Visualization of micro-capillaries using optical coherence tomography angiography with and without adaptive optics

Abstract

The purpose of this work is to investigate the benefits of adaptive optics (AO) technology for optical coherence tomography angiography (OCTA). OCTA has shown great potential in non-invasively enhancing the contrast of vessels and small capillaries. Especially the capability of the technique to visualize capillaries with a lateral extension that is below the transverse resolution of the system opens unique opportunities in diagnosing retinal vascular diseases. However, there are some limitations of this technology such as shadowing and projection artifacts caused by overlying vasculature or the inability to determine the true extension of a vessel. Thus, the evaluation of the vascular structure and density based on OCTA alone can be misleading. In this paper we compare the performance of AO-OCT, AO-OCTA and OCTA for imaging retinal vasculature. The improved transverse resolution and the reduced depth of focus of AO-OCT and AO-OCTA greatly reduce shadowing artifacts allowing for a better differentiation and segmentation of different vasculature layers of the inner retina. The comparison is done on images recorded in healthy volunteers and in diabetic patients with distinct pathologies of the retinal microvasculature.

Keywords: (110.1080) Active or adaptive optics; (110.4500) Optical coherence tomography; (170.3890) Medical optics instrumentation; (170.4470) Ophthalmology.

Fig. 1

Schematic diagram showing the different steps for calculating the Weber contrast value. These steps were performed for each volume and each plexus. The fore and background regions are indicated in white in the images on the right hand side.

Fig. 2

Representative B-scan images of a healthy volunteer extracted from a volume acquired at a location vertical = 0° and nasal = 3° from the fovea. (A) Intensity image (linear grey scale), (B) AO-OCTA image (linear grey scale). Visualization 1 shows a fly through of the B-scan images. The dashed lines show the corresponding borders for the en-face projection images. The different regions containing the vessel beds are indicated with the numbers 1-4. Blue: Posterior border of the topmost layer including the retinal nerve fiber layer (RNFL). Violet: Posterior border of the ganglion cell layer (anterior border is the blue line). Red: Posterior border of the inner plexiform layer. Green: Posterior border of the outer plexiform layer.

Fig. 3

Side by side comparison between AO-OCT intensity images from different vasculature beds (top row: A, B, C, D) and the AO-OCTA images (bottom row: E, F, G, H). From left to right the corresponding depth integrated layers (cf. Fig. 2) are the nerve fiber layer, ganglion cell layer, inner plexiform layer and outer plexiform layer (the red arrow in E indicates a vessel that is not clearly visible in the corresponding intensity image. The red arrow in H indicates areas with low signal intensity, green arrow points to an artifact).

Fig. 4

The mean Weber contrast determined for 4 volumes that were recorded at 4 different locations. Blue: WC for the angiography images. Yellow: WC for the AO-OCT intensity images. Error bars indicate the standard deviation. The x-axis indicates the number of plexus as shown in Fig. 2.

Fig. 5

OCTA images retrieved from a healthy volunteer. (A) En-face projection depth integrated over the nerve fiber layer (Layer No. 1 in Fig. 2, the red arrow indicates a capillary within the nerve fiber layer, (B) En-face projection depth integrated over layer No.2 in Fig. 2, (C) En-face projection between the vessels of layer No. 2 and layer No.3 (The red circles indicate connecting vessels that are perpendicular to the vasculature beds and connect the vessel layers (B) and (D). The blue arrow points to a connecting vessel that is more inclined than the other connecting vessels, the red arrow indicates a vessel that can be seen in the B-scan image (G). (D) En-face projection over layer No. 3, (E) En-face projection between the vessels of layer No. 3 and layer No. 4 (The red circles indicate vessels that connect the central capillary plexus in (D) with the outer capillary plexus in (F), the blue arrow points to a connecting vessel with a large inclination, the yellow arrow points to a vessel that can be observed in the B-scan image displayed in (H). (F) En-face projection over layer No. 4. (G) Representative B-scan averaged over 3 frames extracted from Visualization 2 at the location marked with a white line in (A) (The red arrow points to a connecting vessel that can be observed in (C). (H) Representative averaged B-scan extracted from Visualization 2 at the location marked with a red line in (A) (The yellow arrow points to a vessel that can be seen in (E). The volume was recorded at a location 1.5° temporal and 3° inferior to the fovea.

Fig. 6

Vessel layer posterior to the nerve fiber layer (region 2 in Fig. 2). (A) AO-OCTA image consisting of 25 images that were stitched together. (B) Angiogram recorded with the commercial instrument. Field of view: ~7° × 7°. Depth integration of en-face images was identical for both images and is indicated in Fig. 2.

Fig. 7

Vessel layer below the inner plexiform layer (region 3 in Fig. 2). (A) AO-OCTA image consisting of 25 images that were stitched together. (B) Angiogram recorded with the commercial instrument. Field of view: 7°x7°. Depth integration of en-face images was identical for both images.

Fig. 8

Vessel layer below the outer plexiform layer (region 4 in Fig. 2). (A) AO-OCTA image consisting of 25 images that were stitched together. (B) Angiogram recorded with the commercial instrument. Field of view: 7° x 7°. Depth integration of en-face images was identical for both images.

Fig. 9

(A) Summation of the vessel layers shown in Figs. 6(A), 7(A), 8(A). (B) The different vessel layers shown in Figs. 6(A), 7(A) and 8(A) color coded to indicate the different depth locations acquired with AO-OCTA. Field of view: 7° × 7°.

Fig. 10

AO-OCTA images (left column) and OCTA images (right column) of selected regions of interest compared side by side. The retinal vessels are imaged in the macula region and are depth integrated over region 2 in Fig. 2. The field of view approximately corresponds to the area that can be imaged with the AO-OCTA using a single volume acquisition. (A) AO-OCTA image showing a small vessel (indicated with a red arrow) that proceeds parallel to a larger vessel. (B) The same region as in A) imaged with the commercial instrument. (C) AO-OCTA image showing a continuous vessel loop (indicated with the red arrow). (D) In this image the vessel loop (indicated with the red arrow) seems not to be connected with the main vessel on the left side. (E) AO-OCTA image showing three vessel junctions (indicated with a red arrow). (F) The same region as in E) imaged with the commercial instrument. The vessel junctions cannot be observed. Instead, closed vessel loops can be seen.

Fig. 11

OCTA and AO-OCTA images of a patient with diabetic retinopathy. (A) Overview OCTA image recorded with a commercial instrument (the red square indicates the region of interest that has been imaged using AO-OCTA). (B) Enlarged region of interest (indicated by the red square in (A) depth integrated over the anterior layers (region 1 and 2 in Fig. 2). (C) The same region as in (B) but depth integrated over deeper retinal layers (region 3 and 4 in Fig. 2). (D) OCT B-scan recorded at the center of the region of interest shown in (A). (E) En-face AO-OCTA image depth integrated over the region between the green lines shown in (G). (F) En-face AO-OCT intensity image depth integrated over the same region as in (E). (G) AO-OCTA B-scan showing the microaneurysm (H) AO-OCT intensity image at same location as in (G). The red arrows in the images show the location of a microaneurysm. The green arrows point to a hard exudate. The blue arrows indicate a small capillary that appears to perform a twisted loop and is embedded in highly scattering media.