Disagreement of Radial Peripapillary Capillary Density Among Four Optical Coherence Tomography Angiography Devices

Abstract

Purpose: This prospective study evaluated the agreement among four optical coherence tomography angiography (OCTA) devices in the assessment of radial peripapillary capillary (RPC) density.

Methods: The study included 48 eyes of 48 subjects (14 healthy, 19 glaucomatous, and 15 non-glaucomatous optic neuropathy). Each participant was scanned using four OCTA devices in a random sequence: RTVue XR Avanti (RTVue), DRI OCT Triton (Triton), Revo NX 130 (Revo), and PLEX Elite 9000 (PlexE). All 6 × 6-mm grayscale OCTA images from each device were analyzed for RPC density using a customized algorithm. Agreement between each pair of devices was assessed using intraclass correlation coefficients (ICCs) and Bland–Altman plots.

Results: There was a poor correlation between devices in all comparisons (RTVue–Triton, ICC = 0.34; RTVue–Revo, ICC = 0.31; RTVue–PlexE, ICC = 0.28; Triton–Revo, ICC = 0.31; Triton–PlexE, ICC = 0.17; Revo–PlexE, ICC = 0.34). Significant proportional biases (P < 0.05) and wide limits of agreement with apparent constant biases were identified in all comparisons. The mean difference was greatest for the RTVue–Revo pair (−49.3%; 95% confidence interval [CI], −52.9 to −45.8) and smallest for the Triton–PlexE pair (−7.7%; 95% CI, −10.1 to −5.3).

Conclusions: The RPC densities obtained from each device had poor inter-device agreement and significant biases and cannot be used interchangeably.

Translational Relevance: RPC density obtained from different OCTA devices is not interchangeable; thus, the progression of optic neuropathy should be monitored using the same OCTA device.

Introduction

Optical coherence tomography angiography (OCTA) is an emerging non-invasive diagnostic technology for imaging the microvasculature of the retina and choroid using laser light reflectance of the surface of moving red blood cells to depict vessels through different segmented areas of the eye. Compared to fluorescence angiography and indocyanine green angiography, which require an injectable dye that is time consuming and increases the risk of adverse side effects, OCTA provides higher resolution and visualization of all the vascular layers, including the radial peripapillary and deep capillary network.¹ The radial peripapillary capillary (RPC) plexus plays an important role in evaluating optic disc perfusion, which is useful for the early detection and prediction of the progression of optic nerve diseases, such as glaucoma and non-glaucomatous optic neuropathy.^2,3 

Currently, there are many commercially available OCTA modules, including the AngioVue (RTVue XR Avanti; Optovue, Fremont, CA), AngioPlex (Cirrus HD-OCT 5000; Carl Zeiss Meditec, Jena, Germany), SS-OCT Angio (DRI OCT Triton Swept-Source OCT; Topcon, Tokyo, Japan), Angiography Module (SPECTRALIS OCT2 Module; Heidelberg Engineering, Heidelberg, Germany), AngioScan (RS-3000 Advance OCT; Nidek Co., Gamagori, Japan), AngioPlex (PLEX Elite 9000; Carl Zeiss Meditec), Angio eXpert (OCT-HS100; Canon Medical Systems, Otawara, Japan), and the Angiography SOCT (Revo NX 130; Optopol, Zawiercie, Poland). Previous studies compared the agreement of different OCTA devices evaluating the macular area; however, data on the peripapillary area are limited.^4,5 

This study aimed to investigate the agreement of capillary density of the RPC layer measurements in healthy patients, patients with glaucoma, and patients with non-glaucomatous optic neuropathy among four OCTA devices—namely, the RTVue XR Avanti (RTVue), DRI OCT Triton (Triton), Revo NX 130 (Revo), and PLEX Elite 9000 (PlexE). 

Methods

This study was conducted in compliance with and approved by the Institutional Review Board of the Faculty of Medicine, Chulalongkorn University, Thailand (IRB No. 733/61). All participants were recruited from the outpatient unit of the Ophthalmology Department at King Chulalongkorn Memorial Hospital from May 1, 2020, to October 31, 2020. The study was conducted in accordance with the tenets of the Declaration of Helsinki, and written informed consent was obtained from all participants after explanation of the nature and possible consequences of the study. 

