Optical detection of laser-induced stress waves for measurement of the light distribution in living tissue

Detection of the laser-induced stress wave generated in tissue by irradiation with a short laser pulse is a method for direct measurement of the absorbed energy distribution. In this study we tried to optimize this technique in order to avoid distortion of the stress wave during its propagation from the irradiated volume to the detector. Stress waves are formed by short pulsed (pulse duration 8 ns) irradiation of an absorbing dye solution and of tissue samples with a Q-switched Nd:YAG laser at 532 nm. An optical transducer based on pressure-induced reflectivity changes of a glass-water interface detects the stress wave in front of the irradiated sample surface. It is shown theoretically and experimentally that this kind of detector, where the active area is a small spot close to the irradiated surface, minimizes signal distortion due to acoustic diffraction. The absorption coefficients of the dye solutions could be derived with high accuracy from the slopes of the recorded stress waves. Measurements in layered samples and in liver tissue demonstrated that apart from the optical constants also some information about the structure of the samples can be derived from the stress signals. The optical detector is sensitive enough to registrate pressure amplitudes in the range of 1 bar. This makes it possible to use it for in-vivo applications.