Developing a high-resolution photoacoustic microscopy platform

Existing optical imaging modalities including confocal microscopy, two-photon microscopy and optical coherence tomography do not image optical absorption directly. Photoacoustic imaging (also called optoacoustic imaging) is a new promising modality in biomedical imaging integrating the benefits of optics and acoustics. When biological tissue is irradiated with ultrashort laserpulses with durations of a few nanoseconds the light is absorbed according to the local absorption properties and is converted successively into heat and pressure by means of the thermoelastic effect. The motivation for photoacoustic imaging is to combine ultrasonic resolution with high contrast due to light absorption depending on the physiology of the examined biological tissue. The resolution of conventional photoacoustic imaging systems is not sufficient for in-vitro measurements of small tissue samples or individual cells. In this work, we present a high-resolution photoacoustic microscopy pla tform based on the SASAM acoustic microscope (Kibero GmbH, Germany) that allows high resolution imaging on living cells. The system based on an inverted optical microscope consists of a laser source for optical multi wavelength excitation (diode- or solidstate- laser) which emits nanosecond laser pulses with a wavelength in the near infrared spectrum (optical window). We use different ultrasound transducers in the frequency range up to 300 MHz for detection of the pressure transients. Read out electronics combined with reconstruction algorithms for photoacoustic imaging allows converting the recorded signals into a spatial representation of the absorbed energy. Furthermore, the possibility of using nanoscaled contrast agents for photoacoustic contrast enhancement is presented. In addition to the photoacoustic imaging mode all common optical modalities are implemented. Pure acoustic imaging and optical transmission mode are used for reference imaging.