Diagnostic Imaging Modalities

Primary authors	KEOGT Team

(This chapter is under development)

Optical Coherence Tomography

Introduction

Optical coherence tomography (OCT) is a non-invasive imaging technique used to obtain high-resolution images of the retina, optic nerve head, and anterior segment of the eye. OCT is widely used in ophthalmology for diagnosis and management of various retinal diseases, including age-related macular degeneration, diabetic retinopathy, and glaucoma. In this chapter, we will provide an overview of OCT technology, including its history, types of technology, and future developments in clinical use.

History of OCT technology

OCT technology was first introduced in 1991 by Huang et al. as a high-resolution imaging modality for retinal imaging. The first OCT device was based on a time-domain technique, which used a low-coherence interferometer to measure the backscattered light from the eye. In the following years, OCT technology continued to develop, and a more advanced technique, called spectral-domain OCT (SD-OCT), was introduced in 2002. SD-OCT has almost completely replaced the time-domain technique, providing higher imaging speed and resolution.

Time-domain OCT

Time-domain OCT (TD-OCT) is the first-generation OCT technology that uses a low-coherence interferometer to measure the time delay of reflected light from the back of the eye. TD-OCT acquires images by sequentially varying the reference arm length and recording the interferometric signal as a function of time. The system then performs Fourier transformation to obtain the depth profile of the tissue. TD-OCT has a limited imaging speed and resolution compared to SD-OCT, and it is rarely used in clinical practice today.

Spectral-domain OCT

Spectral-domain OCT (SD-OCT) is the current standard OCT technique used in clinical practice. SD-OCT is based on a spectral interferometry technique that measures the spectral interference pattern of the backscattered light from the targeted tissues. SD-OCT acquires images by simultaneously detecting the interferometric signal from all depths, which allows for faster image acquisition and higher resolution. SD-OCT can provide images with a resolution of up to 5 μm and an imaging speed of up to 70,000 A-scans per second.

Anterior segment OCT

Anterior segment OCT (AS-OCT) is a modified version of SD-OCT that allows for high-resolution imaging of the anterior segment of the eye. AS-OCT uses a longer wavelength light source and a specialized lens to obtain images of the cornea, anterior chamber, and lens. AS-OCT has become an important tool in clinical practice for diagnosis and management of various anterior segment disorders, including corneal dystrophies, angle-closure glaucoma, and cataract.

OCT Angiography

OCT angiography (OCTA) is a novel imaging technique that provides detailed images of the retinal and choroidal vasculature without the need for dye injection. OCTA is based on the principle of motion contrast imaging, where blood flow is detected by measuring the decorrelation of the OCT signal between consecutive B-scans. OCTA has become an important tool in the diagnosis and management of various retinal vascular diseases, including diabetic retinopathy and macular degeneration.

Future technologies

Various new technologies are currently being developed for OCT imaging, including polarization-sensitive OCT (PS-OCT), swept-source OCT (SS-OCT), and full-field OCT (FF-OCT). PS-OCT is a technique that provides information on tissue birefringence, which can be used to detect changes in the tissue microstructure. SS-OCT is a technique that uses a rapidly tunable laser to obtain high-speed and high-resolution images of the retina. FF-OCT is a technique that provides en face images of the retina, allowing for visualization of the entire retinal structure.

Conclusion

OCT has revolutionized the field of ophthalmology by providing non-invasive and high-resolution images of the eye's structure and vasculature. The technology has advanced significantly since its introduction, from time-domain OCT to spectral-domain OCT and anterior segment OCT, and most recently, OCT angiography. With ongoing research and development, new technologies such as polarization-sensitive OCT, swept-source OCT, and full-field OCT will continue to enhance our ability to diagnose and manage ocular diseases. OCT has become a critical tool in the field of ophthalmology, and its continued development holds immense promise for improving patient care and outcomes.

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