Assessment of Retinal Arteriolar Morphology by Adaptive Optics Ophthalmoscopy

Abstract

The advent of adaptive optics ophthalmoscopy (AOO) represents a revolution in digital retinal imaging. Adaptive optics allows retinal microcirculation exploration at a near-histological scale. The best advantage of this technique is represented by the correction of the wave front aberrations observed in all imperfect optical systems, like the eye. The high magnification achieved by AOO allows a differentiation of the arteriolar wall from other perivascular structures, providing more information about the vessel diameter than just the transition in contrast at the border of the blood column of retinal microvessels.

A number of variables are studied for the morphologic analysis of retinal microcirculation: wall thickness, internal and outer diameter, wall-to-lumen ratio (WLR), wall cross-sectional area and wall thickness and lumen irregularity. Blood pressure and age are the main determinants of arteriolar WLR, as found in a large population study. Antihypertensive treatment has an effect on retinal microvascular remodeling, suggesting that adequate control of blood pressure may provide protection form microvascular alterations. The standardization of retinal microvascular measurements is needed, and novel algorithms for the study of already known and novel biomarkers may be soon be available for a better comprehension of the role of retinal microcirculation in cardiovascular disease and the cross-talk between micro- and macrocirculation.

Appendix 4.1 How to Measure Retinal Microvascular Parameters with AoDetect

Up-to-date measurements of retinal arteriolar parameters have been performed on the superotemporal retinal arteriole of the right eye.

Move the blue square along the 400 pixels segment of the arteriole (A) to be analyzed (310 μm, given an eye axial length of 24 mm).

Use the enhanced zoom image on the right side of the screen to optimize the individuation of the arterial segment and the best positioning of the pointer.

Set the pointer (*) in the middle of the arterial lumen (along the axial reflex, white arrow) in order to allow the best algorithm of calculation of the wall thickness and diameter.

Click on the right mouse button to freeze the image.

The blue square will turn red, a yellow line on the transversal axe of the vessel at the height of the central pointer will appear and two light-blue and dark-blue longitudinal lines will delimit, respectively, the internal and external walls along a 50 pixels length (corresponding to 38.7 μm given an eye axial length of 24 mm). These lines are automatically located on the highest gradient peak. To confirm it, click on :

A manual adjustment of the measure can be made on the froze image by viewing the intensity and gradient wave profile. The pointer is moved toward another gradient peak that fits with the delimitation of the wall observed by the operator:

Axial orientation on the yellow line can be equally made. Manual adjustments must be kept at their minimum and only used when clear miscalculations have been automatically made by the software.

Once the measurement has been confirmed, final results of internal and external diameter, both walls’ thickness, wall-to-lumen ratio, and wall cross-sectional area will be displayed in yellow on the left side of the screen (red arrow).

This procedure should be repeated on three consecutive segments granting up to 50% overlap between each consecutive image, as shown in the following pictures.

The same measurements can be done for the venular (V) segment, taking into account only the internal diameter.

The three measurements are averaged, and standard deviation can be calculated in order to obtain the coefficient of variation for the wall thickness and the internal diameter.