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Image reconstruction in 3D optoacoustic tomography system with hemispherical transducer array

Image reconstruction algorithm for three-dimensional laser optoacoustic imaging system (LOIS) is proposed and tested in computer-simulating experiments. It was assumed that acoustic transducers were evenly distributed with 3-degree interval along polar and azimuthal angles on the surface of a 100-mm diameter hemisphere, all optoacoustic sources were located inside the hemisphere of a slightly smaller diameter. At the first stage of calculations, the initial data in a form of spherical surface integrals were converted into the plane surface integrals. We deduced an approximate analytical formula for this conversion. At the second stage, the three-dimensional Radon transform algorithm was applied for reconstruction of optoacoustic sources. Three- dimensional images of computer-simulated spherical objects were generated. Quality of reconstructed images was evaluated with the following four criteria: The noise level on the entire tomogram, the step-transfer function, the loss-of-contrast function and the contrast-dimension relation. These quality criteria may be employed to characterize any tomography systems regardless of the type of technology employed. Image analysis demonstrated that the artifact level associated with data conversion from spherical into planar coordinates did not exceed 10%. A 1-mm spatial resolution could be obtained with the proposed algorithm, provided the signal-to-noise ratio equals approximately 3 on the tomogram. Very small (0.5-mm diameter) and small (3-mm diameter) spherical tumors could be revealed on optoacoustic tomograms if their contrast equals at least 6 and 0.3, respectively.