Swept-source optical coherence tomography (SS-OCT) is an emerging imaging modality used for the diagnosis and monitoring of patients with glaucoma. Compared to spectral domain OCT technology, SS-OCT uses a tunable, longer wavelength of light. This facilitates high resolution imaging of intraocular structure with improved range of depth and significantly increased scan speed. Current research is investigating the role of SS-OCT in imaging intraocular structures relevant to glaucoma, namely, the anterior segment, macula, choroid, and optic nerve including the lamina cribrosa. Future research is needed to establish the role of this new technology and determine how it fits into the larger framework of clinical glaucoma patient care.
Key wordsswept-source – optical coherence tomography – glaucoma – optic nerve – macula – choroid – lamina cribrosa
13 Future Directions: Swept-Source OCT for Glaucoma
Swept-source optical coherence tomography (SS-OCT) is an emerging ophthalmic imaging modality with many applications in the field of ophthalmology. We will concentrate on the role that SS-OCT may play in the diagnosis and monitoring of glaucoma. The technology represents an advancement compared to previous OCT platforms by replacing the fixed wavelength of spectral domain OCT with an ultra-high speed wavelength-tuned or “swept-source” laser, thereby improving range of depth, while shortening scan times. 1 Wide-field, 12 mm × 12 mm swept-source can image both the macula and optic nerve with a single scan. 2 Similarly, in the anterior segment, a single scan can simultaneously image the cornea, iris, and anterior lens. 2 In the United States, the Food and Drug Administration (FDA) approved the PLEX Elite 9000 Carl Zeiss Meditec (Jena, Germany) for research use in 2016 and in 2018 they approved the Triton Topcon SS-OCT system (Topcon, Tokyo, Japan) for clinical use. 3 Both of these can be used for imaging the posterior segment. The Tomey Casia SS-OCT (Tomey, Nagoya, Japan) is dedicated to imaging the anterior segment and is currently pending FDA approval but has been used in Asia since 2012 for both clinical and research purposes. Research continues to develop novel SS-OCT prototypes.
The commercially available SS-OCT devices scan the posterior pole at a rate of 100,000 A-scans/second at wavelengths centered at 1,050 nm with a range of approximately 100 nm, producing an axial resolution of 6.3 to 8 μm and a transverse resolution of 20 μm. Research prototypes have reported axial resolutions down to 5 μm. 2 By comparison, previous spectral domain OCT (SD-OCT) technology uses a fixed wavelength of 840 nm with scan speeds of 25,000 to 100,000 axial scans per second yielding axial resolution in the range of 2 to 7 μm and transverse resolution of 20 μm. 4 SD-OCT represented an improvement in speed and scan quality compared the previous time domain OCT (TD-OCT) technology by acquiring images at 50× the speed of TD-OCT and applying a Fourier transform to the interference spectrum detected. 5
Anterior segment OCT (AS-OCT) utilizes different wavelengths of light to optimize visualization of anterior ocular structures. SS-AS-OCT captures 30,000 A-scans per second, at 1,310 nm wavelength, yielding axial and transverse resolution of 10 and 30 μm, respectively, and permitting 360-degree scans in 2.4 seconds. 6 By comparison, SD-AS-OCT utilizes a central wavelength of 830 nm, with axial resolution of <10 μm, and TD-AS-OCT utilizes 1,310 nm wavelength with axial resolution of 18 μm. 7 Research devices have demonstrated comparable imaging performance between enhanced depth imaging (EDI)-SD-OCT-AS and SS-OCT-AS. 8
13.2 Imaging of Intraocular Structures Using SS-OCT in Glaucoma
Many intraocular structures are relevant to the study of glaucoma, namely, the anterior segment, macula, choroid, and optic disc including the lamina cribrosa. Since SS-OCT has become more widely used, research has focused on establishing its role in relationship to previous OCT imaging modalities in imaging intraocular structures. The majority of studies have focused on clinical applications in glaucoma diagnosis and monitoring while others have focused on the potential of the technology to investigate the underlying pathophysiology.
