Topographic analysis of short posterior ciliary arteries using wide-field swept-source en-face optical coherence tomography

Purpose: To study the topographic distribution of the short posterior ciliary arteries (SPCA) entry sites into the choroid in normal eyes using structural en-face swept source optical coherence tomography (SS-OCT). Methods: Retrospective analysis of SS-OCT scans (wide-field structural SS-OCT 12x12mm) of 13 healthy subjects was performed. Cross-sectional swept-source OCT scans derived from a volume scan were represented as en-face image display following the Choroid-Scleral Interface to obtain en-face OCT. SPCAs in their last scleral location before choroidal entrance were identified manually, counted and localized by two masked observers. Correlations between two masked observers were analyzed using inter- and intra- class correlation. Results: Accuracy for the choroidal inner and outer border segmentation was 95-99%. Eighteen eyes from 13 normal subjects were included for SPCA analysis. The mean number of arteries was 13.8±3.5 per eye. Thirty-six percent were in the center of the posterior pole image; however, 21% were in the temporal part of the posterior pole. Median accuracy of the detection is 0.94. The correlation between the two observers was fair (0.54). Conclusion: Our algorithm allows visualization of the SPCA at the posterior pole of the eye using wide-field en-face SS-OCT. It can also help the clinicians to study the SPCAs in numerous ocular diseases, particularly its relationship with focal choroidal diseases.


Introduction
Choroid is perfused by several posterior ciliary arteries (PCA) that originate from the ophthalmic arteries and traverse the sclera. 1 Posterior ciliary arteries arise from each part of the ophthalmic artery (first segment, second segment, or angle) and course towards the medial, temporal and superior aspects of the sclera before dividing into several arteries. 2 Recently, with advancing imaging, choroidal vasculature is more and more implicated in the understanding and pathogenesis of retinal diseases. [3][4][5] Considering PCAs supplying choroidal vessels, it may be worthwhile to understand vessel entry into the choroid. With the advent of swept-source optical coherence tomography, there is a growing interest in the detection and visualization of short PCAs (SPCAs) in the posterior pole in-vivo. 6 There have been previous reports showing perforating vessels using OCT in myopes. 7,8 However, previous reports analyzed spectral domain OCT over a smaller area 9 .
Swept source OCT (SS-OCT) using a longer wavelength enabled a better visualization of the outer choroid, until the sclera. In some patients the PCAs are visible with structural SS-OCT. 10 Only the SPCAs coming from the lateral PCA are studied since the medial PCA is too far away from the center to be imaged. SPCAs are shown on cross sectional scans, however, topographic analysis in the entire posterior pole of healthy subjects of these vessels have not been done. In histologic section of sclera, there are between 10 and 20 SPCAs per eye followed by almost the same number of posterior ciliary nerves 11 . The usual variability in the number of SPCAs depends on the number of subdivisions of the PCA before it reaches the sclera region. 12 Current challenges include poor visibility of these vessels due to poor resolution and artefacts due to slicing at the chorio-scleral level.
Our aim is to establish an algorithm to visualize SPCAs entry sites and perform a topographic-based quantitative analysis of the SPCAs at the posterior pole using an en-face SS-OCT of the choroid-sclera interface.

Results:
Performance of automated choroid segmentation algorithm: Accuracy of choroid inner boundary (CIB) detection: We observed that the overall mean dice coefficient (DC) obtained between proposed segmentation and the manual reference is 99.76% (0.16%) while that obtained between two manual segmentations is 99.79% (0.13%) indicating the efficacy of the proposed method. Completely details are presented in Table 1.  (Table 2). Overall, the proposed algorithm is observed to be accurate, consistent and performs at par with the manual delineation of the choroid. Location of vessels: The mean number of arteries localized in the patients were 13.8 (SD=3.5). The distribution of number of arteries at the posterior pole is shown in Figure 1.
Total 36% of the arteries were localized at the center of the posterior pole, 7% in the nasal part, 21% in temporal, 10% in the superior square, 11% in the inferior one. Overall, highest number of vessels (more than a third of the arteries) were present in the center of the posterior pole. However, 21% were in the temporal part of the posterior pole. Among the analyzed scans, there were no areas of the scan where the visualization was better which could bias the results.

