Automated Quantification of Nonperfusion in Three Retinal Plexuses Using Projection-Resolved Optical Coherence Tomography Angiography in Diabetic Retinopathy

Abstract

Purpose

The purpose of this study was to evaluate an automated algorithm for detecting avascular area (AA) in optical coherence tomography angiograms (OCTAs) separated into three individual plexuses using a projection-resolved technique.

Methods

A 3 × 3 mm macular OCTA was obtained in 13 healthy and 13 mild nonproliferative diabetic retinopathy (NPDR) participants. A projection-resolved algorithm segmented OCTA into three vascular plexuses: superficial, intermediate, and deep. An automated algorithm detected AA in each of the three plexuses that were segmented and in the combined inner-retinal angiograms. We assessed the diagnostic accuracy of extrafoveal and total AA using segmented and combined angiograms, the agreement between automated and manual detection of AA, and the within-visit repeatability.

Results

The sum of extrafoveal AA from the segmented angiograms was larger in the NPDR group by 0.17 mm² (P < 0.001) and detected NPDR with 94.6% sensitivity (area under the receiver operating characteristic curve [AROC] = 0.99). In the combined inner-retinal angiograms, the extrafoveal AA was larger in the NPDR group by 0.01 mm² (P = 0.168) and detected NPDR with 26.9% sensitivity (AROC = 0.62). The total AA, inclusive of the foveal avascular zone, in the segmented and combined angiograms, detected NPDR with 23.1% and 7.7% sensitivity, respectively. The agreement between the manual and automated detection of AA had a Jaccard index of >0.8. The pooled SDs of AA were small compared with the difference in mean for control and NPDR groups.

Conclusions

An algorithm to detect AA in OCTA separated into three individual plexuses using a projection-resolved algorithm accurately distinguishes mild NPDR from control eyes. Automatically detected AA agrees with manual delineation and is highly repeatable.

Diabetic retinopathy (DR), characterized by capillary nonperfusion, vascular hyperpermeability, and neovascularization, is a leading cause of blindness.^1–3 Capillary nonperfusion is an early finding of DR and increases with severity of DR. The Early Treatment of Diabetic Retinopathy Study (ETDRS) qualitatively evaluated macular ischemia using fluorescein angiography (FA) and found it to have predictive value for progression of disease.^4,5 Such rigorous grading of FA, however, is seldom done in clinical practice. Accurate, objective, and automated evaluation of macular ischemia could be a valuable biomarker for DR that is also practical clinically. With FA, dependence on early transit for macular capillary details, obscuration of details due to dye leakage, and variability of contrast have limited the clinicians' ability to assess nonperfusion objectively. Optical coherence tomography angiography (OCTA), which rapidly provides high-contrast capillary details without the need for dye injection, makes such an objective evaluation of the capillaries more practical.^6–13

Previously, we reported that automatically quantified total avascular area (AA) quantified on a 6 × 6 mm² inner retinal OCTA-detected DR with high sensitivity and specificity, and it agreed with AA seen on FA.⁷ In that study, however, most of the diabetic participants had proliferative DR. Whether AA would detect DR with similar accuracy in less severe disease was uncertain.

Recently, we also reported on projection-resolved (PR) OCTA, a new technique to visualize three distinct retinal plexuses by solving the problem of projection artifacts that blurred the plexuses together.¹⁴ In DR eyes, we found that individual plexuses revealed more areas of capillary dropout than combined inner retinal angiograms.¹⁵ Projection-resolved OCTA, by visualizing the three plexuses separately, increased the sensitivity of detecting capillary nonperfusion.

Taking advantage of these advances, this study evaluated the diagnostic accuracy and repeatability of a distance-transform map-based automated algorithm to quantify AA in three individual plexuses on 3 × 3 mm² PR OCTA macular angiograms. By applying this technique to patients with mild nonproliferative diabetic retinopathy (NPDR), we assessed the value of automatic quantification of nonperfusion in early DR.¹⁶

Methods

Data Acquisition

Healthy volunteers and diabetic participants with mild NPDR were recruited from Casey Eye Institute of Oregon Health and Science University (OHSU). Eyes with nondiabetic macular pathology, media opacity, or other significant eye disease were excluded. Informed consent was obtained, and the study was approved by the Institutional Review Board of OHSU. The study complied with the Declaration of Helsinki and Health Insurance Portability and Accountability Act. Clinicians initially graded the severity of DR using International Clinical Diabetic Retinopathy Severity Scale.¹⁷ A masked grader examined ETDRS seven-field color photographs to confirm disease severity.