13.2.1 Anterior Segment Application of SS-OCT in Glaucoma (Fig. 13‑1)
AS-OCT has been used to examine angle configuration for aid in diagnosis of angle anomalies relevant to glaucoma including narrow angle and plateau iris and has been used to monitor response to treatment, that is, laser peripheral iridotomy. 9 Research in the field of SS-AS-OCT has focused on comparing anterior segment measurements with those obtained with previous technologies including SD-AS-OCT and TD-AS-OCT. 10 , 11 , 12 One of these studies demonstrated that SS-AS-OCT was superior to SD-AS-OCT in visualizing deeper angle structures including the scleral spur. 11 In the area of angle closure, researchers have explored the role of SS-AS-OCT in visualizing peripheral anterior synechia (PAS) and studying changes in iris volume in response to pupillary dynamics and have found that PAS can be reproducibly imaged and the findings are comparable to gonioscopy. 13 , 14 The specifics of these articles are discussed below.
Xu et al compared anterior segment parameters measured by SS-OCT and SD-OCT and found excellent inter-device reproducibility for measurements of angle opening distance (AOD), trabecular iris space area (TISA), and lens vault (LV). 10 However, anterior chamber width (ACW) showed low agreement. The authors hypothesized the difference might be due to variability in scan location between the two devices. 10 They concluded that although both the measurements are reliable, they should not be used interchangeably.
Qiao et al compared the ability of SS-OCT with SD-OCT to distinguish deeper angle structures including Schwalbe’s line, Schlemm’s canal, and scleral spur. 11 They compared scans from 67 healthy subjects obtained with SS-OCT versus SD-OCT, using anatomical markers (i.e., conjunctival blood vessel) to ensure equivalent positioning of both scans. Their results showed that SS-OCT had improved visualization of the scleral spur. They suggest that the improved depth of resolution possible with SS-OCT might be of particular benefit in visualizing angle structures in older patients with higher rates of pinguecula and pterygium.
Chansangpetch et al compared anterior segment parameters measured by SS-OCT and TD-OCT. 12 They found strong correlation between measurements of anterior chamber depth (ACD), LV, and ACW. However, AOD, TISA, and angle recess area (ARA) demonstrated variability, with SS-OCT tending to give larger numbers. They concluded that the measurements from different devices are reproducible but not interchangeable. They hypothesized that differences in measurements might be influenced by differences in speed, resolution, segmentation, or scan location.
Lai et al used SS-OCT to measure the area and degree of PAS. 13 They found good reproducibility of the PAS measurements by SS-OCT and found agreement with PAS assessment by gonioscopy. Synechial and appositional angle closure were demonstrated with dynamic dark-light OCT imaging. The authors propose this could be a new clinical tool for quantitative monitoring of PAS progression.
Mak et al compared iris volume in primary angle closure (PAC) or PAC suspect (PACS) with primary open-angle glaucoma (POAG) in response to light, dark, and pharmacologic dilation. 14 They found iris volume decreased after dilation in POAG and PAC and PACS and the degree of reduction was less in eyes with smaller anterior chamber volume. In addition, they found larger iris volume was associated with a smaller angle width. The authors suggest SS-OCT may provide insights into the pathophysiology of angle closure.
Several studies have explored the use of macular and peripapillary retina SS-OCT scans including their diagnostic value compared with SD-OCT scan for glaucoma. 15 , 16 , 17 In general, studies have shown SS-OCT obtains thinner measurements for most retinal layers compared to SD-OCT. However, the glaucoma discriminating ability overall was comparable. 15 , 17 Another study performed this same comparison using wide-field technology and found that specific layer measurements were more equivalent between SD-OCT and SS-OCT. 16 SS-OCT has also been used to assess asymmetry of the macula as a tool for glaucoma diagnosis. 18 The specifics of these articles are discussed below.