Autocorrelation
In a scatter plot of the Pearson correlations of all 18 subjects (or images), the distribution of the Pearson correlation is nonsymmetrical; hence, the statistical median is the measure of preference to quantify the central tendency of the correlation ( Figure  2a). Thus, the median that we obtained for the 18 Pearson correlations that we computed is 0.9406 (very strong correlation).

Cross correlations
The cross correlation for each image between two masked observers showed moderate to high correlation in 15 cases (Figure 2b) with the median cross correlation of approximately 0.54.
We also sorted the intra-class correlations and averaged the 9th and 10th correlations to obtain the median correlation measure; hence, the median accuracy of the detection is calculated 0.94.

Discussion
Visualizing SPCA using in-vivo could be useful to understand chorioretinal and optic nerve diseases. We report visualization of SPCA branches entering into the choroid using SS-OCT. We localized the arteries with fair reproducibility and repeatability between masked observers. The mean number of arteries in the 12x12mm scans is 13.8±3.5. They are mainly located in the center of the posterior pole.
In histologic section of sclera, there are 10 to 20 SPCAs, depending on the number of divisions before the entrance in the sclera. [12][13][14] This number only takes into account the SPCAs emerging from the lateral PCA and not the medial one. There are also paraoptic PCAs that supply the optic nerve heads. Those are maybe the one identified in the nasal square of the OCT. 15 Motaghiannezam et al. succeeded in imaging the very central SPCA vessels on cross sectional scans. However, distribution of vessels and quantification is not possible using cross sectional scans. Therefore, we used wide-field en-face OCT images and flattened the choroid-sclera interface to visualize, possibly, all entry sites of SPCA vessels into the choroid. 6 Puyo et al. also demonstrated SPCAs with cross sectional SS-OCT at the posterior pole in a patient. They confirmed the location of PCAs on laser doppler holography (LDH) (Figure 3). 16 In-vivo, intrascleral vessels could also be localized using ICG angiography in eyes with choroidal atrophy. 10 In 66.9% of the eyes, it was localized in the foveal area, which is more than in our cohort. However, they included highly myopic eyes which may influence the visibility of scleral vessels. In our study, we included emmetrope subjects (mean spherical equivalent was -0.77(SD=2.7) diopters), in whom scleral vessel visibility may be challenging due to choroidal thickness and retinal pigment epithelium. 10 Flattening the Choroid-Sclera Interface was essential to count the number of scleral arteries. Rothenbuehler et al. could image the perforating scleral vessels but in a very short field (4.5 mm centered in the macula) using EDI. 9 With SS-OCT, we could perform images with wilder fields and deeper visualization.
Our technique has enabled us to recognize the arteries and avoid many false positives that could be either signal from the choroid and non vascular hyporeflective spots in the sclera. As we used many stacks to avoid projections artifacts and confirmed the presence of arteries in correlation with cross sectional B scans, to improve the detection and accuracy. There is already growing interest in analyzing the chorioretinal diseases in relation to scleral vessels. In high myopia, Ishida et al. 8 and Watanabe et al. 17 showed CNV next to the location of the scleral vessels. Moriyama et al. 18 showed displaced PCAs entrance in the choroid around the boundaries of the staphyloma. Pedinielli et al. 7 showed PCAs in the border of focal ectasia. However, in high myopia, the sclera is thin so that superficial PCAs could be mistaken for SPCAs because of the high remodeling of the choroid, sclera, and scleral vessels. 10 Amini et al reported that the flow was not decreased in optic neuritis associated with multiple sclerosis in humans. 19 In choroideremia, scleral pits are scleral ectasias associated with choroidal and retinal interruption. Harvey et al. recently could colocalize scleral pits with the pathway of the posterior ciliary arteries. It could be a sign of the involvement of the scleral arteries in choroideremia. 20 Also, in optic nerve disorders, there is an interest in analyzing the SPCA. In ischemic optic neuropathy associated with giant arteritis, the SPCA are the precise location of the occlusion. 21 In dogs, not only the lumen of the arteries but also their number was decreased in glaucomatous eyes. 22 There are many limitations in our study. Small sample size and poor-quality images in eight eyes is one of the major limitations. We expect that improvement in penetration and resolution will help to improvise the image quality and visualize more vessels. We evaluated this technique only in healthy eyes, however, there could be challenges such as increased choroidal thickness or shadowing in diseased eyes which deteriorate scleral vessel visualization. Also, the trajectory of the arteries in the sclera is either oblique or horizontal. Therefore, the precise location of the hyporeflective structure as the entrance of the artery into the choroid remains uncertain.
In conclusion, the use of the proposed technique on SS-OCT images of the posterior pole could identify short posterior ciliary arteries vascularizing the choroid in healthy subjects. This technique/tool is unique, and will help advance our current research in understanding choroidal vessels in several eye diseases. It can also facilitate our research for the automatic detection of blood vessels with an automated quantitative analysis. Our future research will include the evaluation of this tool in diseased eyes, which will further expand our understanding of vascularization patterns with choroidal pathologies.

