Theoretical and experimental studies of ultrasound-modulated optical tomography in biological tissue

Ultrasound-modulated optical tomography in biological tissue was studied bo th theoretically and experimentally. An ultrasonic beam was focused into bi ological tissue samples to modulate the laser Light passing through the ult rasonic beam inside the tissue. The ultrasound-modulated laser Light reflec ts the local optical and mechanical properties in the ultrasonic beam and p ermits tomographic imaging of biological tissues by scanning. Parallel dete ction of the speckle held formed by the transmitted laser light was impleme nted with the source-synchronous-illumination lock-in technique to improve the signal-to-noise ratio. Two-dimensional, images of biological tissues we re successfully obtained experimentally with a laser beam at either normal or oblique incidence, which showed that ultrasound-modulated optical tomogr aphy depends on diffuse Light rather than on ballistic light. Monte Carlo s imulations showed that the modulation depth decreased much more slowly than the diffuse transmittance, which indicated the possibility that even thick er biological tissues can be imaged with this technique. (C) 2000 Optical S ociety of America.