To extend the depth of field (DOF) in optical-resolution photoacoustic microscopy (OR-PAM), we propose the needle-shaped beam photoacoustic microscopy (NB-PAM) via customized diffractive optical elements to extend the DOF, featuring a well-maintained beam diameter, a uniform axial intensity distribution, and negligible sidelobes. The advantage of using NB-PAM with an improved DOF has been demonstrated by both histology-like imaging of fresh slide-free organs using 266 nm laser and in vivo mouse brain vasculature imaging using 532 nm laser. Our approach provides new perspectives for slide-free intraoperative pathological imaging and various in vivo organ-level imaging applications.
Focal size and depth-of-focus (DOF) are dependent by the numerical aperture (N.A.) of the lens. Consequently, a high-resolution image inherently results in a short DOF. In order to extend the DOF of a high N.A. lens, a novel diffractive optical element is developed to generate needle-shaped beams. The DOF can be enhanced from 12μm (two Rayleigh lengths) to 120μm with a constant diameter of 1.5μm (the same as the focal size). When applied to a virtual biopsy of human skin, the needle-shaped beam can reveal the individual cells in the epidermal layer.
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