Imaging blood flow inside highly scattering media using ultrasound modulated optical tomography

We report the use of ultrasound modulated optical tomography (UOT) with heterodyne parallel detection to locally sense and image blood flow deep inside a highly scattering medium. We demonstrate that the UOT signal is sensitive to the speed of the blood flow in the ultrasound focus and present an analytical model that relates UOT signals to the optical properties (i. e. scattering coefficient, anisotropy, absorption, and flow speed) of the blood and the background medium. We found an excellent agreement between the experimental data and the analytical model. By varying the integration time of the camera in our setup, we were able to spatially resolve blood flow in a scattering medium with a lateral resolution of 1.5 mm. Sensitive, accurate and noninvasive method to measure blood flow is important to study biological processes such as local metabolism, pharmacodynamics, and stroke recovery. Optical methods of flowmetry possess these features, however the lack of spatial resolution with increasing depth due multiple scattering present challenge. Ultrasound modulated optical tomography (UOT), a hybrid of light and ultrasound combines the high sensitivity to blood flow (i. e. attribute of the optical techniques) and the spatial resolution (i. e. the attribute of ultrasound). The ability of UOT for depth resolved blood flow imaging is illustrated in the image above.