Light and ultrasound: ultrasound tagging of light for imaging beyond the scattering limits.

• Online Access: http://hdl.handle.net/2047/D20327426

In optical imaging, the depth and resolution are limited due to scattering. Unlike light, scattering of an ultrasound waves in tissue is negligible. Hybrid imaging methods such as ultrasound modulated optical tomography (UOT) use the advantages of both modalities. UOT tags light by inducing phase change caused by modulating the local index of refraction of the medium. The challenge in UOT is detecting the weak signal. The displacement induced by the acoustic radiation force (ARF) is another ultrasound effect that can be utilized to tag the light. It induces greater phase change, resulting in a stronger signal. Moreover, the absorbed acoustic energy generates heat, resulting in a change in the index of refraction and a strong phase change. Since the speckle pattern is governed by the phase of the interfering scattered waves speckle pattern analysis can obtain information about local displacement and temperature changes in the tissue. We have presented a model to simulate the insonation processes. Simulation results based on fixed-particle Monte Carlo and experimental results show that the signal acquired by utilizing ARF is stronger compared to UOT. The introduced mean irradiance change (MIC) signal reveals both thermal and mechanical effects of the focused ultrasound wave in different timescales. Simulation results suggest that variation in the MIC signal can be used to generate a displacement image of the medium. In addition to the displacement correlated image, the MIC signal can provide images based on the morphology of the tissue. The MIC signal can provide for tumor detection in a healthy tissue.--Author's abstract