The study included subjects ≥18 years old. Participants were stratified into three groups: normal, glaucoma, and non-glaucomatous optic neuropathy. The inclusion criteria for the normal group were as follows: best-corrected visual acuity of 20/40 or better using the Snellen visual acuity test, intraocular pressure (IOP) < 20 mmHg, normal anterior segment and dilated fundus examination, normal optic nerve head finding on optical coherence tomography (OCT), optic nerve head photography, normal standard perimetry test result, and no family history of glaucoma or optic neuropathy. The glaucoma group included patients with evidence of glaucomatous optic neuropathy, which was defined as a vertical cup-to-disc ratio ≥ 0.7 and/or the presence of neuroretinal rim thinning or notching, confirmed by a retinal nerve fiber layer (RNFL) defect identified in the OCT optic nerve head analysis (Cirrus HD-OCT 5000; Carl Zeiss Meditec), as well as compatible glaucomatous visual field defects on reliable static automated perimetry (Humphrey Field Analyzer 3; Carl Zeiss Meditec). The non-glaucomatous optic neuropathy group included patients with other causes of optic neuropathy that were not in the stage of optic disc edema. 

All subjects underwent a complete ophthalmic evaluation, including best-corrected visual acuity, IOP measurement using a Goldmann applanation tonometer, and slit-lamp biomicroscopic examination of the anterior segment, optic nerve head, and dilated fundus examination. Those with poor fixation leading to motion or doubling artifacts, media opacity obscuring retinal vasculature, presence of retinal pathology, and refractive error greater than −6 or +6 diopters were excluded. Each subject was scanned with the four OCTA devices (RTVue, Triton, Revo, and PlexE) in a random sequence at the same clinical visit. 

Image Acquisition

OCTA images were acquired using the following software: RTVue XR AngioVue 2017.1.0.151 for RTVue, IMAGEnet 6 (version 3.01) for Triton, SOCT 10.0 for Revo, and PLEX Elite 9000 (version 2.0) for PlexE. For each device, scans were taken from 6 × 6-mm cubes centered on the optic nerve head. The scan quality was evaluated by a glaucoma specialist (S.C.) at the time of acquisition, and imaging was repeated if required. To evaluate the RPC layer, the slab between the internal limiting membrane and outer limit of the RNFL was used. The slab set was checked and manually corrected for segmentation errors, if present, before exporting the images. For PlexE, the RPC slab image was extracted using the Advanced Retina Imaging Network test algorithm system (Peripapillary Nerve Fiber Layer Microvasculature Density v0.7, developed by ZEISS Algorithm Development; available at www.arinetworkhub.com). 

All scans were checked for artifacts. Poor-quality images, defined by low signal strength (signal strength index < 45 for RTVue, signal quality < 45 for Triton, and signal strength < 7 for both Revo and PlexE), signal void area, wrong segmentation, blink, or motion artifact, were excluded from the study. One eye from each subject was used in the study. In subjects with unilateral disease, the pathologic eye was assessed. In subjects with bilateral disease and for whom the images of both eyes were eligible, the eye was selected randomly. 

RPC Density Analysis

For each device, OCTA images were exported for analysis. Pixel density was analyzed to obtain vessel density using a customized MATLAB algorithm (MathWorks, Natick, MA). Vessel density was calculated in terms of capillary density to focus on the pixel containing perfused capillaries. To measure capillary density, we removed large vessels from the images. The details of image normalization and thresholding have been published previously.⁶ In brief, this was performed by first thresholding OCTA grayscale images to create binary images by replacing all pixel intensities > 0.55 with the value of 1 (white) and the remaining pixels with the value of 0 (black). After large blood vessels were removed from the binary image, local adaptive thresholding was performed using a sampling window size of 15 × 15 pixels to account for local differences in brightness throughout the image. In addition, after determining the center of each image, two concentric circles (3.45 mm and 1.95 mm in diameter) were placed to produce an annular area with a width of 0.75 mm (Figure). The capillary density of the RPC layer (RPCD) was defined as a percentage by dividing the number of pixels associated with capillaries in this ring by the number of pixels in the entire ring. 

Figure.

 

Peripapillary capillary density map of the RNFL plexus obtained from four devices. The left panels show the preprocessing images. The right panels show the image processing. The region of interest is an annulus area between two concentric circles (in yellow) with an inner diameter of 1.95 mm and an outer diameter of 3.45 mm. Major blood vessels (in cyan) were removed from the vessel density calculation. (Top left) RTVue. (Top right). Triton. (Bottom left) Revo. (Bottom right) PlexE.

Statistical Analyses

Each pair of devices was assessed for correlation between RPCD measurements for all patients using intraclass correlation coefficients (ICCs) for a two-way, mixed-effects model with absolute agreement. Bland–Altman plots were used to evaluate the agreement between pairs of devices. The analysis was repeated for the normal, glaucoma, and non-glaucomatous optic neuropathy subgroups. Orthogonal linear regression was performed to assess the proportional bias and constant offset. The analyses were performed using Stata 16.0 (StataCorp, College Station, TX) and R 3.3 for Macintosh (R Foundation for Statistical Computing, Vienna, Austria). 

