Model-based optical resolution optoacoustic microscopy

Model-based reconstruction techniques have been successfully implemented in optoacoustic tomography and acoustic-resolution microscopy to retrieve improved image quality over delay-and-sum methods. In scanning optical resolution optoacoustic microscopy (OR-OAM), no reconstruction methods are employed while post-processing is usually limited to basic frequency filtering and envelope extraction with the Hilbert transform. This results in considerable deterioration of the acoustically-determined resolution in the axial (depth) direction. In addition, when OR-OAM is used for transcranial mouse brain imaging, the skull strongly attenuates high ultrasonic frequencies and induces reverberations, which need to be accounted for during the reconstruction process to avoid image distortions and further deterioration of the axial resolution. Here we show a basic implementation of a model-based reconstruction to increase the axial resolution in OR-OAM. The model matrix is calculated using Field II for free field conditions, taking into account the shape and bandwidth of the spherically focused transducer. Assuming the confinement of the optoacoustic sources within the limits of the optical focus, one may calculate the model matrix by assuming a line source of small absorbing spheres equal in size to the optical beam. In addition, a plate model used in the recently reported virtual-craniotomy deconvolution algorithm is incorporated into the model matrix to tackle the transcranial acoustic transmission problem. The free-field model-based results are compared against the plate model for transcranial brain data obtained in-vivo.