Two 3 × 3 mm scans with 2-mm depth were obtained in one eye of each participants within a visit using a commercial spectral-domain OCT system (RTVue-XR; Optovue, Fremont, CA, USA) with a center wavelength of 840 nm, a full-width half maximum bandwidth of 45 nm, and an axial scan rate of 70 kHz.⁷ In the fast transverse scanning direction, 304 axial scans were sampled to obtain a single 3-mm B-scan. Two repeated B-scans were captured at a fixed position before proceeding to the next location. A total of 304 locations along a 3-mm distance in the slow transverse direction were sampled to form a three-dimensional data cube. All 608 B-scans in each data cube were acquired in 2.9 seconds.

Based on the volumetric OCT reflectance signal, the scanning software computed a signal strength index (SSI), which is often used as an indicator of scan quality.

Data Processing

Blood flow was detected using the split-spectrum amplitude decorrelation (SSADA).^18–20 Projection artifacts were suppressed by the PR OCTA algorithm.¹⁴ Optical coherence tomography structural images were obtained by averaging two repeated B-scans. The structural and angiography data were generated simultaneously on each scan. For each scan, one x-fast scan and one y-fast scan were registered and merged through an orthogonal registration algorithm to remove motion artifacts.^21,22

A directional graph search algorithm identified structural boundaries²³: the inner limiting membrane (ILM), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), and outer nuclear layer (ONL). The superficial plexus was defined as the slab within the inner 80% of the ganglion cell complex (GCC) and minimum thickness of 61 μm. The intermediate plexus was defined as vessels in the outer 20% of GCC and inner 50% of INL. The deep plexus was defined as vessels in the remaining slab internal to the OPL and minimum thickness of 37 μm. We chose a proportional rather than fixed segmentation scheme because the three plexuses merge at the edge of the foveal avascular zone (FAZ),²⁴ and a fixed segmentation scheme can result in arbitrary and meaningless differences in FAZ size between the vascular plexuses. In the normal eye, this scheme shows each plexus reaching the edge of the FAZ and avoids differences in the measurement of FAZ of three plexuses caused solely by the segmentation method (Campbell JP, Zhang M, Hwang TS, et al., manuscript submitted, 2016).

These three plexuses have been well characterized histologically in nonhuman primates and recently in human cadaveric eyes.^24,25–28 Top view, maximum decorrelation projection of each slab created en face OCTA of each plexus.²³ A combined angiogram showing the more superficial plexuses placed on top with different color showed the relationship between the plexuses.^18,23

The image processing strategy developed in this work for nonperfusion quantification is mainly composed of three steps: preprocessing; vessel distance transform; and morphologic operations. This is outlined in Figure 1.

Figure 1.

Overview of the proposed algorithm for quantification of capillary nonperfusion.

First, the original angiogram (Figs. 2A, 3A) was first enhanced using a two-scale (σ = 1 and 2 pixels) Frangi vesselness filter (step 1 in Figs. 1, 2B, 3B).²⁹ This filter enhances vessels by obtaining vesselness measure on the basis of eigenvalues of the second-order local structure of the angiogram (Hessian) and suppresses background noise.^30–32

Figure 2.

Reflectance-adjusted thresholding on a normal eye with a low signal strength region (yellow circle), caused by a vitreous opacity. The low signal region seen on the original angiogram (A) persists with vessel filter (B) and binary filter with fixed threshold (C), falsely simulating capillary dropout. An en face map of the GCL and IPL reflectance amplitude map (D) is filtered and scaled to create the reflectance-adjusted threshold (E). Applying this, the binary vessel mask (F) does not show an area of false capillary drop out in the low reflectance area.

Figure 3.

Nonperfusion detection in the superficial plexus of an NPDR eye. The original angiogram (A) is enhanced by a vesselness filter (B) and processed with a binary vessel mask with reflectance-adjusted thresholding (C), which is used to create a vessel distance map (D) by applying distance transform. Then morphologic operations removed regions with vessel distance less than four pixels (40 μm) (E) and then they eroded by a five-pixel-wide square kernel and threw away areas with minimum area smaller than eight pixels or a minor axis smaller than two pixels (F). Then remaining regions were dilated by seven-pixel-wide square kernel pixels (G). (H) Resulting avascular area (light blue) overlaid on the enhanced angiogram.