Methods
This study was carried out according to the Declaration of Helsinki and approved by the University of Pittsburgh Institutional Review Board. Informed consent was obtained for each healthy volunteer who underwent swept source OCT imaging. Every subject underwent best-corrected visual acuity assessment, a complete ophthalmologic examination, and a wide-field SS-OCT 12x12mm in the Plex Elite 9000 device (Carl Zeiss Meditec, Dublin, CA) with fovea as the center. Swept source OCT scans were exported as complete 8-bit volumes. Each OCT volume consists of 1024 B-scans each of resolution 1024x1536. We included healthy subjects. Every subject with diabetes, high blood pressure, or any systemic disease or systemic treatment was excluded. Every subject with a history of maculopathy or retinopathy was excluded as well as high myopic eyes.

Estimation of choroidal inner (CIB) and outer (COB) boundary
The structures in the OCT volume including sclera appear curved due to the spherical shape of the eye. Consequently, to get enface slices of the sclera it is required to restructure the OCT volumes to make the sclera flat. In other words, the scleral region below choroid-sclera interface (CSI) should be flattened. To this end, COB detection is a crucial step in localizing sclera below the choroid. We adopted a modified version of our previous reported method 23 to segment COB. The previous method had two main limitations: (i) detection of CIB and COB was based on two separate approaches and the structural similarity (SSIM) technique employed earlier for obtaining initial COB estimate was computationally intensive; and (ii) individual B-scan processing was performed and inter-slice dependency within the B-scans of a OCT volume was not considered while detecting CIB and COB which may not result in seamless (spatially consistent) boundaries across B-scans. In view of this, in the modified methodology we made two new contributions to address the aforementioned limitations. In particular, they include (i) obtaining initial estimates of both CIB and COB based on single technique i.e., using a two-step exponentiation enhancement method inspired by idiosyncrasies of the OCT imaging; and (ii) performing volumetric smoothing, to correct spurious detection in initial estimates of CIB and COB, leveraging the inherent neighborhood dependency among the B-scans of an OCT volume. The schematic of the proposed method for detecting CIB and COB is depicted in Figure 4, and corresponding implementation output of each step are shown in Figure 5 based on a representative OCT B-scan (Figure 5b) of an OCT volume (Figure 5a). The coordinate orientation of the XYZ axis of OCT volume is also depicted in Figure 4a. In particular, the proposed method involves (i) Detecting initial CIB estimate sequentially in each Bscan and stack all estimates in 3D; (ii) Obtaining final estimate of CIB by smoothing in orthogonal direction to the B-scan orientation; (iii) Detecting initial COB estimate sequentially in each B-scan and stack all estimates in 3D; and (iv) Obtaining final COB estimate by adopting step (ii) of CIB to COB.
Step-by-step details of the method are described in the following.

