<title>Direct measurement of laser fluence distribution and optoacoustic imaging in heterogeneous tissues</title>

Abstract

Determination of laser light distribution in tissues is an important aspect of any laser treatment of living tissues. It is especially essential for clinical laser procedures where profile of light distribution changes during irradiation and requires adjustments of laser conditions. Time- resolved stress detection (TRSD) technique is developed for the monitoring of light distribution in laser irradiated biological tissues, and for the measurement of tissue optical properties. The z-axial profile of a transient stress generated in tissue under confined stress conditions of irradiation corresponds to z-axial distribution of absorbed laser energy in the irradiated volume. The shapes and amplitudes of stress transients induced in layered biological tissues carry information about absorption and scattering properties in each tissue layer. TRSD technique does not require any special preparations, such as tissue slicing, to determine differences in optical properties of layered tissue. The temporal profile of the acoustic signal formed by laser heating can be detected from tissue front or rear surfaces. These two options provide necessary flexibility for an investigator to measure various tissues in vivo. We present time-resolved measurements of acoustic transients induced in heterogeneous phantom tissues by nanosecond laser pulses. Results indicate a capability of TRSD technique to resolve differently absorbing tissue layers with 10 micrometers resolution. Sensitivity of this technique, its advantages and limitations are discussed. Capability of this technique to measure optical properties in layered tissues is defined by a combination of three factors: absorption coefficient, depth of certain layer and its thickness. Thickness of tissue layers with different optical properties can be measured with about 1-micrometers precision. Monte-Carlo simulations of light propagation in layered tissues yielded a good agreement with experimental results.