Characterizing close-focus lenses for microendoscopy

Abstract

Microendoscopes are commonly used in small lumens in the body, for which a focus near to the distal tip and ability to operate in an aqueous environment are paramount for navigation and disease detection. Commercially available distal optic systems below 1mm in diameter are severely limited, and custom micro lenses are generally very expensive. Gradient index of refraction (GRIN) singlets are available in small diameters but have limited optical performance adjustability. Three-dimensional (3D) printed monolithic optical systems are an emerging option that may be suitable for enabling high performance, close-focus imaging. In this manuscript, we compared the optical performance of three custom distal optic systems; a custom-pitch GRIN singlet, 3D-printed monolithic doublet, and 3D-printed monolithic triplet, with a nominal working distance (WD) of 1.5mm, 0.5mm and 0.4mm in 0.9% saline. These short WDs are ideal for microendoscopy in collapsed or flushed lumens such as pancreatic duct or fallopian tube. The GRIN singlet had performance limited only by the fiber bundle relay over 0.9mm to 1.6 mm depth of field (DOF). The 3D printed doublet was able to achieve a comparable DOF of 0.71mm, while the 3D printed triplet suffered the most limited DOF of 0.55mm. 3D printing enables flexible design of monolithic multi-element systems with aspheric surfaces of very short WDs and relative ease of integration.

Keywords: 3D printing; Endoscopy; Lens design; microendoscope; multi-modal imaging.

Fig 1

Distal optic layout and modulation transfer functions for (a) GRIN Singlet, (b) 3D printed Doublet, and (c) 3D printed Triplet. The blue lines represent the on-axis field, and the golden rays represent the off-axis field (sagittal rays are represented by the dotted lines, tangential rays by the solid lines). The vertical line on the MTF plots signifies the frequency cutoff imposed by the fiber bundle’s core-to-core spacing (3.3μm) converted to line pairs(lp) per mm (151.2lp/mm).

Fig 2

Transmittance of (left) the pre-pyrolysis LSR and (right) post-pyrolysis LSR in the visible spectrum

Fig 3

Schematic of the 2PP-enabled 3D-printing setup and printing process

Fig 4

Photographs and electron micrographs of 3D printed doublet (a, b, respectively) and triplet (c, d, respectively). Scale bar is 200μm.

Fig 5

Diagram of Test Setup

Fig 6

Images of the 26 lp/mm resolution pattern taken at the lens-specific WD for the a) GRIN singlet, b) 3D doublet, and c) 3D triplet.

Fig 7

Diagram depicting how a lens with a smaller AFOV but larger WD can result in a larger FOV than a lens with a larger AFOV and shorter WD.

Fig 8

Images of the USAF Target taken at the corresponding WD’s for the a) GRIN singlet showing Group 5, b) 3D printed doublet showing Group 5, and c) 3D-printed triplet showing Group 4 Elements 4, 5, and 6.

Fig 9

Images of a histological slide of human fallopian tissue taken in a microscope setup with the a) GRIN singlet b) 3D printed doublet and c) 3D printed triplet.