Pre-processing:
In the OCT image acquisition process, the attenuation of the signal happens more towards the deeper regions, resulting in poor contrast between the vessel region and stromal region in the choroidal layer. Further, there is inherent speckle noise due to coherence of light. To overcome the aforementioned limitations and improve the signal-to-noise ratio, we apply certain preprocessing steps. In particular, second order median filtering with 6x6 tiling and adaptive histogram equalization are applied on the B-scans to reduce the inherent speckle noise and enhance the contrast of the image, respectively (Figures 5c & 5d). Further, the exponentiation enhancement employed in the subsequent steps compensates for the signal attenuation Common enhancement method: The core techniques employed to obtain initial estimates of CIB and COB are exponentiation and nonlinear enhancement. We employed these operations for binarizing OCT B-scan for choroidal stromal-luminal area quantification 24 . These operations are performed on raw OCT intensity scale. Accordingly, for a grayscale intensity B-scan image , raw intensity for each pixel ( , ) is obtained by 4 . ( The exponential enhanced image of is obtained by 25 ) , (2) where is the exponent and we empirically chose = 10. In general, exponentiation enhancement compensates for the signal attenuation suffered at deeper posterior segment layers during OCT image acquisition. Further, the nonlinear enhanced image of ℎ is given by ( , ) = 2 ℎ ( , ) .
The aforementioned operations applied while estimating CIB is slightly different from those applied while estimating COB which will be described in the following sections.

Choroid inner boundary (CIB) detection:
CIB detection is performed to localize the choroid region below retina to facilitate COB detection. As alluded earlier, we first obtain an initial CIB estimate for each B-scan and then smooth them across B-scans to obtain a final smoothed CIB estimate. Initial CIB estimate: We employed exponential and non-linear enhancement operations (equations (1), (2) and (3)) on a flipped OCT image (Figure 5e).
These operations enhance retinal substructures while attenuating the intensities corresponding to the choroid and the sclera (Figure 5f). Specifically, intensities of the retinal pigment epithelium (RPE) and other retinal layers get saturated. This is an inverse operation described for estimating stromal-luminal ratio of the choroid reported earlier. 24 Subsequently, thresholding is performed using a mid-grayscale intensity threshold of 128 to obtain a binarized image. This results in closely placed large connected components of the retinal region. There could be some small spurious components as well. Accordingly, to spurious components and to detect only large connected components belonging to the retinal layers, morphological operations including close, open and connected components algorithm ( (Figure 5g)) is employed. Subsequently, to obtain the initial CIB estimate (Figure 5h-5i). Note that the aforementioned operations will be again employed in the detection of COB in the subsequent steps.
Smoothing in the orthogonal direction: We noticed some spurious detections in the initial CIB estimates of some of the B-scans of the OCT volume especially when there is deformed or depleted retinal pigment epithelium (RPE). In view of this, we proposed to perform smoothing on the stack of all initial CIB estimates of the volume in the orthogonal direction ( − plane) to the plane containing B-scans. In particular, we want to leverage the neighborhood interdependency among the boundary estimates to correct the deviations. For each array of CIB estimates in the orthogonal direction, we applied robust locally estimated scatterplot smoothing (RLOESS) 26 . Following it, we applied tensor voting 23,27 (adopted from our previous work 23 ) in the original Bscan direction ( − plane) to smoothen out minor deformations representing the final CIB estimate (Figure 5j).

