**Abstract**

Adaptive optics ophthalmoscopy opened a new era in the medical retina field. The possibility of obtaining high-resolution retinal images of photoreceptors and retinal vessels addresses new perspectives in retinal physiology and pathophysiology. The overwhelming incidence of diabetes in the global population justifies the need to develop and refine methods of diagnosing early retinal changes, in order to preserve vision and avoid complications. The current grading of diabetic retinopathy is based on clinical changes only. Nevertheless, imaging tools such as optical coherence tomography and optical coherence tomography angiography are also used for screening of this pathology. The corroboration of the information provided by these imaging methods may lay the foundations for a new approach to the definition and diagnosis of diabetic retinopathy.

**Keywords:** diabetic retinopathy, retinal imaging, adaptive optics ophthalmoscopy, wall-to-lumen ratio, cone mosaic, rtx1™

## **1. Introduction: Adaptive optics in retinal imaging**

The principle of adaptive optics (AO) belongs to Babcock since 1953. Fifteen years afterward, the technique started to be used in military setups. Its main purpose was obtaining good satellite surveillance images, even in unfavorable meteorological situations. In 1970, the Soviet and American military managed the real-time correction of atmospheric turbulences when studying laser sources and stars [1, 2]. In 1996, the first AO ophthalmoscope allowed *in vivo* imaging of the human retina, compensating for static and dynamic aberrations of the optical system of the human eye. AO technology has three main components: a Hartmann-Shack sensor, to measure distortion, a deformable mirror to compensate for the distortion, and a control system to calculate the required compensation.

AO has been allowing *in vivo* studies of human retinal photoreceptors mosaic and vessels at a two-micron transversal resolution. As it is lessening the effect of optical aberrations on any measurement, it can be combined with almost any imaging device. AO imaging at histological resolution of the retina opened new perspectives toward early detection, monitoring, and treatment of retinal diseases. The devices above are allowing noninvasive *in vivo* imaging of retinal structures:

• retinal nerve fiber layers; axons of ganglion cells can be studied with AO scanning laser ophthalmoscopy (AOSLO) [3, 4], AO optical coherence tomography (AOOCT) [5], and with AO flood illumination ophthalmoscopy [6];


The following section focuses on an overview of AO retinal imaging methods, namely AO flood illumination imaging, adaptive optics scanning laser ophthalmoscopy, and adaptive optics optical coherence tomography.
