OCT Captures Optical Sections that Match Corneal Curvature

A team from Paris’ Langevin Institute, a joint unit consisting of researchers from the City of Paris Industrial Physics and Chemistry Higher Educational Institution (ESPCI) and the Centre National de la Recherche Scientifique (CNRS), have developed an instrument to improve the monitoring and detection of eye diseases, as well as diabetes, which changes nerve density in the cornea. The instrument increases the size of the field of view to a point where OCT images capture the curved layers of a person’s cornea with cell-level detail.

The curved field device provides high-resolution optical sections with an area 10× larger than the clinical devices medical professionals currently use for corneal diagnostics. This combination of high resolution and expanded viewing area makes it easier to count localized cells and nerves, vital to making diagnoses and decreasing the potential for missing a diseased area or area where doctors could locate indicators signifying disease.

By delivering more accurate evaluations of corneal health, the device has potential to improve surgical predictions and outcomes, said Kristina Irsch, a physicist in the orthoptics program at Sorbonne University and coauthor of the study that introduces the instrument.

A high-resolution view and large imaging area are of particular importance in cataract procedures. Cataract surgery decreases the number of present endothelial cells. If that number falls too low, a person may require a corneal transplant.

“Doctors frequently perform endothelial cell counting before cataract surgery to ensure there will be enough endothelial cells to preserve the cornea after surgery,” Irsch said.

Traditional imaging approaches acquire a flat slice (pink) that crosses through several corneal layers at once, limiting the field of view (left). The new curved-field OCT approach matches the curvature of the cornea to provide a larger imaging area (right).
Courtesy of Viacheslav Mazlin, The Langevin Institute.

The new technique the instrument makes possible is based on OCT, an existing imaging technique that can in application acquire cross-sectional images of the retina, using interference between light from a sample and light from a mirror located in an additional optical reference arm. The process relies on this interference, and a 2D camera, to capture thin optical slices that are parallel to the surface of the eye.

Using this method to acquire a flat slice across the cornea, which is curved, would slice through multiple corneal layers at once, limiting the field of view.

As a solution, the researchers replaced the flat mirror in the additional optical reference arm with a curved lens. This optically flattened the cornea, capturing optical sections that matched the cornea’s curvature.

In this full-field OCT configuration, a 2D camera is able to simultaneously capture all pixels in the viewing area, making the process immune to artifacts that may arise with other OCT configurations.

“The ability to use full-field and curved-field OCT to image eyes in people — where the eye is constantly moving — became possible recently, thanks to the development of advanced cameras with higher speeds and better light detection capabilities,” said Claude Boccara, research team leader from the Langevin Institute.

The team first tested its instrument on a flat target and model eye, and then imaged a healthy human cornea while in motion by centering the instrument over the corneal apex, which is the point of maximum curvature. At the same time, researchers placed the device on a motorized xyz translation stage they could control with a joystick.

The alignment took several minutes. In the setup, the team captured an image in less than a second. The team ultimately collected nerve and endothelial cell slices with a viewing area unprecedented in size — more than 1 sq mm.

“Our device is universal and may prove useful for studying any type of transparent sample exhibiting a curved structure,” said Viacheslav Mazlin, corresponding author of the study from the Langevin Institute.

As they work to incorporate features to improve the experience of the clinician user, the researchers say the device is ready for use in clinical research. These features include automatic cell counting and simplified aligning procedures. The team is also planning to further increase the viewing area.

The research was published in Optica (www.doi.org/10.1364/OPTICA.396949).