Iterative reconstruction algorithm for reduction of echo background in optoacoustic images

Recently, the influence of acoustic inhomogeneities on optoacoustic images has gained wide attention in biomedical optoacoustics. Resolution and accuracy of optoacoustic images was found to be improved when a model taking inhomogeneous speed of sound into account was included into the reconstruction algorithm. However, scattering of optoacoustic transients on inhomogeneities of the acoustic impedance was not yet paid much attention to. We show that the same inhomogeneities which are responsible for the contrast in echo ultrasound imaging reduce the contrast in optoacoustic imaging. Absorption of light below the tissue surface results in optoacoustic transients which propagate into the tissue and get backscattered from acoustic inhomogeneities. The echoes interfere with the direct optoacoustic signals and lead to a strong background if the optoacoustic image alone is reconstructed. We show that simultaneous reconstruction of an optoacoustic and an echo image allows to reduce the echo background in the optoacoustic image. For this purpose, we iteratively apply optoacoustic and echo ultrasound Fourier algorithms, together with a special regularization technique. Simulations and experimental results show the validity of the algorithm, and demonstrate the impact of this new method.