Opto-acoustic measurement of the local light absorption coefficient in turbid media: 1. Monte-Carlo simulation of laser fluence distribution at the beam axis beneath the surface of a turbid medium

In-vitro study of the temperature dependence of the optoacoustic conversion efficiency in biological tissues

“…Let a nanosecond laser pulse fall onto the interface between a transparent medium and the tissue under study. Then, the amplitude of the OA signal excited in the tissues can be expressed as [26]:…”

“…This effect was studied theoretically and experimentally in some papers (see, for example, [31 - 33]). In this case, the coefficient k depends on the diameter of the laser beam and optical coefficients (absorption and reduced scattering coefficients) [26]. Therefore, the amplitude of the excited OA signal will be determined already by three temperature dependent factors: E max , m a , and G [see formula (1)].…”

“…However, in the case of a narrow laser beam, d £ l * (l * = 1/m s ¢ is the photon transport mean free path in the medium), which for most tissues in the visible and near-IR range is fulfilled when d K 0.5 mm, the quantity k (m a /m s ¢ ) is a constant at 0.01 < m a /m s ¢ < 0.3 [26]. Thus, the diameter of the probe beam can be chosen so that in the subsurface layer of the medium the volume density m a E max of the absorbed laser radiation have similar values at different scattering coefficient and be linearly dependent on the light absorption coefficient m a .…”

“…Therefore, in the backward detection regime [26], under the conditions of the narrow laser beam and the finite duration of the laser pulse, the OA signal amplitude is proportional to the light absorption coefficient m a and depends very little on the reduced light scattering coefficient m s ¢ . Then, the temperature dependence of the OA signal amplitude resembles the temperature dependence of the parameter m a G [the dependence of E max on the temperature, which is related with variations of the light scattering coefficient in the biological tissue, can be eliminated from expression (1)].…”

“…Therefore, the influences of light scattering, light absorption, local laser fluence distribution and Grunaisen parameter on the temperature dependent amplitude of OA signals excited at tissue surface could not be separated. On the other hand, when OA conversion occurs within a small region of a turbid medium, the OA signal amplitude is proportional to the light absorption coefficient of this region and depends very little on light scattering [26]. Thus, the dependences of OA signal amplitude on optical properties of the tissue are different when OA conversion occurs at the tissue surface and inside the tissue.…”