Yang et al compared macular thickness and diagnostic ability of macular ganglion cell layer plus inner plexiform layer (mGCIPL), macular ganglion cell complex (mGCC), and circumpapillary retinal nerve fiber layer (cpRNFL) between wide scan SS-OCT and standard macular SD-OCT. Although the diagnostic accuracy was equivalent between SS-OCT and SD-OCT scans, the measurements of specific layers were not. They found that mGCIPL and mGCC were thinner in both normal and glaucomatous eyes using SS-OCT compared to SD-OCT, while the opposite trend was noted with the cpRNFL. Of the three layers, cpRNFL had the highest sensitivity and specificity for differentiating glaucoma for both SS-OCT and SD-OCT (area under the curve [AUC] = 0.83 and 0.85, respectively). The authors hypothesized differences in retinal subfield measurements might be due to different segmentation algorithms and scan quality. 15
Lee et al compared macula scans by SD-OCT and SS-OCT including measurements of mGCIPL and macular RNFL (mRNFL) in glaucoma and evaluated their diagnostic ability. 17 They found the ability to distinguish glaucoma from normal was comparable; however, the specific layers and retinal subfield measurements were not. Although these differences varied by layer and retinal subfield and also between glaucoma and control, overall SS-OCT was noted to produce thinner measurements than SD-OCT, similar to findings of Yang et al. The highest AUC was noted to be SD-OCT mGCIPL in the outer temporal zone (AUC = 0.894). 17
Hong et al compared glaucoma-discriminating abilities of SD-OCT macular and disc scans with SS-OCT wide scans which encompass both structures. They found that the AUC for standard macula and disc scans versus wide scans were not significantly different. 16 Unlike Yang et al and Lee et al, they found excellent agreement between retinal layer measurements performed by the two imaging modalities. The best individual parameter for differentiating glaucoma from normal was total cpRNFL using standard SD-OCT disc scans (AUC = 0.902).
Another study by Lee et al used SS-OCT technology to evaluate the glaucoma-differentiating ability of measurement asymmetry between macular layer thickness between fellow eyes. Their design was based on the premise that normal eyes tend to display a high degree of symmetry, which is disturbed by the glaucoma disease-state. They compared mRNFL, mGCIPL, mGCC, and total retina thickness between the two eyes. They found that overall glaucoma patients had thinner retinal layers and higher asymmetry. The AUCs of the average thickness differences ranged from 0.748 to 0.894. Authors reports their results were similar to previous studies using SD-OCT and might enhance diagnostic capabilities, although their study did not compare the OCT technologies. 18
13.2.3 Choroidal Application of SS-OCT in Glaucoma
The choroid is thought to play a role in the vascular component of glaucoma. A recent paper used optical coherence tomography angiography (OCTA) to evaluate choroidal microvascular dropout in patients with POAG. They found the prevalence of choroidal microvascular dropout was significantly higher in POAG patients with disc hemorrhage, central visual field loss, and severe glaucomatous nerve damage. 19 Although choroidal imaging is not currently used in clinical evaluation of patients with glaucoma, research has explored the role of SS-OCT and SD-OCT in measuring macular and peripapillary choroidal thickness in glaucoma patients as a diagnostic tool. 20 , 21 , 22
Hirooka et al used EDI SD-OCT to show choroidal thinning correlated to worsening glaucoma severity, particularly notable in the area nasal to the fovea in patients with normal-tension glaucoma (NTG). 20 Song et al compared peripapillary and macular choroidal thickness (PCT and MCT) between glaucoma and normal using SS-OCT and found both were thinner in glaucoma; however, only PCT was significantly thinner after controlling for confounding factors such as age, axial length, and disc area. 21 They did not find any correlation with glaucoma severity, suggesting this technology might be helpful for diagnosis but not so for monitoring disease progression. They conclude their results are similar to previous EDI SD-OCT studies. However, they did not compare the performance of the modalities.
Zhang et al also measured choroidal thickness by SS-OCT in glaucomatous and normal eyes and found that it was thinner in the peripapillary and macular regions in glaucoma compared with control. 22 However, when they accounted for age and longer axial length, they found glaucoma was in fact not independently associated with choroidal thickness. An interesting secondary finding was that the relationship between older age and thinner choroid may be stronger in glaucoma compared with controls suggesting that glaucoma might modulate age-related changes to choroidal thickness.