Photoacoustic signal enhancement of heterogeneously distributed optical absorbers by using speckle illumination

We investigated application of speckle illumination to a heterogeneous sample where small optical absorbers are sparsely distributed. Although such sample is difficult to visualize by homogeneous illumination, speckle illumination has advantages. For example, speckle light has a heterogeneous and granular intensity distribution, which is similar to that of optical absorbers in a heterogeneous sample. In addition, speckle light has higher intensity spots compared to homogeneous illumination. These characteristics can be used for signal enhancement compared to homogeneous illumination. Moreover, speckle illumination enables us to perform heterogeneity evaluation of the heterogeneous sample since the signal is varied depending on spatial overlap between the speckle light and the optical absorbers. In this research, we used acoustic resolution photoacoustic microscope (AR- PAM) to investigate application of speckle illumination to a heterogeneous sample. In the AR-PAM system, speckle light is generated by using a diffuser. As heterogeneous samples, black-dyed microspheres and red blood cells (RBCs) in Matrigel were used. By illuminating speckle light to the sample and changing the speckle pattern, variation of the signal was observed. In the measurement of the microspheres, we confirmed that some speckle patterns provide higher signals compared to homogeneous illumination. In addition, we also confirmed that normalized signal variance is depended on concentration of RBCs. This can be used as heterogeneity evaluation.