Determination of tissue optical properties by piezoelectric detection of laser-induced stress waves

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Abstract
A technique is described for the measurement of optical properties in clear and turbid media based on time-resolved detection of acoustic transients. Thermal expansion of the irradiated volume of a sample heated by short laser pulses causes a pressure-rise that is proportionally to the laser fluence and the absorption coefficient in the sample. The exponential profile of the acoustic signal formed by the initial stress distribution corresponds to z-axial light distribution in the irradiated volume. Therefore, the absorption and scattering properties of tissue can be determined from the profile and amplitude of the acoustic signals induced by the laser pulses. Stress waves generated in phantom aqueous medium and biological tissues by laser pulses were detected by a broad-band lithium niobate acoustic transducer. The results indicate that absorption coefficients in soft biological tissues in the near infrared spectral range are significantly (5 - 10 times) lower as compared to previously reported from integrating sphere measurements.

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