Choroid outer boundary (COB) detection:
COB detection also involves processing of B-scans in two stages including scan-wise detection of initial COB estimates and correction of the spurious detections if any in the volume by performing smoothing in the orthogonal direction to the plane containing B-scans.
Initial COB detection: We primarily replaced the structural similarity (SSIM) index part in our earlier work with the exponentiation and nonlinear enhancement operations (equations (1), (2) and (3)). However, unlike during CIB detection, they are applied in the original orientation without flipping the B-scan (Figure 5k-5n). Subsequently, tensor voting 23,27 is employed to generate a continuous smooth initial COB estimate (Figure 5o). Smoothing in the orthogonal direction: Similar to CIB initial estimate, we noticed that the initial COB estimate deviates from the actual boundary in few B-scans. Hence, to surmount those outliers, we again performed RLOESS 26 smoothing on the stack of initial COB estimates in the direction orthogonal to the plane containing OCT B-scans ( Figure 5p).
The detected CIB & COB for the whole OCT volume under consideration is depicted in Figure 5q.
Scleral en-face image extraction: As alluded earlier, the structures in the OCT volume including sclera take spherical shape due the anatomical structure of the eye. Therefore, it is required to flatten COB (choroid-sclera interface) to obtain scleral enface slices. To this end, once the COBs for the entire OCT volume is obtained, the pixel coordinates of the OCT volume are transformed so that the COBs of the all B-scans lie on a plane. On the representative OCT volume, Figure 6 depicts the OCT volume with flattened COB, the OCT volume with peeled off retinal and choroidal layers, and finally the extracted scleral en-face image.

Validation of choroid segmentation algorithm:
Choroidal segmentation algorithm was validated for choroidal inner border (CIB) and choroidal outer border (COB) separately using random 100 scans from five healthy datasets (total 500 scans). The performance of automated detection of CIB and COB was validated by considering the intraobserver variability as the reference. Manual delineations were performed twice by a masked expert using the ImageJ software. In particular, first an initial piece-wise linear boundary was marked by picking sparse locations on the boundary using the `Segmented Line' tool. Subsequently, a smooth boundary was obtained by fitting a spline to the initial boundary by 'Fit Spline' operation (Edit→Selection→Fit Spline).

Dice coefficient (DC) to validate choroid segmentation:
We adopted widely accepted Dice coefficient (DC) metric to compare accuracy algorithmic segmentation of CIB and COB vis-à-vis corresponding manual segmentations and is defined in the following. The value of DC=1 (100%) indicates that two measurements are equal and DC=0 indicates that two measurements are completely uncorrelated.
We considered DC between the two manual segmentations as the yardstick and compared with DC between the proposed algorithmic segmentation and the reference manual segmentation obtained by averaging two manual segmentations Localizing the arteries: The OCT volumes were then rebuilt with A-scan positions adjusted in z to align the chorio-scleral junction to a plane. A substack of 5 to 30 enface projections was chosen manually, to select the best stacks showing the arteries. The observer marked manually the arteries which could be followed between the stacks and which entered the choroid. Nine-squared squares (each 4x4 mm2) were determined from the en-face image. The number of PCAs in each sector was determined using their location. Measurements of the vessels in this manner were performed twice by the two masked observers. Masked observers were masked to their first measurements as well as other observer's measurements. Eyes with identification of less than 10 PCAs due to poor resolution were secondarily excluded from the analysis because it is the minimum number of SPCAs encountered from histological studies. 12

Statistical analysis
Correlation analysis (e.g., Pearson correlation) has been used for determining choroidal vascular density [3][4][5] and developing a feature learning technique. 28 In our approach, interclass (interobserver variation) and intraclass correlations were determined. Statistical analysis was made using ExcelStats. Suppose X and Y are the pixel-based positional information of an artery on a 2D image plane, by considering the bottom left corner as the origin of the image pixels in a cartesian coordinate system. Then, we define Z as the positional information of the artery that was determined by a human expert such that Where determines the robustness of the artery detection. We then used correlation between Zt and Zt+1 as a measure of determining the artery positions on an image, where Zt and Zt+1 satisfy the above equation at trial t with n images (we used n=18).
Then the Pearson correlation is calculated in autocorrelation and cross correlation. Qualification of the correlation is based on the empirical characterization of Divaries correlation coefficient 29,30 which defines the following groups: below 0.20 --> very weak correlation 0.20 to 0.40 --> weak correlation 0.40 to 0.60 --> moderate correlation 0.60 to 0.80 --> strong correlation above 0.80 very strong correlation.
Distribution of arteries (percentages) was analyzed in 9 sectors (each square = 4x4 mm 2 ) over the posterior pole.