Ultrasound-modulated optical tomography is a promising and noninvasive method for biomedical imaging. The
advantage of this technology is its combination of optical contrast and ultrasonic resolution. In order to reconstruct the
tissue imaging effectively and reliably, the propagation of the light modulated by ultrasound in the tissue should be
understood extensively. In our opinion, there are three light transport processes in tissue as follows: Firstly, the incident
light goes from the surface to the focused region. If the distance, tagged as Iis long enough (Z>>mfp, mean free path).
the light transport obeys diffuse theory. Secondly, the diffuse light can be modulated in the focused region at Z due to the
light-sound interaction. Finally, the modulation light from the Z can be regarded as a spot light source which emits the
ballistic or snake photons to reach the surface and so as to be collected by a detector outside of tissue in the third process.
the propagation of the diffused light modulated by ultrasound play an important role in particularly because it reflects
some information about the optical and ultrasonic properties of tissue. Based on the Monte Carlo simulation, the relations
to the modulation light intensity and its modulation depth contributed by the tissue thickness, optical properties, etc. are
figured out and supported by an equivalent experiment and at an extended condition also agree with the diffuse theory.