Large depth of field and high-resolution optical coherence tomography angiography with a needle-shaped beam

Optical coherence tomography angiography (OCTA) has emerged as a highly competitive technique for visualizing blood perfusion without the need for exogenous contrast agents. However, for high-resolution optical imaging, the pursuit of diffraction-limited resolution is often accompanied by a trade- off in the form of a reduced depth of field (DOF). This constraint impedes the rapid acquisition of high-resolution images of samples with irregular surfaces. Achieving these goals usually involves laborious axial scanning at multiple planes and complicated image processing procedures, or using non-diffracting beams such as Bessel or Airy beams to simultaneously achieve large DOF and high resolution, but the imaging quality is usually limited by the severe sidelobes and low efficiency of the beams. To solve this problem, we developed a needle-shaped beam (NB) through a spatially multiplexed phase pattern that creates many axially closely spaced foci. The phase mask was created on a diffractive optical element (DOE) which was fabricated on a 500-mu m- thick fused silica substrate via four rounds of lithography. The DOE was introduced into the OCTA system and converted the conventional Gaussian beam (GB) OCTA system into the NB-OCTA system, which has both long DOF and high resolution, maintaining 8 mu m lateral resolution over a depth range of 620 mu m, allowing real-time, nonstitched, large-field (6.4x4 mm) imaging of samples with uneven surfaces. Imaging of mouse brains with natural curvature was demonstrated in our work, compared to GB-OCTA, NB-OCTA captures the high- resolution distribution of blood vessels within a larger depth range.