Optimization of tissue irradiation in optoacoustic imaging using a linear transducer: theory and experiments

摘要

Optoacoustic images from rather large tissue samples, such as the human extremities, the breast, or large organs, are preferably obtained in reflection mode. In the past it has been assumed that irradiating the tissue directly below or even better through an acoustic receiver results in an optimum image contrast. Our theoretical and experimental results however show that when a linear array transducer is used, this is not always true. The optimum location of irradiation depends on the depth of the tissue structures to be imaged and on various sources of image background, namely random optical absorption in the bulk tissue surrounding the region of interest, reconstruction artifacts, and acoustic backscattering. It turns out that the influence of absorption in the bulk tissue becomes minimal when irradiating close to the transducer aperture, the opposite however is the case for image artefact background. Its influence becomes minimal if the fluence in the tissue is homogeneously distributed obtained for an irradiation far away from the transducer. Echo background, which results from backscattered optoacoustic transients, additionally limits the imaging depth in reflection mode optoacoustic imaging. Therefore, the irradiation geometry when using a linear array transducer has to be adapted to the depth of the imaged structures.