Two-dimensional opto-acoustic tomography transducer array and image reconstruction algorithm

Opto-acoustic tomography (OAT) utilizes laser pulses to create acoustic sources in tissue and wide-band detection of pressure profiles for the image reconstruction. A new laser optoacoustic imaging system (LOIS) for breast cancer detection and two-dimensional visualization is described. A Q-switched Nd:YAG laser was used for generation of optoacoustic profiles in phantoms and tissues in vitro. Acoustic pulses were detected by a 12 element linear array of piezoelectric transducers. Each transducer was made of 0.5-mm thick PVDF slabs with dimensions of 4.3 mm x 12.5 mm. Signal-to-noise ratio was calculated and the sensitivity of optoacoustic system was evaluated. The axial (in-depth) resolution and the lateral resolution of the system were determined. The axial resolution of the receiving array was limited by its frequency band and was estimated to be approximately 1 mm. The lateral resolution was about 2.5 times the lateral dimension of the "tumor" and defined by the finite aperture of the array and relatively large size of the single transducer. The time of full data acquisition was limited by the time allowed in clinical procedure of about 5-10 minutes. The procedure of signal processing is described. It includes high-pass signal filtering, compensation for acoustic diffraction, detection of the irradiated surface position and rejection of the reverberating signal. Radial back-projection algorithm for image reconstruction was developed and included in the computer code. Two-dimensional opto-acoustic images of simulated spheres and objects inside tissue phantoms are presented. The contrast of these images and limits of detection and localization of deeply embedded tumors are discussed.