Ultrasound-modulated optical microscopy

Optical imaging modalities provide functional, structural, and molecular information about tissue based on optical contrast mechanisms. Technical advantages, such as nonionizing radiation, easy operation, low cost, and high sensitivity favor these technologies for biomedical applications. Optical imaging offers excellent intrinsic optical contrast but suffers from poor spatial resolution at depths beyond one transport mean free path inside the biological tissue, due to strong scattering of light. Thus many in vivo optical microscopy techniques offer micron-scale resolution at shallow (a few hundred mum) imaging depths in the skin. On the other hand, ultrasound imaging offers excellent spatial resolution and imaging depth, both of which are scalable with ultrasound parameters. However, it provides poor contrast for soft tissue imaging. Ultrasound-modulated optical imaging (UOI) is a promising hybrid imaging modality that combines the advantages of both ultrasound and optical imaging.;This work develops an ultrasound-modulated optical microscope (UOM) for imaging several millimeters deep inside scattering biological tissue with excellent spatial resolution and optical contrast. This microscope is based on a long-cavity confocal Fabry-Perot interferometer for real time detection of multiply scattered light modulated by high frequency (30 MHz to 75 MHz) focused ultrasound pulses. Using 75MHz in tissue-mimicking phantoms, at an imaging depth of about 2 mm, an axial resolution better than 30 Jim can be achieved, with a lateral resolution of 38 lam. Experimental results and analytical calculations show that modulation depth and image contrast decrease with an increase in ultrasound frequency. This microscope images blood vasculature in highly scattering tissue samples from a mouse and a rat. Therefore UOM is a promising tool for studying the morphology of blood vasculature and blood-associated functional parameters, such as oxygen saturation.;It remains a challenge to provide both optical absorption and scattering contrasts using a single optical microscope. This work demonstrates that UOM can map both optical absorption and scattering contrasts. Another challenge is to provide optical scattering contrast in deep tissue with high spatial resolution. This work successfully investigates imaging optical scattering contrast with high spatial resolution in macroscopic (deep tissue) regime of UOI using experiments, analytical calculations, and Monte Carlo simulations.