Optical Coherence Tomography Angiography

Excerpt

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 eye area, 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 using OCT to generate structural anatomy images. OCT has become widely adopted in ophthalmology since its introduction in 1991 and has continually improved. Until optical coherence tomography angiography (OCTA), 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, were generally used to evaluate retinal vasculature and choroidal vasculature, respectively.

OCTA 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 for any intravascular dyes. OCTA technology allows for the ability to image flow in the retinal and choroidal vasculature through en-face, depth-encoded slabs. These slabs are presented alongside structural OCT B-scans, which obtain cross-sectional images. Together, they provide detailed flow imaging of the deep retinal vascular plexus and choriocapillaris, which were poorly visualized with previous imaging modalities.

Distinguishing the differences between Doppler OCT and OCTA is critical. Although they both use phase information, Doppler OCT quantifies blood flow in larger vessels and measures total retinal blood flow using phase shift. In contrast, OCTA analyzes scatter from a static background tissue to create angiograms.

Historical Development of Optical Coherence Tomography

OCT is the optical equivalent of ultrasound, which generates images using time delay and light echo magnitudes. Michel Duguay originally proposed using echoes of light to examine biological tissue at AT&T Bell Laboratories when he published "Light Photographed in Flight" in American Scientist in 1971 and was the first to show that high-speed shutters made it possible to "see inside biological tissues."

The field of femtosecond optics was further developed by Erich Ippen of the Massachusetts Institute of Technology (MIT) in the mid-1970s. His group collaborated with Dr. Carmen Puliafito of the Massachusetts Eye and Ear Infirmary; together, they studied femtosecond laser effects on the retina and the cornea. Moreover, working with DeSilvestri of Milan, Italy, and Margolis and Oseroff from the Department of Dermatology at Massachusetts General Hospital, Duguay's initial work to "see inside tissues" was further developed. These investigators 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 eye's axial length, 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. However, a significant breakthrough was made through the ongoing research into low-coherence interferometry of David Huang, an MD/PhD student, in 1991. Huang showed the practical applicability of coherence interferometry using an 800-nm low-coherence laser diode. Consequently, higher sensitivities were achieved, which yielded information on eye structures (eg, the lens and the iris). The first OCT images were published by Huang in Science in 1991. Unpublished concepts of a similar system were also shown by Tanno et al in Japan.

Swanson et al developed the first in vivo retinal images in 1993, and Fercher et al of Vienna demonstrated a similar retinal system. 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 macular edema were developed by Michael Hee, an expert programmer who used the early Apple Macintosh computers.

Michael Hee was largely responsible for major developments 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 company, set up by C Puliafito, E Swanson, and J Fujimoto in 1992, was acquired by Humphrey Zeiss 2 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 faster machines with better-resolution images; 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 various subspecialties, including ophthalmology, cardiovascular medicine, dermatology, neurology, gastroenterology, dentistry, otolaryngology, urology, pulmonology, and gynecology, with new applications found every year.