A backward-mode optical-resolution photoacoustic microscope for 3D imaging using a planar Fabry-Pérot sensor

Optical-resolution photoacoustic microscopy (OR-PAM) combines high spatial resolution and strong absorption-based contrast in tissue, which has enabled structural and spectroscopic imaging of endogenous chromophores, primarily hemoglobin. This makes OR-PAM an important tool for preclinical vascular research. Conventional piezoelectric ultrasound transducers often need to be placed far away from the signal source due to their opacity, which results in reduced acoustic sensitivity. Optical ultrasound sensors are an alternative as their transparency allows them to be positioned close to the sample for minimal source-detector distances. In this work, a backward-mode OR-PAM system based on a planar Fabry-Pérot ultrasound sensor and coaxially aligned excitation and interrogation beams was developed. Two 3D imaging modes, using raster-scanning for enhanced image quality or continuous-scanning for fast imaging, were implemented and tested on a leaf skeleton phantom. In fast imaging mode, a scan-rate of 100,000 A-lines/s could be achieved. In raster-scanning mode, 3D images of a zebrafish embryo were acquired in vivo. The transparency of the FP sensor in the visible and near-infrared wavelength region makes it potentially suitable for combined functional and molecular imaging using OR-PAM and multi-photon fluorescence microscopy.