Optical Coherence Tomography Angiography

Optical coherence tomography (OCT) is a noninvasive imaging technique that uses low-coherence interferometry to produce depth-resolved imaging. A beam of light is used to scan an area of the eye, say the retina or anterior eye, and interferometrical measurements are obtained by interfering with the backscatter or reflectance from ocular structures with the known reference path of traveling light. This modification of classic Michelson interferometry allows for the generation of structural images of anatomy when using OCT. OCT has become widely adopted in the field of ophthalmology since its introduction in 1991 and has since continually been improved. Until optical coherence tomography angiography (OCT-A), conventional structural OCT images predominantly provided visualization of anatomic changes with low contrast between small blood vessels and tissue within retinal layers. Thus, other imaging modalities such as fluorescein or indocyanine green angiography was generally used to evaluate retinal vasculature and choroidal vasculature, respectively.

OCT-A uses the principle of diffractive particle movement of moving red blood cells to determine vessel location through various segments of the eye without the need of any intravascular dyes. OCT-A technology allows for the ability to image flow in the retinal, and choroidal vasculature can be displayed through en face, depth-encoded slabs. These slabs are presented alongside structural OCT B scans. Together, they provide detailed flow imaging of the deep retinal vascular plexus and choriocapillaris, which were not well visualized with previous imaging modalities.

It is important to distinguish the differences between Doppler OCT and OCT-A. Although they both use phase information, Doppler OCT quantifies blood flow in larger vessels and measures total retinal blood flow using phase-shift while OCTA analyzes scatter from a static background tissue to create angiograms.

History of the development of Ocular Coherence Tomography

Optical coherence tomography is the optical equivalent of ultrasound to generate images using time delay and light echo magnitudes. The use of echoes of light to examine biological tissue was originally proposed by Michel Duguay at the AT&T Bell Laboratories, published by him as "Light photographed in flight" in the American Scientist in 1971. He was the first to show that using high-speed shutters; it was possible to "see inside biological tissues." The field of Femtosecond optics was further developed by Erich Ippen of the Massachusetts Institute of Technology in the mid-1970s. His group collaborated with Dr. Carmen Puliafito of the Massachusetts Eye and Ear Infirmary, and they studied femtosecond laser effects on the retina and the cornea. Together with S DeSilvestri of Milan, Italy, and R. Margolis and A. Oseroff from the Department of Dermatology at the Massachusetts General Hospital, they further developed Duguay's initial work to "see inside tissues." They initially used lasers at a 625-nm wavelength and later progressed to using longer 1300-nm wavelengths, which allowed the reduction of scattering. The first application of low-coherence interferometry, which was used to measure the axial length of the eye, was reported by Fercher et al. of the Medical University of Vienna, Austria, in 1988.

An Electrical Engineering undergraduate, John Apostolopoulos, used low-coherence laser diodes in 1989 to describe the potential ophthalmic applications of this technology, although the sensitivity was limited. The breakthrough came after ongoing research into low-coherence interferometry by David Huang, an MD/Ph.D. student, in 1991. He showed the practical applicability of coherence interferometry using an 800-nm low-coherence laser diode. Higher sensitivities were achieved, which yielded information on eye structures such as the lens and the iris. The first OCT images were demonstrated by David Huang in Science in 1991. Unpublished concepts of a similar system were also demonstrated by Tanno et al. in Japan.

Swanson et al. developed the first in vivo retinal images in 1993, and a similar retinal system was demonstrated by Fercher et al. of Vienna. Practical advances were then rapidly made by the MIT group working with Carmen Puliafito and Joel Schuman of the New England Eye Center of the Tufts University School of Medicine in Boston. OCT examination protocols for circumpapillary scanning for the assessment of glaucoma and to assess macular edema were developed by Michael Hee, an expert programmer who used the early Apple Macintosh computers.

Michael Hee was largely responsible for the major development in the 1990s, publishing more than 30 papers during his doctorate. His 1997 doctorate thesis, "Optical coherence tomography of the eye," remains a seminal reference work on OCT in ophthalmology. The first OCT atlas was organized by Carmen Puliafito in 1996 (Optical Coherence Tomography of Ocular Diseases, Slack, 1995). The Advanced Ophthalmic Diagnostics (AOD) company set up by C Puliafito, E Swanson, and J Fujimoto in 1992, was acquired by Humphrey Zeiss two years later and went on to develop machines that were introduced into clinical use, the first machine being introduced in 1996. As with many new techniques, clinical adoption by the ophthalmic community was slow in the latter 1990s, with only 180 units in use until 2000. By 2004, the company had developed further machines that were faster and had better resolution images, and by 2004, more than 10 million OCT imaging procedures had been obtained worldwide. OCT has since become a standard of care in the ophthalmic community. OCT imaging is now used in ophthalmology, cardiovascular medicine, dermatology, neurology, gastroenterology, dentistry, otolaryngology, urology, pulmonology, gynecology, and other subspecialties with new applications being found every year.