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Curved-field OCT offers larger-scale imaging of the cornea

28 Jul 2020

Langevin Institute platform enables more precise cell and nerve counts to tackle disease.

A project at the Langevin Institute, Paris, has developed an OCT platform able to image curved tissue sections more effectively than conventional versions of the technology.

The curved-field OCT (CF-OCT) approach could assist clinicians to make precise observations of cells and nerves in the human cornea, assisting with the monitoring and diagnosis of disease. The work was published in Optica.

Conventional OCT captures flat optical sections of the target sample, so if that sample is in fact a curved area of tissue the effective view of each layer is reduced. In the cornea, each flat OCT slice also takes in several different tissue layers at once, limiting the usefulness of the approach.

To achieve curved optical sectioning instead, the Langevin Institute team replaced the flat mirror in the optical reference arm of conventional time-domain OCT architecture with a curved optical lens. This allowed the coherence plane of the platform to also be a curve, and meant that the optical sections could become arbitrary curvatures in the cornea.

Although a conceptually simple approach, ocular and head movements of a patient being examined with OCT mean that keeping the apex of the curved mirror matched to the apex of the patient's cornea is challenging.

The team tackled this problem by mounting the interferometer on a 3-axis motorized stage controlled by a joystick. When imaging the curved subbasal nerve plexus and endothelium of the cornea, fringe patterns generated from light returning from the reflective corneal surfaces themselves were monitored to indicate misalignment, and then compensated for by shifting the interferometer.

Cell-level resolution and large viewing area

After testing the device on a flat target and a model eye, the project used it to image the cornea of a healthy person. It found that aligning the platform using the motorized translation stage and joystick took only a few minutes, after which image capture was completed in 3.5 microseconds.

Significantly, the device was able to successfully capture nerve and endothelial cell slices of the cornea with an unprecedented viewing area larger than 1 square millimeter, according to the Institute.

"CF-OCT revealed the 2 to 4-micron-thick corneal nerves within a 1.13 x 1.13 millimeter field of view, which is about 10x larger in area than clinical in vivo confocal microscopy," commented the team in its published paper.

Larger FOV is beneficial in ophthalmology as it reduces the chance of missing an area affected by corneal disease. It also allows more precise counts of the cells in the cornea endothelium, a standard part of assessment for cataract surgery carried out to ensure that the cornea can recover after the procedure.

At present, changing the curvature of optical sectioning requires changing the lens in the reference arm and realigning, but the project will now investigate using a combination of static and tunable lenses to provide more flexible curvature adjustment.

The platform might then be applied to areas outside ophthalmology, allowing non-contact exploration of other in vivo and ex vivo human and animal tissues exhibiting a curved structure. But the current set-up is already suitable for use in clinical research, according to the project.

"The cell-level resolution and large viewing area available from our instrument are ideal for monitoring corneal diseases like endothelial dysfunction and general diabetic conditions, understanding their evolution at the biological scale, and quantitatively evaluating the efficacy of novel treatment strategies," commented Viacheslav Mazlin of the Langevin Institute.

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