Quantitative assessment of the retinal microvasculature using optical coherence tomography angiography

Abstract

Optical coherence tomography angiography (OCTA) is clinically useful for the qualitative assessment of the macular microvasculature. However, there is a need for comprehensive quantitative tools to help objectively analyze the OCT angiograms. Few studies have reported the use of a single quantitative index to describe vessel density in OCT angiograms. In this study, we introduce a five-index quantitative analysis of OCT angiograms in an attempt to detect and assess vascular abnormalities from multiple perspectives. The indices include vessel area density, vessel skeleton density, vessel diameter index, vessel perimeter index, and vessel complexity index. We show the usefulness of the proposed indices with five illustrative cases. Repeatability is tested on both a healthy case and a stable diseased case, giving interclass coefficients smaller than 0.031. The results demonstrate that our proposed quantitative analysis may be useful as a complement to conventional OCTA for the diagnosis of disease and monitoring of treatment.

Fig. 1

Overview of the quantitative OMAG algorithm.

Fig. 2

Representative OMAG images to illustrate the quantitative analysis algorithm. (a) Original en face OMAG image. (b) Vessel area map, a binarized vasculature image using hessian filter and adaptive threshold. This image is used for VAD, VDI, and VCI quantification. (c) Vessel skeleton map, which is obtained by iteratively deleting the pixels in the outer boundary of the vessel area map until one pixel remained along the width direction of the vessels. This image is used for VSD and VDI quantification. (d)Vessel perimeter map, which is obtain by detecting the edge of vessels in the vessel area map and deleting pixels that are not on the edge of vessels. This image is used for VPI and VCI quantification.

Fig. 3

Demonstration of VCI quantification. (a)–(j) are area images (a), (c), (e), (g), and (i) and perimeter images (b), (d), (f), (h), and (j), with increasing visual complexity. Plot (k) shows their corresponding quantification of VCI.

Fig. 4

Quantitative OMAG on a normal case. (a) OMAG en face image with montage scanning protocol, nine smaller cubes are stitched together to achieve a $6.72\mathrm{mm}\times 6.72\mathrm{mm}$ FOV. (b) Illustration of segmentation and color code strategy. Retinal vasculature was segmented into superficial slab, deep slab, and the avascular outer retina slab. Red represents superficial plexus, green represents deep plexus, and blue for outer retinal slab. All three slabs were integrated together and represented in a color image in (a), excluding retinal nerve fiber layer. (c) Vessel density map. This map shows the VAD of image (a), calculated using a kernel moving across the entire OMAG image. (d) Vessel diameter map. This map shows the local averaged VDI using the same calculation method as (c). (e) Vessel complexity map. This map shows the local averaged VCI using the same calculation method as (c). (f)–(h) Integration of quantitative maps (c)–(e) with binary vessel area map [Fig. 2(b)].

Fig. 5

Quantitative OMAG on a NPDR case. (a) FA image. (b) Whole retina OMAG image, correspond to the red rectangular region in the FA image. Scanning area is $6.72\mathrm{mm}\times 6.72\mathrm{mm}$. (c) Flow impairment zone quantification. Capillary drop out region was detected from vessel skeleton image and presented in green color, superimposed on (b). (d) Vessel density map. (e) Vessel diameter map. (f) Vessel complexity map. (g)–(i) Integration of quantitative maps (d)–(f) with binary vessel area map.

Fig. 6

Quantitative OMAG analysis of a BRVO case with repeated scans. (a) and (d) OMAG image of occluded region, covering a FOV of $4.3\mathrm{mm}\times 4.6\mathrm{mm}$. (b) and (e) Vessel area map. (c) and (f) Vessel skeleton map. (g) and (j) Vessel density map integrated with vessel area map. (h) and (k) Vessel diameter map integrated with vessel area map. (i) and (l) Vessel complexity map integrated with vessel area map.

Fig. 7

Repeatability of Quantitative OMAG on a normal case. (a) Whole retina OMAG image of a normal case, covering an area of $6.72\mathrm{mm}\times 6.72\mathrm{mm}$. (e) Repeated scan of the same subject at same location, whole retina OMAG image. (b) and (f) Vessel density map integrated with the binary vessel area map. (c) and (g) Vessel diameter map integrated with the binary vessel area map. (d) and (h) Vessel complexity map integrated with the binary vessel area map.

Fig. 8

(a) Illustration of region of interest selection. An original OCTA image with a FOV of $6.72\mathrm{mm}\times 6.72\mathrm{mm}$. (b) Illustration of quadratic analysis. Red represents superior quadrant, green represents nasal quadrant, blue represents in inferior quadrant, and yellow represents temporal quadrant. (c) ROI selection example of a circle. (d) ROI selection example of a rectangle. (e) ROI selection example of an ellipse. (f) ROI selection example of a circle.

Fig. 9

Quantitative OMAG analysis of a MacTel2 case. (a) FA image of the MacTel2 subject. (b) SS-OCT OMAG image of the whole retina, covering $3\mathrm{mm}\times 3\mathrm{mm}$ area, corresponding to the white rectangular region in FA image. (c) Vessel density map integrated with vessel area map on the whole retina. (d) Vessel diameter map integrated with vessel area map on the whole retina. (e) Vessel complexity map integrated with vessel area map on the whole retina. (f) OMAG image of the deep capillary plexus. (g) Vessel density map integrated with vessel area map of the deep capillary plexus. (d) Vessel diameter map integrated with vessel area map of the deep capillary plexus. (e) Vessel complexity map integrated with vessel area map of the deep capillary plexus.

Fig. 10

Quantitative OMAG comparison between the MacTel2 and normal cases. (a) $3\mathrm{mm}\times 3\mathrm{mm}$ OMAG image of the whole retina of a MacTel2 subject. (b) Vessel density map. (c) Vessel diameter map. (d) Vessel complexity map. (e) $3\mathrm{mm}\times 3\mathrm{mm}$ OMAG image of the whole retina of a normal subject. (f) Vessel density map. (g) Vessel diameter map. (h) Vessel complexity map.

Fig. 11

Three OCTA scans of a patient with MacTel2 monthly apart. a1–a3 shows the superficial retina for the first, second, and third scans, respectively. b1–b3 shows the deep retina for three scans, respectively. c1–c3 shows the outer retina for three scans, respectively. d1–d3 shows the vessel density map for three scans, respectively and e1–e3 shows the vessel diameter maps for three visits while f1–f3 shows the vessel complexity maps for all three visits.

Fig. 12

Illustration of the quadratic analysis. 2-mm circle is selected excluding the FAZ. Superior, inferior, temporal, and nasal quadrants are labeled in different colors as showed in text.