Imaging fine structures of the human trabecular meshwork in vivo using a custom design goniolens and OCT gonioscopy

Abstract

The trabecular meshwork (TM), located within the iridocorneal angle, is a target for many glaucoma treatments aimed at controlling intraocular pressure. However, structural variations between individuals are poorly understood. We propose a newly designed gonioscopic lens optimized for high-resolution imaging to image fine structures of the human TM in vivo. The body of the new lens is index-matched to the human cornea and includes a choice of two gonioscopic mirrors (59° and 63°) and matching air-spaced doublets placed on the anterior surface of the goniolens. The new design allows a diffraction-limited image plane at the iridocorneal angle structures. The goniolens design was built and then placed on the subjectś eyes coupled to the cornea with goniogel and a 3D adjustable mount. Images were obtained using a commercially available OCT device (Heidelberg Spectralis). The optical resolution was measured in a model eye as 40.32 and 45.25 cy/mm respectively for each mirror angle. In humans, dense OCT scans with minimum spacing oriented tangential to the iris and ICA were performed on 7 healthy subjects (23-73 yrs). The TM was successfully imaged in all subjects. The custom goniolens improved the contrast of the uveoscleral meshwork structures and corneoscleral meshwork revealing limbus parallel striations, not visible with previous goniolens designs. Transverse OCT images were constructed along the segmentation line, providing an enface image of the TM structures including corneoscleral beams, previously only imaged in vivo using custom adaptive optics systems.

Fig. 1.

Schematic table of design changes and performance of the goniolens modeled using Zemax Optic Studio optical design software for a 6.4 mm incident beam. Ray tracing of the design for each sequential step for improvement in shown in row 1. Performance after each sequential change is described by the spot diagram at the image plane, shown in row 2, the theoretically calculated Airy radius in row 3 and the modeled Root Mean Square (RMS) and geometric radii in rows 4 and 5.

Fig. 2.

Schematic of new two-mirror goniolens design optically optimized for high-resolution imaging. The body of the goniolens is index matched to the cornea to reduce aberrations. Optimized air-spaced doublets for each mirror are used to focus light at the TM.

Fig. 3.

Optical performance of the index matched fluid filled goniolens design, for the 59° mirror, over a 1 mm2 field at the image plane. Left: Optical layout and ray tracing of the goniolens design for the center field. Right: Spot diagrams at the different field locations, as indicated in the footprint diagram in the black box. Airy radius is 9.349 µm as indicated by the black circle in each spot diagram. RMS radii for the 5 field locations are 1.352 µm (0, 0), 1.578 µm (1, 0), 3.701 µm (0, 1), 3.624 µm (0, −1) and 1.578 µm (−1, 0).

Fig. 4.

A) Exploded view of the MgF₂ goniolens manufacturing process showing each one of its parts separately on the left and the fully assembled goniolens on the right. B) Design of the 3D printed goniolens shell (first and second image) and fully assembled model of the goniolens (third image), and picture, right, of a fabricated 3D metal printed goniolens lens (not blackened to show interior) with the mounted air-spaced doublet focusing lens for the 63° mirror.

Fig. 5.

Gonioscopic OCT enface images of a USAF target placed in a model eye at the iridocorneal angle with a) the modified clinical goniolens providing a resolution of 35.92 cy/mm, and the 3D printed goniolens b) at 59° providing a resolution of 40.32 cy/mm, and c) at 63° with a resolution of 45.25 cy/mm. Target is left-right inverted due to the reflection at the goniolens mirror. Blue arrows indicate the achievable resolution.

Fig. 6.

Images of S1 with OCT gonioscopy and the MgF₂ custom goniolens using the 59° mirror aimed at the superior-nasal angle. A) Cropped Scanning Laser Ophthalmoscope image acquired by the OCT device at the start of the OCT scan, indicating the location and size of the OCT volume with a green square and a green arrow indicating the location and direction of a b-scan. B) Cropped gonioscopic b-scan of the TM. The image overlay indicates the direction of the a-scans in blue and the b-scans in green. The segmentation along the interface of the anterior chamber and the TM is shown in red on the image and the 3D overlay. Segmentation is used to construct the transverse enface image. C) Cropped gonioscopic b-scan focusing on the layer-like structures observed in the TM. D) Constructed enface image from the segmented OCT volume (as seen by looking down on the red line) focused on the same area seen in panel B). Red arrows indicate lamellar structures within the TM.

Fig. 7.

Gonioscopic OCT images with the 3D printed goniolens for subject 1 at the inferior-temporal angle. A) SLO image through the goniolens. The green box indicates the location and size of the OCT volume, and the green arrow indicates the orientation of the b-scan. B) SLO image overlapped with the constructed enface image at a depth of 13 µm from the anterior TM. Blue box indicates the location of the expanded and cropped enface image seen in panel C). D) Gonioscopic OCT b-scan oriented tangential to the iris and ICA as seen by the green arrow in A). Yellow box indicates the region that has been expanded in panel E). Red arrows point at lamellar structures.

Fig. 8.

Gonioscopic OCT images with the 3D printed goniolens for subject 4. A) SLO image through the goniolens. The green box indicates the location and size of the OCT volume, and the green arrow indicates the orientation of the b-scan. B) SLO image overlapped with the constructed enface image at a depth of 13 µm from the anterior TM. Blue box indicates the location of the expanded and cropped enface image seen in panel C). D) Gonioscopic OCT b-scan oriented tangential to the iris and ICA as seen by the green arrow in A). Yellow box indicates the region that has been expanded in panel E). Red arrows point at lamellar structures.