Chromatic Swept-Source Corneal Confocal Microscopy

Purpose : Methods for the visual or quantitative exploration of the cellular morphology of the cornea based on inherent three-dimensional features open up completely new diagnostic possibilities. However, previous approaches to confocal microscopy of the cornea using mechanical 3D scanning strategies are slow and thus only partially suitable for in vivo investigations. Addressing these shortcomings, we introduce a novel scanning scheme for rapid three-dimensional confocal microscopy to study the cellular morphology of the cornea in vivo.

Methods : We induce longitudinal chromatic aberration using specifically designed high-dispersion optics and exploit it by means of a swept-source laser. The laser operates at 1060 nm with a sweep range of 70 nm and sweep rates of up to 400 kHz. This way, the slowest mechanical scanner is replaced by an optical scanner, providing an extremely fast method for focal positioning in the tissue. But, the rather small wavelength range requires a highly dispersive material. We identified As40Se60 as promising. The optical microscope setup was simulated in OpticStudio (Zemax, An Ansys Company, Kirkland, USA).
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Results : We were not able to achieve the required chromatic aberration with a single lens. However, if a non-magnifying telescope consisting of two oppositely oriented objectives systems, both containing two As40Se60 lenses and one N-BK7 catalog lens, is inserted in the microscope, a chromatic aberration of 143 µm is feasible. But, the material’s high refractive index leads to strong Fresnel reflections, reducing the initial intensity to about 1 % after passing the optics twice. Hence, a dedicated anti-reflective coating is crucial. Then, a total transmittance of 79 % is achievable.

Conclusions : Utilizing a dedicated laser line scanning will enable the display of sectional images in real-time. These are comparable to slit lamp images but with cellular resolution. Furthermore, with an x-y scanning protocol, corneal volumes can be acquired in less than one second. The project thus paves the way for cellular 3D slit-lamp microscopy and, in perspective, provides ophthalmologists with further methods for dedicated diagnostics of ocular surface diseases. This paper summarizes the concept of this novel approach and presents intermediate results.