To distinguish flow signal from the background noise, we previously used a fixed threshold.⁷ However, the flow noise floor depends on the OCT reflectance signal,^14,33,34 which may vary among scans and even within a single scan. Consequently, with the fixed cutoff, signal strength instability can cause within-visit variation, leading to false detection of capillary dropout from the locally reduced signal (e.g., by vitreous opacity; Fig. 2C). In this study, we established a reflectance-adjusted threshold using en face structural OCT of the ganglion cell layer (GCL) and IPL, which have moderate reflectance and are nearly free of cysts and exudates, making them good reference layers. The mean projection of reflectance between GCL and IPL (Fig. 2D) was taken as logarithm (S) and filtered with a 15 × 15 pixel Gaussian operator G, with an SD of 8 pixels and scaled (step 2 in Fig. 1) to generate a threshold image (T_xy; Fig. 2E)

The filter size and SD were set empirically. The scaling parameters a and b were trained from scans from two control eyes with in-scan signal variation and two NPDR eyes, identifying maximum avascular area in NPDR while only identifying the FAZ as an avascular area in control eyes. In this study, a was 8 × 10⁻⁵ and b was 8 × 10⁻². A relatively high threshold (maximum of T) was assigned to the 0.6-mm-diameter central area to facilitate a noise suppression in FAZ (Fig. 2E). The center of FAZ was identified manually.

This threshold image was then compared with the enhanced angiogram to determine capillary and static tissue from the enhanced angiogram, generating a binary vessel mask by thresholding the angiogram adaptively (step 3 in Figs. 1, 2F, 3C)

Figure 2 shows that false detection of capillary nonperfusion can be avoided by this reflectance-adjusted threshold (Fig. 2F) over fixed thresholding (Fig. 2C).

Next, a vessel distance map was obtained by applying Euclidean distance transform to the binary vessel mask (Fig. 3D). Euclidean distance transforms of a binary image assign a number that is the distance between that pixel and the nearest nonzero pixel of the binary image³⁵

The value in the vessel distance map represents the distance of a given pixel (x, y) to its nearest vessel (x′, y′). This strategy does not give disproportionate value to larger vessels like the vessel density map, allowing more accurate detection of capillary nonperfusion. This method ignores the physiologic capillary-free area along the large vessels in normal eyes, which had been frequently detected as a nonperfusion area with our previous simple thresholding method.³⁶

Finally, the nonperfusion map was acquired by thresholding the vessel distance map (DT > 4; Fig. 3E) and applying morphologic operations. The morphologic operations included (1) an erosion by a five-pixel-wide square kernel, (2) elimination of areas smaller than eight pixels or whose minor axis length was smaller than two pixels (Fig. 3F), and (3) dilation by a seven-pixel-wide square kernel (Fig. 3G). These parameters were empirically determined so that these operations limit AA to relatively large, smooth contiguous areas and avoid detecting nonphysiologic areas as AA. This procedure quantified each of the three plexuses and the total inner retinal angiogram. The total AA (TAA) and the extrafoveal AA (EAA), defined as the AA outside the 1-mm central circle, were separately tabulated to minimize the contribution of normal variations in foveal avascular zone size. These values were corrected for magnification variation associated with axial length variation as previously described.³⁷

Data Analysis

The Mann-Whitney U-test was used to compare the detected AA between the patients with NPDR and the healthy controls. The diagnostic accuracy of each parameter was assessed by sensitivity with a fixed specificity and the area under the receiver operating characteristic curve (AROC). Repeatability of the detected AA was assessed by evaluating the pooled SD for eyes that had two scans with an SSI greater than 54. We also compared the detected AA in different plexuses in NPDR. All statistical tests were done using SPSS, version 20 (IBM, Armonk, NY, USA).

The AA in healthy controls identified by the automated procedure was compared with manual grading for detection accuracy. The manual results were delineated by using the free selection tool in GIMP 2.8 (https://www.gimp.org; provided in the public domain by The Gimp Team). The Jaccard coefficient, defined as the area of intersection divided by the area of union, measured the similarity between the automated result and manual result. The false-positive and -negative errors were also computed.

Results

Eyes from 13 healthy volunteers (mean [SD] age: 43 [13] years, 10 women) and 13 participants with mild NPDR (mean [SD] age: 59 [8] years, 7 women) were imaged, and one eye from each participant was randomly chosen for inclusion in the study. The SSI of the control group ranged from 59 to 88 and in the NPDR group from 54 to 81. No eye in the study had cysts in the inner retina. In the control group, the automated algorithm detected AA only inside the central 1-mm circle in combined one-layer or three-layer angiograms (Fig. 4), with the exception of one eye that had an extrafoveal AA of 0.01 mm². The AA in normal controls detected by automated algorithm agreed with manual grading with Jaccard indices of 0.85, 0.82, and 0.81 for superficial, intermediate, and deep angiograms, respectively (Table 1), and was independent of signal strength variation between or within scans (Fig. 5).

Figure 4.

Avascular area in two scans of a normal eye with high SSI. The AA is limited to the foveal avascular zone and is similar in size and shape in all scans whether in individual plexuses or combined inner retinal angiogram. The EAA is defined as AA outside the white 1-mm circle.

