Optical Coherence Tomography; applications to retinal and biofilm imaging

Optical microscopy enables us to characterize samples, whether industrial or biological, with micrometer resolution on scales of the order of mm ³ . Unlike fluorescence imaging, the most popular microscopy technique in biology in particular, label-free imaging uses the intrinsic properties of materials and other biological tissues to characterize their structure and possibly their function. The properties of light absorption, scattering (i.e. the ability of an object to change the direction of incident light) or phase shift (i.e. the ability to delay incident light) can be used and combined to finely characterize a sample.

Generally speaking, in imaging, it is often desirable to maximize the sample observation volume, while maintaining the finest possible resolution. However, axial resolution, in the direction of wave propagation, decreases very rapidly with numerical aperture, making it difficult to image large fields of view while maintaining sufficient axial resolution. In ultrasound imaging, this problem is solved by measuring the propagation time it takes for the acoustic wave to make a round trip from the source to the detector, in order to find the depth of a given structure.

Optical coherence tomography (OCT) can be described as the optical equivalent of ultrasound imaging. It allows us to image structures that backscatter light - essentially due to optical index contrast - and to measure the depth of these structures by measuring the propagation time of photons in the sample. Unfortunately, this is not so straightforward in optics due to the high speed of light, and it is necessary to use an indirect measurement thanks to the low-coherence properties of optical interference. This makes it possible to measure propagation time differences between photons scattered by the sample and those reflected by a reference mirror whose axial position is controlled. In this way, axial resolution in OCT depends solely on the coherence properties of the light and no longer on the imaging parameters. This makes it possible to maximize the sample volumes imaged.

The aim of this article is to describe the OCT technique and compare the different configurations available. In order to fully understand the issues and principles underlying this technique, general principles of microscopy and interferometry are first presented.

The various possible OCT configurations are detailed in the article, before focusing on the most widely used configuration, known as Fourier-domain OCT. The article details the formation and characteristics of the images obtained, as well as applications in retinal and bacterial biofilm imaging.