Results

Fifty-one subjects were initially enrolled; however, three subjects were excluded due to inadequate image quality (i.e., motion artifact and signal block area) in at least one of the devices, resulting in 48 subjects being used for the final analysis. These subjects included 14 normal eyes, 19 eyes with glaucoma, and 15 eyes with non-glaucomatous optic neuropathy. The mean age ± SD was 57.3 ± 14.0 years, ranging from 19 to 79 years. In the glaucoma group, seven subjects had primary open-angle glaucoma, seven had normal-tension glaucoma, four had primary angle-closure glaucoma, and one had secondary glaucoma. The diagnoses of non-glaucomatous optic neuropathy included seven cases of optic neuritis, four cases of compressive optic neuropathy, two cases of ischemic optic neuropathy, one case of optic atrophy from previous chronic papilledema, and one case of traumatic optic neuropathy. The demographic and clinical characteristics of the participants are presented in Table 1. 

Table 1.

 

Demographic and Clinical Characteristics

 

The mean ± SD RPCD values for all subjects were 9.73% ± 3.92%, 27.89% ± 7.23%, 59.07% ± 14.16%, and 35.56% ± 5.50% for the RTVue, Triton, Revo, and PlexE, respectively. Table 2 presents the RPCD for each device stratified by subgroup. The overall ICC across the four devices was 0.31 (95% confidence interval [CI], 0.16–0.47). The ICCs of the six pairs ranged from 0.17 (Triton–PlexE) to 0.34 (RTVue–Triton and Revo–PlexE). 

Table 2.

 

Radial Peripapillary Capillary Densities (%) Obtained From Four OCTA Devices

 

The Bland–Altman plots showed wide limits of agreement for all pairs of comparison, including −31.3% to −5.0% between RTVue and Triton, −73.2% to −25.5% between RTVue and Revo, −37.1% to −14.6% between RTVue and PlexE, −57.0% to −5.3% between Triton and Revo, −23.9% to 8.5% between Triton and PlexE, and −0.6% to 47.6% between Revo and PlexE (Table 3, Supplementary Fig. S1). Constant biases were apparent with mean differences of −18.2% (95% CI, −20.1 to −16.2) between RTVue and Triton, −49.3% (95% CI, −52.9 to −45.8) between RTVue and Revo, −25.8% (95% CI, −27.5 to −24.2) between RTVue and PlexE, −31.2% (95% CI, −35.0 to −27.4) between Triton and Revo, −7.7% (95% CI, −10.1 to −5.3) between Triton and PlexE, and 23.5% (95% CI, 19.9–27.1) between Revo and PlexE. Table 3 summarizes the comparison of RPCDs among the four devices. 

Table 3.

 

Bland–Altman Analyses and ICCs for OCTA Device Measurements

 

The Bland–Altman plots also showed a negative proportional bias in the RTVue–Triton, RTVue–Revo, Triton–Revo, and RTVue–PlexE pairs, as well as a positive proportional bias in the Triton–PlexE and Revo–PlexE pairs (Supplementary Fig. S1). Orthogonal linear regression revealed significant proportional biases in all pairwise comparisons (all P < 0.05). The RTVue–PlexE and Triton–PlexE pairs had the smallest proportional bias (Table 4). 

Table 4.

 

Proportional Biases Among Four Optical Coherence Tomography Angiography Devices

 

Discussion

The RPCDs obtained from the OCTA images were highest in the Revo, followed by the PlexE, Triton, and RTVue. There was poor correlation between all pairs of devices, with the ICCs ranging from 0.17 (Triton–PlexE) to 0.34 (RTVue–Triton and Revo–PlexE).⁷ In all pairwise comparisons, there were relatively wide limits of agreement, with the Triton–Revo pair having the widest limits and the RTVue–Triton pair having the narrowest limits. In addition, constant and proportional biases were apparent in all comparisons. The mean difference was the greatest in the RTVue–Revo pair and the least in the Triton–PlexE pair. 

The use of OCTA for the examination of the optic nerve head is increasing worldwide. RPCD changes have been documented in glaucoma^3,8,9 and other optic neuropathies.^2,6,10,11 In accordance with previous studies, all devices in our study consistently showed that the RPCD in eyes with glaucoma and non-glaucomatous optic neuropathy were lower than that in normal eyes. 

