An optoacoustic detection method suitable for depth profiling of optical absorption of layered or continuously varying tissue structures is presented. Detection of thermoelastically induced pressure transients allows reconstruction of optical properties of the sample to a depth of several millimeters with a spatial resolution of 24 µm. Acoustic detection is performed using a specially designed piezoelectric transducer, which is transparent for optical radiation. Thus, ultrasonic signals can be recorded at the same position the tissue is illuminated. Because the optoacoustical sound source is placed in the pulsed-acoustic near field of the pressure sensor, signal distortions commonly associated with acoustical diffraction are eliminated. Therefore, the acoustic signals mimic exactly the depth profile of the absorbed energy. This is illustrated by imaging the absorption profile of a two-layered sample with different absorption coefficients, and of a dye distribution while diffusing into a gelatin phantom.
A classical medical ultrasound system was combined with a pulsed laser source to allow laser-induced ultrasound imaging (optoacoustics). Classical ultrasound is based on reflection and scattering of an incident acoustic pulse at internal tissue structures. Laser-induced ultrasound is generated in situ by heating optical absorbing structures, such as blood vessels, with a 5 ns laser pulse (few degrees or fraction of degree), which generates pressure transients. Laser-induced ultrasound probes optical properties and therefore provides much higher contrast and complementary information compared to classical ultrasound. An ultrasound array transducer in combination with a commercial medical imaging system was used to record acoustic transients of both methods. Veins and arteries in a human forearm were identified in vivo using classical color doppler and oxygenation dependent optical absorption at 660 nm and 1064 nm laser wavelength. Safety limits of both methods were explored. Laser-induced ultrasound seems well suited to improve classical ultrasound imaging of subcutaneous regions.
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