Imaging of layered structures in biological tissues with opto-acoustic front surface transducer

Optoacoustic tomography is a promising technique for imaging skin layered structures and vascular and pigmented lesions in vivo. The imaging of skin is complicated by necessity to perform laser irradiation and acoustic detection from the same site at the surface. We designed an opto-acoustic transducer incorporating a fiber-optic light delivery system and a LiNbO3 piezoelectric detector in one device. The results of test experiments in phantoms, chicken cockscomb, and human hand yielded the following parameters of the opto-acoustic transducer: (1) sensitivity of 0.98 V/bar, (2) temporal resolution of acoustic detection of 5 ns. This fast response time allows one to achieve an in-depth resolution of 10-15 mu m. A small size of the detector provides lateral resolution of 200 mu m Feasibility studies demonstrated that the current design of the optoacoustic transducer permits monitoring of the light absorbing heterogeneities at the depth up to 4 mm. The principle scheme of the opto-acoustic transducer design, theoretical background and experimental testing is presented. Theoretical model of the wide-band ultrasonic detection is developed The principles of the opto-acoustic image reconstruction under conditions of significant diffraction of acoustic wave are described. A basic algorithm for reconstruction of 3-D images from the laser-induced acoustic waves recorded in backward mode is also presented.