Imaging of iris vasculature: current limitations and future perspective

Abstract

Fluorescein and indocyanine green angiography have been the traditional ways to image the vasculature of the iris in the last few decades. Because of the invasive nature of these procedures, they are performed in rare situations, and thus, our understanding about iris vasculature is very limited. Optical coherence tomography angiography (OCTA) is a noninvasive imaging method that enables the detailed visualization of the retinal and choroidal vascular networks. More recently, it has been also used for the examination of the iris vasculature in healthy and disease eyes. However, there is a lack of uniformity in the image acquisition protocols and interpretations in both healthy and pathological conditions. Artifacts of iris OCTA include shadowing, motion, segmentations errors, mirror effects. OCTA devices have an eye-tracking system designed for the posterior segment and the applications of these systems on the anterior segment can determine motion lines, vessel duplication, and vessel discontinuity. OCTA of the iris should always be performed under ambient room lighting to create miosis and reduce iris vasculature changes during the examination. In the near future, eye-tracking systems specifically designed for the iris vessels could permit the follow-up function, and the development of new OCTA metrics could reveal interesting applications of this new imaging technique.

Fig. 1. Anterior segment color picture showing iris features.

Dashed line demarcates the collarette region. Crypts are also visible among the radial streaks. Some iris freckles and Wolfflin nodules are dysplayed. The peripheral circular lines correspond to the contraction furrows.

Fig. 2. Spectral-domain (SD) and swept-source (SS) optical coherence tomography (OCT) of the iris.

A SD-OCT of the iris in a 30 years-old man acquired with Optovue Angiovue, showing the anterior border/stromal layer (thin arrow) and the iris pigment epithelium (thick arrow). Arrowhead shows the failure to identify the anterior iris surface beneath the Schwalbe’s line. B Swept source-optical coherence tomography of the iris in a 36 years-old woman acquired with Topcon Triton showing an excellent visualization of the peripheral iris.

Fig. 3. Multimodal imaging of a light-colored iris of a 52-years-old healthy patient with the normal undilated pupil.

A Anterior segment colour picture shows a light blue-green iris. B, C Early phase fluorescein and indocyanine green angiography (FA and ICGA) of the iris display the arterial vascular filling starting from the root of the iris with a radial orientation. D, E Late phase FA and ICGA show a dense plexus of capillaries in the pupillary region. Note a late leakage of the dye on late phase FA (D). F Optical coherence tomography angiography performed before dye injection clearly shows the minor arterial circle found along the border of the pupil linked with the radially oriented vessels.

Fig. 4. Multimodal imaging of a pigmented iris of a 57-years-old healthy patient with normal undilated pupil.

A Anterior segment colour picture shows a brown iris and corneal gerontoxon. B Early phase fluorescein angiography (FA) of the iris shows how the pigment impairs the evaluation of the vasculature. C Early phase indocyanine green angiography (ICGA) allow the imaging of some vessels, including a vessel with anomalous orientation (white arrow). D, E Late phase FA and ICGA still show a difficult evaluation of the iris vasculature. Note a late leakage of the dye in the superior pupillary margin on late phase FA. (F) Optical coherence tomography angiography performed before dye injection shows flow artifacts and multiple masking effects making the quality of the image very low.

Fig. 5. Spectral-domain optical coherence tomography angiography (SD-OCTA) of the iris of two young patients (30 and 32 years-old) with different pigmentation of the iris, acquired with Optovue Angiovue.

A Anterior segment slit lamp picture shows a light-coloured iris. B En face SD-OCTA of the iris allows a good visualization of the minor arterial circle along the border of the pupil linked with the radially oriented iris vessels within the stroma. Note the roundish peripheral shadowing effect at the iris root on en face scan due to the reflection of the cornea as shown on B-scan. C Manually adjusted B scan segmentation including all the iris tissue. D Anterior segment slit lamp picture reveals a brown dark iris. E En face SD-OCTA of the iris shows how the anterior pigment layer determines shadowing and flow artifacts that impair the iris vasculature evaluation. Note the roundish peripheral shadowing effect at the iris root on en face scan due to the reflection of the cornea as shown on B-scan. F Manually adjusted B scan segmentation including all the iris tissue.

Fig. 6. Spectral-domain optical coherence tomography angiography of the iris of a 32 years-old man acquired with Optovue Angiovue.

A Anterior segment colour picture showing a brown-green iris. (B) Custom segmentation manually adjusted to include all the iris tissue showing a better visualization of the radially oriented iris vessels. Note the roundish peripheral shadowing effect at the iris root on en face scan due to the reflection of the cornea as shown on B-scan. C Custom segmentation manually adjusted to include only the pigment epithelium of the iris. Note the projection of the superficial iris vessels on en face scan with a more pronounced roundish shadowing effect and the red flow signal within the segmentation slab of the B scan.

Fig. 7. Swept source-optical coherence tomography angiography of the iris of a 47 years-old woman acquired with Topcon Triton OCTA with the spacer and without the anterior segment dedicated lens.

Macular 4.5 × 4.5 cube acquired moving the focus on the + level, showing en face and corresponding B-scans with automatic segmentations of superficial plexus (A), deep plexus (B), outer retina plexus (C), and choriocapillaris slab (D).

Fig. 8. Swept source-optical coherence tomography angiography of the iris in a 61 years-old man with brown iris acquired with Topcon Triton plus with spacer and without anterior segment lens before and after pupil dilation.

Both scans were acquired with the same light room conditions. (A) 6 × 6 macular cube acquired before pupil dilation. (B) 6 × 6 macular cube acquired after pupil dilation. Note how the visualization of the iris vessels was worse after dilation.