Weighted synthetic aperture focusing for optoacoustic microscopy with scanning illumination and detection

Scanning optoacoustic microscopy operates in two distinct regimes optical resolution microscopy relies on a focused illumination and acoustic resolution microscopy that forms images by focusing the received acoustic field. Recently, a number of approaches have been proposed that combine those two modes of operation to create a highly scalable technique that can image at multiple penetration scales by gradually exchanging microscopic optical resolution in superficial tissues with ultrasonic resolution at diffuse (macroscopic) depths. However, scanning microscopy schemes commonly employ acquisition geometries that impede the use of synthetic aperture techniques to achieve meaningful images due to non-stationary illumination patterns and strong non-uniformity of the excitation light field. Here we present a Weighted Synthetic Aperture Focusing Technique (W-SAFT) as a universal framework that effectively accounts for the non-uniform distribution of both the excitation light field and spatial sensitivity field of the detector. As a result, W-SAFT maintains optical resolution performance at superficial depths while improving the acoustic resolving capacity for deeper tissues. The dynamic range of the optoacoustic data is compressed using a general fluence decay term applied to the W-SAFT operator, allowing a more uniform visualization of the entire imaged volume. Our three-dimensional algorithm makes use of the sample's surface to account for the heterogeneity produced when scanning a finite-size light beam. We tested a GPU implementation of W-SAFT with numerical simulations and showcase its performance on experimental data acquired from targets embedded in tissue mimicking phantoms.