Spectral optoacoustic imaging uses two or more optical wavelengths for the excitation of ultrasonic waves in order to obtain spectrally resolved images of tissue. We developed a wavelength-multiplexing method that enables the simultaneous generation and detection of optoacoustic waves with two optical wavelengths emitted from a pump laser - optical parametric oscillator system. Two-dimensional images were taken with a linearly scanning optoacoustic transducer from phantoms and from tissue in vivo. The time required for signal acquisition depended solely on the pulse repetition rate of the laser. At presence, with a 10 Hz system, this time was between 10 and 20 seconds. Pairs of images at two wavelengths show an exact overlap of the imaged structures. Wavelength-dependent variations of optical absorption coefficients are visualized by calculating ratio and difference images. The presented experimental results demonstrate the feasibility of the method to generate maps of blood oxygenation.
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