OCTA devices lack consistency in the measurement of macular capillary plexus density. Two studies evaluated the agreement between OCTA devices in measuring superficial capillary plexus (SCP) and deep capillary plexus (DCP) perfusion over the macular region in healthy subjects. One study assessed seven OCTA devices in 18 patients and the other assessed four OCTA devices in 16 patients. Both studies found that, although foveal avascular zone (FAZ) measurements were consistent across devices,⁴ capillary density measurements of both plexuses were significantly different.^4,12 Studies in eyes with choroidal neovascularization (CNV) have also revealed poor agreement in CNV vessel density^13,14 despite excellent agreement between CNV area measurements.¹⁴ Our study is the first to evaluate the agreement of OCTA devices in measuring RPCDs. In addition, we found poor agreement in both healthy eyes and eyes with optic nerve head pathology. These results are in line with the inconsistency of vessel density measurements over macular regions reported in previous studies. 

The RPCDs obtained from the RTVue, Triton, Revo, and PlexE devices all exhibited significant biases. There are several possible factors that could explain these biases. First, different light sources were used to obtain images—namely, spectral domain (SD)-OCTA for the RTVue and Revo and swept-source (SS)-OCTA for the PlexE and Triton. SS-OCTA utilizes a higher scanning speed coupled with a longer wavelength and less sensitivity roll-off; thus, it provides deeper, wider, denser, and faster scans compared with SD-OCTA. The different scanning speeds of the devices can cause variations in vessel detection and image resolution. Second, different algorithms are used in OCTA devices to contrast the retinal blood flow detected by B-scans: RTVue uses split-spectrum amplitude-decorrelation angiography, Revo uses SD-OCT angiography (SOA), PlexE is based on microangiography (OMAG), and Triton uses the OCTA ratio analyses (OCTARA) algorithm.^15–17 Third, the repeat numbers of the B-scan were varied. Finally, although we utilized the same landmark definition to outline the RPC slab, each OCTA platform uses a unique segmentation algorithm, which can lead to signal discrepancy. 

The size of the image can also affect the agreement between vessel density measurements. A study evaluating the interchangeability of three sizes of OCTA angiocubes found that there was high reproducibility of the FAZ area but a poor correlation of SCP, DCP, and choriocapillaris densities over the parafoveal area.¹⁸ However, our study employed an image size of 6 × 6 mm, as this is a scan protocol shared among the four devices; therefore, the results may not be generalizable to other angiocube sizes. 

The stratification of subjects into normal, glaucoma, and non-glaucomatous optic neuropathy groups during enrollment was performed to cover a wide range of optic nerve head conditions. The agreement and correlation among devices in each subgroup displayed similar tendencies compared to the all-subject analysis, with poor to fair ICCs and poor agreement (Table 2). For most pairs, the mean difference was similar among the normal, glaucoma, and non-glaucomatous optic neuropathy groups. However, the mean difference in the Triton–PlexE pair was the lowest for the normal subgroup (−2.15%) and increased for the glaucoma (−8.39%) and non-glaucomatous optic neuropathy (−11.93%) subgroups. This suggests that the disparity is more pronounced in eyes with optic nerve head disorders. 

The results of our study indicated that the vessel densities obtained from the RTVue, Triton, Revo, and PlexE cannot be used interchangeably. Not only did each pair of devices have a large constant offset, but the proportional bias was also significant. This means that the disparity among the devices varies depending on the measurement value. We recommend that RPCDs be monitored using the same device to assess disease progression in each patient. Our findings support the need for a company to develop a device-based normative database to facilitate the interpretation of RPCD across devices. 

A strength of this study is that it is the first to investigate the agreement of RPCDs among four commonly available OCTA devices. These results could be applied at any institutions using these devices. Furthermore, we covered various types of optic nerve head disorders, including glaucoma and non-glaucomatous optic neuropathies. We also used a standard imaging algorithm to convert the images obtained from different devices into common values that could be compared and analyzed for agreement among devices. However, this study had several limitations. First, the sample size of the subgroups was small. Therefore, the subgroup analyses should be interpreted with caution. Second, the peripapillary annulus was the only region evaluated in this study. The results cannot be applied to assess capillary density in other regions, such as the optic nerve head. Finally, this study conducted a quantitative comparison among the devices, whereas a qualitative comparison was not performed. 

In conclusion, the RPCDs obtained using the RTVue, Triton, Revo, and PlexE devices had poor agreement and significant biases and cannot be used interchangeably. 

Acknowledgments

Supported by the Ratchadapiseksompotch Fund, Faculty of Medicine, Chulalongkorn University, Thailand (RA62/063). 

Disclosure: M. Sawaspadungkij, None; S. Apinyawasisuk, None; Y. Suwan, None; M. Aghsaei Fard, None; A. Sahraian, None; J. Jalili, None; S. Chansangpetch, None