Optical Coherence Tomography (OCT) - Technology and biomedical applications

Optical coherence tomography (OCT) is an optical imaging technique used primarily in the biomedical field. OCT has been commercially available since 1996 for retinal imaging, just a few years after its first laboratory demonstration. It is now a standard and indispensable technique in ophthalmology. OCT is beginning to be used in other areas of medicine, notably in interventional cardiology for the treatment of coronary artery disease. By providing non-invasive, micrometer-scale images, OCT performs an "optical biopsy", making it possible to obtain information on biological tissues from imaging, instead of the conventional biopsy followed by histopathological examination. OCT is thus of interest in gastroenterology for the detection of incipient cancers, and in dermatology to improve the diagnosis of skin lesions.

Often described as the optical analogue to ultrasound, OCT probes biological tissues with light instead of ultrasound, and maps their reflectivity in depth. In ultrasound, tissue structures are localized by measuring the travel time of echoes. Because of the propagation speed of light, around 1.5.10 ⁵ times greater than that of sound in tissue, such a measurement is not feasible in optics. An indirect measurement method is implemented in OCT, based on interferometry in weakly coherent light. After reflection by the tissue, the light beam interferes with a reference beam from the same light source. From the interferometric signal detected, we gain access to the depth reflectivity profile of the tissue, known as an "A-scan" by analogy with ultrasound. Based on several adjacent A-scans, 2 or even 3-dimensional images can be obtained. Reflective structures located at different depths can be distinguished, if their distances are greater than the coherence length of the light detected. The lower the temporal coherence of the light, the better the depth resolution in OCT. In practice, this resolution is between 1 and 20 µm. Penetration depth in biological tissue is mainly limited by light scattering. In highly scattering media, such as skin, it is around 1 mm. This penetration is superior to that of other high-resolution optical imaging techniques such as confocal microscopy.

Since its emergence in the early 1990s, OCT has attracted growing scientific activity, with around 85,000 publications in 30 years, increasingly focusing on medical applications (half of all publications in 2021). Some 50 companies (3/4 of them startups) are (or have been) involved in commercializing OCT devices, almost half of them designed for ophthalmology. OCT performance has improved rapidly and spectacularly, both in terms of image quality (resolution, field of view, contrast) and image acquisition speed. Various extensions to OCT have...