Optical coherence tomography – additional contrast mechanisms, future trends, and cutting‐edge technological developments

In ophthalmology, structural OCT measurements are now used routinely in the clinic to help with the diagnosis and management of anterior‐segment and retinal disorders. More recently, the development of additional or complementary contrast mechanisms, as well as novel approaches targeted at enhancing resolution, have opened up new opportunities for ophthalmic OCT applications. For example, different algorithms have been proposed to contrast the retinal and choroidal vasculature and analyze blood flow, which extend clinical spectral‐domain OCT (SD‐OCT) from structural to functional imaging capabilities. Such OCT‐based angiography (OCT‐A) relies on sequential capture of SD‐OCT images at the same tissue region and the subsequent analysis of temporal variations in the OCT signal. OCT‐A algorithms differ in this analysis, specifically in how they create contrast between flow and static tissue (i.e., how they distinguish vascular from other structures). Common is the assumption that the change in the OCT signal over time is mainly due to the motion of backscattering particles, such as erythrocytes. With so‐called polarization‐sensitive OCT (PS‐OCT), polarized light has also been employed to create tissue‐specific contrast. PS‐OCT has shown great potential for functional and quantitative imaging and exploits the fact that various tissues, particularly birefringent structures in the cornea and retina (e.g., corneal collagen fibrils and the retinal nerve fiber layer), can change the state of polarization of incident light. Other technological developments have focused on enhancing OCT's resolution. Note that while OCT enables high‐resolution cross‐sectional views (where the axial resolution is determined by the spectral bandwidth of the light source), it has a limited lateral or en face resolution. Integrating adaptive optics (AO) is one method to improve the lateral resolution of retinal OCT. This lecture will discuss recent technological advances and cutting‐edge developments, leading to additional contrast mechanisms and functional extensions (e.g., OCT‐A, PS‐OCT) or improved lateral resolution (e.g., AO‐OCT).