Ultrasound-modulated optical tomography in biological tissue was studied both theoretically and experimentally. An ultrasonic beam was focused into biological tissue samples to modulate the laser light passing through the ultrasonic beam inside the tissue. The ultrasound-modulated laser light reflects the local optical and mechanical properties in the ultrasonic beam and permits tomographic imaging of biological tissues by scanning. Parallel detection of the speckle field formed by the transmitted laser light was implemented with the source-synchronous-illumination lock-in technique to improve the signal-to-noise ratio. Two-dimensional images of biological tissues were successfully obtained experimentally with a laser beam at either normal or oblique incidence, which showed that ultrasound-modulated optical tomography depends on diffuse light rather than on ballistic light. Monte Carlo simulations showed that the modulation depth decreased much more slowly than the diffuse transmittance, which indicated the possibility that even thicker biological tissues can be imaged with this technique.
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