Developing a technique for combining light and ultrasound for deep tissue imaging

Sammanfattning: Biological tissues are strongly light-scattering and absorbing media that limit the depth of optical-based imaging. Even though, optical imaging provides good optical contrast information of biological content that is beneficial for clinical diagnostics application. Ultrasound was used to assist the optical-based imaging technique in overcoming the poor spatial resolution and shallow imaging depth difficulties. Ultrasound can penetrate deep into tissue. When the tissue is illuminated, the ultrasound modulates the light inside the ultrasound focus. The modulated light that shifted by one ultrasound frequency is called the tagged photons. Measure of the tagged photons intensity distribution would provide local light irradiance information in the tissue, and this is the principle of ultrasound optical tomography (UOT). The UOT system can have a spatial resolution as good as the ultrasound focus. The goal of this thesis work is to check the feasibility of UOT at 606 nm. The thesis work was first started with characterizing tissue models with controlled optical properties, homogeneity and macroscopic geometry. Then the performance of an ultrasound scanner was investigated. The UOT experiment was carried out on multiple tissue models with different thicknesses but the same optical properties. When the laser energy was 30 nJ, the current UOT system can detect UOT signal for a 40 mm thick tissue model. And it was also capable of performing a 1D line measurement on a 10 mm thick highly scattering medium with an absorbing inclusion in the center, the dimension of the inclusion can be estimated from the UOT measurements.