Real-time biomedical optoacoustic imaging

Abstract

In optoacoustic imaging, short laser pulses irradiate scattering biological tissue and adiabatically heat hidden absorbing structures, such as blood vessels, to generate pressure transients by means of the thermoelastic effect. These acoustic transients propagate to the tissue surface and are recorded with transducers to reconstruct high contrast images of the initial absorbed energy distribution exactly resembling the absorbing structures. Two groups of new real-time optoacoustic imaging systems were developed: (1) Optical systems exploring Schlieren optical detection and acoustic lens system for 3D imaging. (2) Laser excitation combined with classical medical ultrasound system for comparison of the two complementary techniques. In medical applications real-time imaging avoids motion artifacts (heartbeat and breath), facilitates imaging procedure and allows instant diagnosis. In the first optical system, the Schlieren transducer images the pressure transient in a fluid filled cuvette below the tissue with an ns-flash lamp and reconstructs images on a computer for visualization. The second optical system uses an acoustic lens to directly reconstruct a 3D image of the original pressure distribution into a water container. This copied pressure image is optically dark field imaged at two angles to provide a stereo image pair of the original absorbing structures. Both optical systems operate at real-time frame rates of 10-20 Hz and provide high resolutions up to 30-100 μm. Both systems allow illumination at the position of sensing through the transducer water tank (backward mode optoacoustic imaging). This is advantageous for all body parts not accessible from two opposite sides or containing bones. An additional part of basic research included piezoelectric ultrasound transducer design. Piezoelectric polyvinylidene-fluoride (PVDF) is widely established for high bandwidth single element transducers in optoacoustics. A new PVDF sensor for optoacoustic depth profiling featuring transparent conductive indium-tinoxide electrodes (ITO electrodes) was developed, which allows backward mode operation and quantification of optical properties. Medical ultrasound, a widely used diagnostic tool, is limited by low acoustic contrast, which particularly deteriorates or inhibits imaging of smaller structures in near skin regions. Two systems combining laserexcitation and commercial ultrasound are presented exploring high optical contrast and sub-millimeter acoustical spatial resolution for in vivo biomedical optoacoustic imaging. Variation of the laser wavelength allows spectroscopy and functional imaging of blood oxygenation level based on oxygen dependent hemoglobin absorption spectra. The sophisticated combined system features 64-channel parallel acquisition of an image using a single laser pulse. The online image reconstruction based on FFT takes 100 ms and therefore allows real-time operation. The in vivo optoacoustic images acquired from human finger, arm and legs show high contrast detailed blood vessel structures, which are hard to see on the corresponding ultrasound echography images. The two techniques extract complementary information, which strongly suggests a combination (overlay) of the two techniques in a single device. In conclusion, real-time and 3D capability of optoacoustics has been demonstrated with the newly developed high contrast biomedical optoacoustic imaging systems.