Spectral optoacoustic imaging provides information about functional tissue properties in addition to tissue structure. To acquire simultaneously images at two wavelengths a wavelength-multiplexing method is proposed that uses two outputs from a pump laser - OPO (optical parametric oscillator) system. Two-dimensional optoacoustic images were created using a synthetic aperture focusing technique. A clear discrimination of phantom objects due to their different
absorption properties at the two wavelengths was achieved. First in vivo images of blood vessels indicate that this method may be useful in generating maps of blood oxygenation.