**1. Introduction**

Retinal imaging is limited due to optical aberrations caused by imperfections in the optical media of the eye. Consequently, diffraction limited retinal imaging can be achieved if optical aberrations in the eye are measured and corrected. Information about retinal pathology and structure on a cellular level is thus not available in a clinical setting but only from histologi‐ cal studies of excised retinal tissue. In addition to limitations such as tissue shrinkage and distortion, the main limitation of histological preparations is that longitudinal studies of dis‐ ease progression and/or results of medical treatment are not possible.

Adaptive optics (AO) is the science, technology and art of capturing diffraction-limited im‐ ages in adverse circumstances that would normally lead to strongly degraded image quality and loss of resolution. In non-military applications, it was first proposed and implemented in astronomy [1]. AO technology has since been applied in many disciplines, including vi‐ sion science, where retinal features down to a few microns can be resolved by correcting the aberrations of ocular optics. As the focus of this chapter is on AO retinal imaging, we will focus our description to this particular field.

The general principle of AO is to measure the aberrations introduced by the media between an object of interest and its image with a wavefront sensor, analyze the measurements, and calculate a correction with a control computer. The corrections are applied to a deformable mirror (DM) positioned in the optical path between the object and its image, thereby ena‐ bling high-resolution imaging of the object.

Modern telescopes with integrated AO systems employ the laser guide star technique [2] to create an artificial reference object above the earth's atmosphere. Analogously, the vast ma‐

jority of present-day vision research AO systems employ a single point source on the retina as a reference object for aberration measurements, consequently termed guide star (GS). AO correction is accomplished with a single DM in a plane conjugated to the pupil plane. An AO system with one GS and one DM will henceforth be referred to as single-conjugate AO (SCAO) system. Aberrations in such a system are measured for a single field angle and cor‐ rection is uniformly applied over the entire field of view (FOV). Since the eye's optical aber‐ rations are dependent on the field angle this will result in a small corrected FOV of approximately 2 degrees [3]. The property of non-uniformity is shared by most optical aber‐ rations such as e.g. the well known primary aberrations of coma, astigmatism, field curva‐ ture and distortion.

of the cornea, the pupil, and the lens, project light from the outside world to the sensory retina (Fig. 1, left). The amount of light that enters the eye is controlled by pupil constric‐ tion and dilation. The human retina is a layered structure approximately 250 µm thick [23, 24], with a variety of neurons arranged in layers and interconnected with synapses

Dual Conjugate Adaptive Optics Prototype for Wide Field High Resolution Retinal Imaging

http://dx.doi.org/10.5772/53640

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**Figure 1.** Schematic drawings of the eye (left) and the layered retinal structure (right). (Webvision, http://webvi‐

Visual input is transformed in the retina to electrical signals that are transmitted via the optic nerve to the visual cortex in the brain. This process begins with the absorption of photons in the retinal photoreceptors, situated at the back of the retina, which stimulate several interneurons that in turn relay signals to the output neurons, the retinal ganglion cells. The ganglion cell nerve fiber axons exit the eye through the optic nerve head (blind

Unlike the regularly spaced pixels of equal size in a CCD chip the retinal photoreceptor mo‐ saic is an inhomogeneous distribution of cone and rod photoreceptors of various sizes. The central retina is cone-dominated with a cone density peak at the fovea, the most central part of the retina responsible for sharp vision, with a decrease in density towards the rod-domi‐ nated periphery. Cones are used for color and photopic (day) vision and rods are used for

Blood is supplied to the retina through the choroidal and retinal blood vessels. The choroi‐ dal vessels line the outside of the eye and supply nourishment to the photoreceptors and outer retina, while the retinal vessels supply inner retinal layers with blood. Retinal capilla‐ ries, the smallest blood vessels in the eye, branch off from retinal arteries to form an intricate network throughout the whole retina with the exception of the foveal avascular zone (FAZ). The FAZ is the capillary-free region of the fovea that contains the foveal pit where the cones are most densely packed and are completely exposed to incoming light. Capillaries form a superficial layer in the nerve fiber layer, a second layer in the ganglion cell layer, and a third

sion.med.utah.edu/book/part-i-foundations/simple-anatomy-of-the-retina/)

(Fig. 1, right).

spot).

scotopic (night) vision.

layer running deeper into the retina.

A method to deal with this limitation of SCAO was first proposed by Dicke [4] and later de‐ veloped by Beckers [5]. The proposed method is known as multiconjugate AO (MCAO) and uses multiple DMs conjugated to separate turbulent layers of the atmosphere and several GS to increase the corrected FOV. In theory, correcting (in reverse order) for each turbulent lay‐ er could yield diffraction limited performance over the entire FOV. However, as is the case for both the atmosphere and the eye, aberrations do not originate solely from a discrete set of thin layers but from a distributed volume. By measuring aberrations in different angular directions using several GSs and correcting aberrations in several layers of the eye using multiple DMs (at least two), it is possible to correct aberrations over a larger FOV than com‐ pared to SCAO.

The concept of MCAO for astronomy has been the studied extensively [6-12], a number of experimental papers have also been published [13-16], and on-sky experiments have recent‐ ly been launched [17]. However, MCAO for the eye is just emerging, with only a few pub‐ lished theoretical papers [3, 18-21]. Our group recently published the first experimental study [21] and practical application [22] of this technique in the eye, implementing a labora‐ tory demonstrator comprising multiple GSs and two DMs, consequently termed dual-conju‐ gate adaptive optics (DCAO). It enables imaging of retinal features down to a few microns, such as retinal cone photoreceptors and capillaries [22], the smallest blood vessels in the reti‐ na, over an imaging area of approximately 7 x 7 deg2 . It is unique in its ability to acquire single images over a retinal area that is up to 50 times larger than most other research based flood illumination AO instruments, thus potentially allowing for clinical use.

A second-generation Proof-of-Concept (PoC) prototype based on the DCAO laboratory demonstrator is currently under construction and features several improvements. Most sig‐ nificant among those are changing the order in which DM corrections are imposed and the implementation of a novel concept for multiple GS creation (patent pending).