Table 1.

Agreement Between Automated Detection and Manual Delineation of the Macular AA in Normal Controls

Figure 5.

Two normal control eyes with relatively low SSI scans in individual plexuses and combined inner retinal angiogram. No avascular area outside the foveal avascular zone is detected.

In the NPDR group, the automated algorithm identified AA that was frequently incongruent between the three plexuses. As a result, it sometimes failed to detect any AA outside the FAZ when applied to the combined inner retinal angiogram in eyes with AA in individual plexuses (Fig. 6). The AA in the superficial and deep plexuses tended to be larger than in the intermediate plexus (Table 2).

Figure 6.

Two eyes with mild NPDR. Individual plexuses show incongruent areas of capillary nonperfusion that are not detected with combined inner retinal angiogram.

Table 2.

Avascular Area in Diabetic Retinopathy and Control Eyes

There was no significant difference in EAA or TAA between the control and NPDR groups when the algorithm was applied to the combined one-layer angiogram. When three plexuses were individually evaluated, the EAA and TAA in each of the three plexuses were significantly larger in the NPDR group compared with the control group (Table 2).

Holding the specificity at 95%, the three plexuses showed that the sum of EAA was the most sensitive with the best diagnostic accuracy at an AROC of 0.99 (Table 2). Total AA had lower sensitivity and diagnostic accuracy.

Within-visit repeatability was assessed using eyes with two scans in the same visit with an SSI greater than 54 (Table 3). Twenty-two eyes from control participants and 17 eyes from NPDR participants had two scans with adequate signal strength. The pooled SD of AA ranged from 0.000 to 0.024 mm² in controls and from 0.035 to 0.051 mm² in NPDR patients (Table 3). For parameters with a statistical difference between the NPDR and control group, the pooled SDs were smaller than the difference between the two groups. For all other parameters, the pooled SDs were smaller than the population SDs (comparing Table 2 and Table 3).

Table 3.

Within-Visit Repeatability of AA Measurements

The participants in the NPDR group were older than the control group. The possible relationship between age and AA was examined with a Pearson test. The correlation coefficients of superficial, intermediate, and deep plexuses for EAA and participant age were 0.129 (P = 0.674), 0.033 (P = 0.915), and 0.19 (P = 0.534), respectively. The correlation coefficients of superficial, intermediate, and deep plexuses for TAA were 0.179 (P = 0.558), 0.051 (P = 0.868), and 0.233 (P = 0.443).

Discussion

We improved the method for automated quantification of nonperfusion using OCTA by combining several image processing techniques. Projection-resolved OCTA allowed visualization of three distinct retinal plexuses and detection of capillary abnormalities not visible in combined inner retinal angiograms. A vesselness filter removed the spurious pixels that can interfere with the detection of actual gaps between vessels. The use of a vessel distance map minimized disproportionate contribution of large vessels in determining vessel density abnormality and eliminated the undesirable detection of a normal avascular area along large vessels of the superficial plexus as AA. By using local reflectance signal strength as a reference for separating flow signal and background noise, false-positive detection of nonperfusion is minimized, and the repeatability of measurement was improved. Morphologic operations further reduce false-positive detection of AA.

By applying these techniques to eyes with mild NPDR, the total nonperfusion area outside the central 1-mm circle could distinguish NPDR from control eyes with high diagnostic accuracy, even in eyes where the combined inner retinal angiogram did not reveal any nonperfusion. This study confirms the potential of automatically quantified AA using OCTA as a biomarker in DR, even in less severe disease.

The FAZ is known to have significant variability in normal and diabetic patients.^38–40 With segmentation into individual plexuses, the FAZ size can be further affected by segmentation scheme. Exclusion of the central 1-mm circle factored out this normal variation of FAZ and allowed good diagnostic accuracy of AA quantification.

In this cohort of mild NPDR patients, the EAA was larger in superficial and deep plexuses compared with the intermediate plexus, suggesting that nonperfusion in the intermediate plexus may occur later in the disease process of DR. A prospective study with a larger cohort is needed to verify this.

This study is limited by small numbers and a cross-sectional design. The two groups were not age or sex matched. However, we did not observe a significant age effect in the measured AA in any vascular plexus. Furthermore, the DR group lacked eyes with significant macular edema. Large cysts and exudates may interfere with this approach of quantifying AA. More advanced techniques may be required to generalize this method to eyes with macular edema.

Also, the study had a limited field of view of 3 × 3 mm² area. The significance of peripheral pathology in DR is well established.⁴¹ Although this approach had excellent sensitivity and specificity for detecting DR, a wider field of view with faster machines is likely necessary to further improve the utility of OCTA and its biomarkers in the management of DR.