The Future of Retinal Imaging

Daniel Q. Li; Netan Choudhry

Disclosures

Curr Opin Ophthalmol. 2020;31(3):199-206. 

In This Article

Advancements in Optical Coherence Tomography

Since its introduction in the 1990s, OCT has revolutionized the practice of ophthalmology and transformed the way ophthalmic diseases are diagnosed, monitored and treated. Further innovations in OCT technologies are expected to aid our assessment and management of ophthalmic diseases further, faster and in more detail.

Scientific Background

Effectively an 'optical ultrasound', OCT borrows the same echo-location principle as ultrasonography to noninvasively produce cross-sectional images within tissues. Unlike ultrasonography, the wavelength of light is much shorter than that of sound, and therefore OCT generates images of superior resolution, equivalent to a low-power microscope.[18] A broad-bandwidth light source is divided into a sample beam and reference beam (the time delay and intensity of which is known), and their reflections from the sample tissue produce characteristic patterns of light interference from which depth profile of the tissue can be determined. In the original time-domain OCT systems, the reference beam was varied in time using a moving mirror. This limited the scanning speed, with a scan rate of 400 axial scans per second with an axial resolution of 8–10 μm in tissue.[19] In 2006, spectral-domain OCT (SD-OCT) systems were introduced, which simultaneously measured interference patterns as a function of frequency using a high-speed spectrometer. This technique achieved scan rates of 20,000–52,000 axial scans per second and a resolution of 5–7 μm in tissue.[20,21]

Swept-source Optical Coherence Tomography

In 2012, swept-source OCT (SS-OCT) became available in clinical practice. SS-OCT uses a tunable (frequency-swept) laser to sequentially emits various frequencies in time, and the interference spectrum is measured by photodetectors instead of a spectrometer. This significantly increases the acquisition speed to 100 000 axial scans per second, while providing superior sensitivity for higher quality images.[22] In addition, light source in most commercially available SS-OCT machines is centered around 1050 nm, which is longer than the average wavelength of 840 nm used by most SD-OCT machines. Longer wavelength provides three main advantages. First, it allows for deeper tissue penetration to visualize more posterior structures, including the details of the vitreoretinal interface, the whole thickness of the choroid, the optic nerve head and even the sclera (Figure 2). Detailed visualization of these structures has shown growing applications in the assessment and early detection of several retinal diseases.[23,24] Second, longer light wavelength allows for improved penetration through media opacities (such as cataract, blood and vitreous debris) and reduced light scattering between phase transitions (such as between gas, oil, blood, vitreous and so on).[22] This allows for clear visualizations in conditions that would have been difficult to image on SD-OCT, such as the immediate postvitrectomy period. As an example, we have previously demonstrated consistent early assessment of macular hole closure on postoperative day-1 in gas-filled eyes using SS-OCT.[25] Third, as 1050 nm falls outside of the visible light spectrum, the retina is not stimulated during testing, which enables unperturbed OCT testing with reduced motion affect.[26]

Figure 2.

24 mm posterior pole montage using Sweptsource OCT of the left eye of a 46 year-old Asian female with myopic foveoschisis demonstrating clear vitreoretinal interface traction on the macula with subretinal and intraretinal fluid. Nerve fiber and inner retinal schisis are visible on the nasal and temporal margins of the image.

Megahertz Optical Coherence Tomography

The recently introduced Fourier-domain mode-locked OCT has enabled high-quality SS-OCT imaging at unprecedented speeds. Aptly named 'Megahertz OCT', they feature a scan rate of up to 5.2 Megahertz in research prototypes, with 4-spot parallel illumination to achieve effectively 20.8 million axial scans per second.[27] This is a significant advantage because in retinal imaging, acquisition times need to fall below a few seconds to prevent distortions from eye motion. Current-generation commercial OCT systems suffer from a small area of predefined scan region because of limited scan speed. With ultra-high-speed imaging, a wider field of view can be visualized, with high-density cross-sectional scans extending beyond the central macular region into the peripheral retina.[28] In addition, it enables high-quality three-dimensional volume scans that can be reconstructed to provide widefield enface visualization of the retina.[29] In a small pilot study, Reznicek et al.[30] demonstrated the feasibility of using widefield Megahertz OCT in diabetic retinopathy screening. However, further studies need to be done to assess the clinical utility of Megahertz OCT in different retinal diseases.

