ABSTRACT
The choroid is the most vascular tissue in the eye and it plays an important role in the pathophysiology of various common chorioretinal diseases such as central serous retinopathy, age-related macular degeneration and degenerative myopia. Quantitative assessment of the choroid has been quite challenging with traditional imaging modalities such as indocyanine green angiography and ultrasonography due to limited resolution and repeatability. With the advent of optical coherence tomography (OCT) technology, detailed visualization of the choroid in vivo is now possible. Measurements of choroidal thickness have also enabled new directions in research to study normal and pathological processes within the choroid. The aim of the present study is to review the current literature on choroidal imaging using OCT.
Introduction
The choroid is a pigmented vascular tissue that was first histologically examined in the 17th century, and to date has been studied by various imaging methods. It extends from the ora serrata anteriorly to the optic nerve head posteriorly. According to histopathological examination, the choroid has a mean thickness of 0.15 mm anteriorly and 0.22 mm posteriorly; anatomically it forms the posterior portion of the uveal tract, which continues anteriorly with the ciliary body and the iris. From retina to sclera, the choroid comprises Bruch’s membrane, the choriocapillaris, the medium diameter choroidal vessels, and the large diameter choroidal vessels.1A structurally and functionally healthy choroid is essential for retinal function. The central retinal artery nourishes the inner two-thirds of the choroid, whereas the choroidal vasculature nourishes the outer third. Abnormal choroidal circulation leads to retinal photoreceptor dysfunction and death.2 It has been shown that the choroid has a vital role in the pathophysiology of many diseases such as central serous retinopathy (CSR), age-related macular degeneration (AMD), pathological myopia and Vogt-Koyanagi-Harada (VKH) disease.3,4,5,6 Thus, the clear and accurate identification of choroidal changes will allow the proper assessment of many posterior segment diseases. However, unlike the retina, choroidal structures are not found in distinct, ordered layers and they lack contrasting reflective properties; therefore, for many years it was not possible to examine the choroid in as much detail as the retina.
Conclusion
The EDI-OCT technique used with SD-OCT devices enables in vivo cross-sectional visualization of the choroid that is simple, reproducible, high-resolution and noninvasive, and has provided a better understanding of the choroidal changes that occur in many pathologies. However, this technique also has some limitations. One of the major drawbacks of the technique is that software allowing automated measurement of the choroid has not yet been developed. Manual measurement takes time and can result in inaccuracies. Eye tracking systems are necessary on OCT devices to ensure that the simultaneous images are taken at the same position. Another shortcoming of EDI-OCT is that in certain eyes, especially those with media opacities, relatively clear images of the retina can be obtained but the choroidoscleral junction cannot be clearly visualized.The continuing search for new choroidal imaging technology has led to both software and hardware innovations like swept source OCT, Doppler OCT, long wavelength SD-OCT and en face OCT, and will remain a topic of interest for many years to come.Peer-review: Externally and internally peer-reviewed.Authorship ContributionsConcept: Taha Sezer, Muhammet Altınışık, İbrahim Arif Koytak, Mehmet Hakan Özdemir, Design: Taha Sezer, Muhammet Altınışık, İbrahim Arif Koytak, Mehmet Hakan Özdemir, Data Collection or Processing: Taha Sezer, Muhammet Altınışık, Analysis or Interpretation: Mehmet Hakan Özdemir, İbrahim Arif Koytak, Literature Search: Taha Sezer, Muhammet Altınışık, Mehmet Hakan Özdemir, Writing: Mehmet Hakan Özdemir, Taha Sezer, Muhammet Altınışık.Conflict of InterestNo conflict of interest was declared by the authors.Financial DisclosureThe authors declared that this study received no financial support.
