound-modulated optical tomography at clinically useful depths using the photorefractive effect

For years, ultrasound-modulated optical tomography (UOT) has been proposed to image optical contrasts deep inside turbid media (such as biological tissue) at an ultrasonic spatial resolution. The reported imaging depth so far, however, has been limited, preventing this technique from finding broader applications. In this work, we present our latest experimental explorations that push UOT to clinically useful imaging depths, achieved through optimizing from different aspects. One improvement is the use of a large aperture fiber bundle, which more effectively collects the diffused light, including both ultrasound-modulated and unmodulated portions, from the turbid sample and then sends it to the photorefractive material. Another endeavor is employment of a large aperture photorefractive polymer film for demodulating the ultrasound-induced phase modulation. Compared with most UOT detection schemes, the polymer film based setup provides a much higher etendue as well as photorefractive two-beam-coupling gain. Experimentally, we have demonstrated enhanced sensitivity and have imaged through tissue-mimicking samples up to 9.4 cm thick at the ultrasonically-determined spatial resolutions.