Anterior Segment Optical Coherence Tomography

As the speed and resolution of OCT imaging rapidly improved in recent years, anterior segment OCT (AS OCT) has evolved from providing an overview of the anterior segment to now near histological-level details of anterior segment structures. As a result, the clinical application of AS OCT is rapidly expanding. At the ocular surface, OCT can distinguish between pterygium and pseudo-pterygium, and aid in the diagnosis and management of ocular surface neoplasia.[31] In dry eye disease, OCT can be used to assess the relevant anatomy (Meibomian glands, tear duct and lacrimal gland structures), as well as the tear film and tear meniscus, including alterations with different types of ocular lubricants.[32–35] AS OCT is useful for the assessment and monitoring of corneal thinning, corneal infiltrates, corneal dystrophies and especially keratoconus with improved reproducibility compared to Placido-Scheimpflug imaging.[36–38] AS OCT also provides more precise topographical and anatomical measurements in the preoperative, intraoperative and postoperative evaluations of Descemet membrane endothelial keratoplasty, Descemet stripping automated endothelial keratoplasty and different refractive surgeries, which have shown to impact surgical outcomes.[39] In clinical glaucoma practice, AS OCT is a useful adjunct to gonioscopy, as well as a substitute when gonioscopy is not feasible because of corneal disease or lack of patient co-operation. In addition to visualizing the anterior chamber angle, AS OCT can visualize structures of the aqueous outflow system, and 1 day OCT-guided placement of devices in microinvasive glaucoma surgery may result in an improvement in the IOP control.[37,40] Applications of AS OCT also extend to imaging of the conjunctiva (for the assessment of bleb morphology following trabeculectomy),[41] sclera (for the design of scleral lenses[42] and monitor scleral changes following vitrectomy[43] or repeated intravitreal injections[44]), limbus (to observe the palisades of Vogt)[45] and extraocular muscles (for preoperative planning of complicated strabismus surgeries).[46] Novel developments of AS OCT that are still in the research phase include polarization sensitive OCT, which provides tissue-specific contrast based on light's polarization state,[47] and OCT elastography, which detects subtle displacement of tissue when forces are applied for studying strain dynamics and tissue-shape changes.

Hand-held Optical Coherence Tomography

Most commercial OCT systems today have a fix-mounted, table-top design, where patients need to sit in front of the machine and have their head stabilized in a chinrest and headrest. This is difficult for bedridden patients, patients with certain mental or physical disabilities and young children or infants. In order to address the needs of these patients, developments have been made to package OCT into portable systems with handheld probes.[48] Bioptigen (Morrisville, NC), for example, has developed a U.S. Food and Drug Administration-approved hand-held SD OCT unit (the Envisu C-class system) consisting of a handheld imaging probe connected via a 1.3-m cable to a console mounted on a rolling cart. This device now has been used widely to image eyes of neonates, infants and children, and in settings, such as animal research laboratories and intraoperative imaging examinations.[49] It has proven especially useful in neonatal populations for the study of ocular development and for diseases, such as retinopathy of prematurity.[50,51] More recently, researchers at Duke University (Durham, NC) have developed an ultralight, low-cost OCT scanner featuring a redesigned 3D-printed spectrometer.[52] It is roughly 15-times lighter and smaller than current commercial OCT systems, with a total parts cost of $5037. In its first clinical trial performed on 120 eyes, the new OCT scanner demonstrated comparable contrast-to-noise ratio compared to a Heidelberg Engineering Spectralis OCT (Heidelberg Engineering GmbH, Heidelberg, Germany) and was able to resolve features necessary for accurate clinical diagnosis. As hand-held OCT technology becomes more miniaturized, mobile and lower-cost, it can serve in a wide range of new settings, including teleophthalmology for low-resource areas, developing countries and military medicine.

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