Abstract:

Photoacoustic microscopy (PAM) is a promising imaging modality because it is able to reveal optical absorption contrast in high resolution on the order of a micrometer. It can be applied in an endoscopic approach by implementing PAM into a miniature probe, termed as photoacoustic endoscopy (PAE). Here we develop a miniature focus-adjustable PAE (FA-PAE) probe characterized by both high resolution (in micrometers) and large depth of focus (DOF) via a novel optomechanical design for focus adjustment. To realize high resolution and large DOF in a miniature probe, a 2-mm plano-convex lens is specially adopted, and the mechanical translation of a single-mode fiber is meticulously designed to allow the use of multi-focus image fusion (MIF) for extended DOF. Compared with existing PAE probes, our FA-PAE probe achieves high resolution of 3-5 mum within unprecedentedly large DOF of >3.2 mm, more than 27 times the DOF of the probe without performing focus adjustment for MIF. The superior performance is demonstrated by imaging both phantoms and animals including mice and zebrafishes in vivo. Our work opens new perspectives for PAE biomedical applications.

Fig. 1. Imaging of a leaf phantom with rotary scanning of leaves. (a) Schematic of the imaging based on rotary scanning of the leaves. 3D rendering images (b) and photoacoustic YZ MAP images (c) of the MIF and SF images. 3D videos of the MIF (Video 1) and SF (Video 2) images are available. (d) 2D XY slices at around the center of Z in (b). (e) Comparison of the 1D profiles along the dashed lines in (d). Scale bars: 500μm . MIF, multi-focus image fusion; SF, single focus.

Fig. 2. In vivo imaging of mouse eye blood vessels. (a) Photoacoustic XY MAP images of the MIF and SF images. (b) The depth-encoded image of the MIF image in (a). 3D videos of the MIF (Video 3) and SF (Video 4) images are available. ΔZ represents the distance from the focal plane of the SF image (fa=3.7 mm in this case). (c,d) Comparison of the zoom in images of the dashed boxes in (a). (c) For ΔZ = −0.6 mm; (d) for ΔZ=−0.5 mm. (a) and (b) share the same scale bar in (a). MIF, multi-focus image fusion; SF, single focus.

DOI: 10.1109/TMI.2023.3250517