Schlemm’s canal (SC) is a tube-like structure located at the inner part of the corneoscleral junction which goes around the cornea. It plays an important role in intraocular pressure (IOP) regulation by draining the excess aqueous humor from the anterior chamber of the eye to the episcleral veins.1 SC is closely associated with glaucoma, the second leading cause of blindness worldwide.2 Anatomical alterations in Schlemm’s canal are linked to elevated IOP, the most important modifiable risk factor for such disease.3,4 Studies have shown that narrowing or progressive collapse of the SC observed in glaucomatous eyes may be responsible for the increased aqueous outflow resistance and the elevated IOP.5,6 SC dimensions are correlated with the presence of primary open angle glaucoma (POAG).1,7−11 Surgical procedures, such as canaloplasty,12 viscocanalostomy,13–15 and micro-stent implantation,16–18 were developed targeting on mechanical dilation of the SC to lower the IOP, whilst therapeutic interventions that facilitate SC dilation have also been investigated to achieve similar results.19 Moreover, mounting evidence pointed that the flow to the aqueous veins is pulsatile due to the change in SC dimensions.20,21 Accurate visualization and real-time assessment of SC dimensions via in vivo imaging will enable better understanding of the SC and its role in regulating the flow in the aqueous humor outflow (AHO) facility.
Optical coherence tomography (OCT) is a non-invasive imaging technique that offers high-resolution and three-dimensional (3D) visualization of the SC structure and can be applied to a wide variety of clinical problems at the anterior segment (AS).22,23 Several studies have reported on in vivo SC quantifications using spectral-domain (SD) AS OCT operated at 850 nm spectral window,9,24−29 but most of the studies shared the common problem of poor SC discerning ability due to the shadowing artifact casted by superficial vessels as well as blurring effect induced by eye motion. This is mainly due to two reasons. First, commercial SD OCT systems adopted in these studies relied on the line-scan camera with an A-scan speed limited to 70 kHz. At this speed, one single volumetric scan may take at least several seconds to complete, and the image quality will be inevitably deteriorated by motion artifacts. Second, photons at a shorter wavelength experience higher scattering loss when travelling deeper into the tissue. Previous study suggested that the SC may reside as deep as 1.1 mm under the limbus area. At this depth, the backscattered photons from the SC may suffer from greater attenuation which leads to a lower signal-to-noise ratio of the SC structure. Moreover, structures that reside on top of the SC, such as the epi-scleral and intra-scleral vessels as well as the pigmentation in the limbus, may further attenuate the signal. On the contrary, swept-source (SS)-OCT utilizes light sources at longer wavelengths (1060nm or 1310nm) that allow for better signal penetrability to detect deeper structures. Combined with the longer imaging depth and alleviated sensitivity roll-off effect, it facilitates the visualization of deep tissue structures such as SC, iridocorneal angle, sclera spur, and trabecular meshwork (TM). Furthermore, the swept sources can offer a higher A-scan speed which may substantially reduce the motion artifacts. Nevertheless, the resolution offered by SS-OCT is usually inferior to its SD counterpart, and it is unclear whether it will affect the quantification of SC. To date, it is still unclear what optical wavelength window is optimal for OCT imaging of SC, and a quantitative comparison of SD- and SS-OCT on SC imaging performance is still lacking.
In this study, we quantitatively compared the performance of three commercial OCT systems, Zeiss CIRRUS 5000 (CIRRUS, 840nm, SD-OCT), Zeiss PLEX Elite 9000 (PLEX, 1060nm, SS-OCT) and Tomey CASIA SS-1000 (CASIA, 1310nm, SS-OCT), in SC imaging and analyzed the potential impact factors. The cross-sectional area (CSA) of SC was extracted via manual segmentation and compared among different devices. Besides, we proposed three performance metrics, namely the contrast, the coverage, and the continuity of the SC, to evaluate the discernibility of SC in these three devices.