The calibration dependencies of the optoacoustic (OA) transformation efficiency on tissue temperature are obtained for the application in OA temperature monitoring during thermal therapies. Accurate measurement of the OA signal amplitude versus temperature is performed in different ex vivo tissues in the temperature range 25°C to 80°C. The investigated tissues were selected to represent different structural components: chicken breast (skeletal muscle), porcine lard (fatty tissue), and porcine liver (richly perfused tissue). Backward mode of the OA signal detection and a narrow probe laser beam were used in the experiments to avoid the influence of changes in light scattering with tissue coagulation on the OA signal amplitude. Measurements were performed in heating and cooling regimes. Characteristic behavior of the OA signal amplitude temperature dependences in different temperature ranges were described in terms of changes in different structural components of the tissue samples. The accuracy of temperature reconstruction from the obtained calibration dependencies for the investigated tissue types is evaluated.
Optoacoustic (OA) imaging is based on the generation of thermoelastic stress waves by heating an object in an optically heterogeneous medium with a short laser pulse. The stress waves contain information on the distribution of structures with enhanced optical absorption that can be used for early cancer diagnostics. This technique has already been applied in-vivo for breast cancer diagnostics and yielded higher contrast of obtained images than that of X-ray or
ultrasonic images. The resolution was comparable with that yielded by ultrasonic imaging. Therefore, OA imaging is a very promising technique and it is being rapidly developed. Research in the area is now mostly targeted to the development of OA wave detection systems and image reconstruction algorithms. In this work a new design of receiving array transducer, that allows to enhance image resolution is proposed. The array consists of 64 focused piezo-elements made of PVDF slabs imposed on a spherical surface. Resolution yielded by the array in different directions is determined. Several tissue irradiation geometries and laser wavelengths are considered for optimization of the OA image contrast. Obtained results are used for maximum imaging depth studies. All the investigations include both numerical modelling and experiment.
Theoretical and experimental investigations of point spread function of focused 32 elements array transducer for laser optoacoustic (OA) tomography have been carried out. The elements were imposed in the radial plane of a cylindrical surface. Specially developed software was used for numerical modeling the problem of OA tomography.
Diode-pumped Q-switched Nd:YAG laser was employed for thermo-optical excitation of probe acoustic transients. The correspondence between numerically calculated and measured temporal profiles of acoustic transients detected by a single transducer over wide area around it focal zone was demonstrated. The maps of spatial sensitivity for transducers with aperture angles 30° and 60° were determined experimentally and theoretically. The possibility of localization of
the array sensitivity area at the beam waist in the plane perpendicular to the imaging plane has been shown. Back projection algorithm was employed for image reconstruction based on experimentally obtained OA transients from point sources located at different distances from the array. The size of the images allowed to determine transverse and longitudinal resolutions in the imaging plane.