**5.1. Cone photoreceptor imaging**

Imaging of the cone photoreceptor layer (Fig. 10) is accomplished by focusing on deeper ret‐ inal layers. The variation in cone appearance from dark to bright in Fig. 10 is an effect of the directionality [40] or waveguide nature of the cones. The retinal photoreceptor mosaic pro‐ vides all information to higher visual processing stages and is many times directly or indi‐ rectly affected or disrupted by retinal disease. It is therefore of interest to study various parameters, e.g. photoreceptor spacing, density, geometry, and size, to determine the struc‐ tural integrity of the mosaic. An example of this is given in Fig. 11, where the cone density of the mosaic in Fig. 10 has been calculated. Cone spacing, where possible, was obtained from power spectra of 128×128 pixel sub-regions with a 64 pixel overlap. Spacing (*s*) was converted to density (*D*) using the relation *D* = sqrt(3) / (2s2 ), and the density profile was constructed by fitting a cubic spline surface to the distribution of density values.

#### **5.2. Retinal capillary imaging**

**Figure 9.** Zemax simulation of Hartmann spot image (left) and actual WFS image (right).

Retinal images are obtained by illuminating a 10×10 degree retinal field using a 4-6 ms spectral‐ ly filtered (575±10 nm) Xenon flash. A Canon EF 135mm f/2.0 L photographic lens is used to fo‐ cus reflected light from the dichroic beamsplitter onto the science camera, a 2452×2056 pixel Stingray F-504B monochromatic CCD with a square pixel cell size of 3.45 µm (Allied Vision Technologies GmbH, Stadtroda, Germany). The physical size of the full chip corresponds to a retinal FOV of 8.28×6.94 deg with a pixel resolution of 0.059 mrad (0.974 µm on the retina).

AO retinal imaging reveals information about retinal structures and pathology currently not available in a clinical setting. The resolution of retinal features on a cellular level offers the possibility to reveal microscopic changes during the earliest stages of a retinal disease. One of the most important future applications of this technique is consequently in clinical prac‐ tice where it will facilitate early diagnosis of retinal disease, follow-up of treatment effects,

Both the DCAO demonstrator and the PoC prototype feature a narrow depth of focus, ap‐ proximately 25 µm and 9 µm in the retina, respectively. This allows for imaging of different retinal layers, from the deeper photoreceptor layer to the superficial blood vessel and nerve fiber layers. Images are flat-fielded using a low-pass filtered image to reduce uneven illumi‐ nation [39]. A Gaussian kernel with σ = 8 - 25 pixels is chosen depending on the imaged reti‐ nal layer. A smaller kernel is used for images of the photoreceptor layer and a larger kernel is consequently used for images of superficial layers. Final post-processing is performed by convolving an image with a σ = 0.75 pixel Gaussian kernel to reduce shot and readout noise. As the PoC prototype is still under construction all retinal images shown below have been

*4.2.4. PoC flash and imaging modules*

14 Adaptive Optics Progress

**5. Retinal imaging**

and follow-up of disease progression.

acquired with the DCAO demonstrator.

Retinal capillaries, the smallest blood vessels in the eye, are difficult to image because of their small size (down to 5 µm), low contrast, and arrangement in multiple retinal planes. Even good-quality retinal imaging fails to capture any of the finest capillary details. The pre‐ ferred clinical imaging method is fluorescein angiography (FA), an invasive procedure in which a contrast agent is injected in the patient's bloodstream to enhance retinal vasculature contrast. The narrow depth of focus of both the DCAO demonstrator and the PoC prototype allows for imaging of retinal capillaries by focusing on the upper retinal layers. It is a noninvasive procedure with performance similar to FA [22]. An unfiltered camera raw image of the capillary network surrounding the fovea, the central region of the retina responsible for sharp vision, is shown in Fig. 12, and a flat-fielded image is shown in Fig. 13.

**Figure 10.** DCAO image of cone photoreceptor layer. Variation in cone appearance from dark to bright is an effect of the directionality or waveguide nature of cone photoreceptors.

often not detectable using red-free fundus photography until there is more than 50% nerve fiber loss [41]. Although DCAO imaging does not yet provide information about RNFL thickness it can be used to obtain images with higher resolution and contrast than red-free

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

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

17

**Figure 13.** Image in Fig. 12 after flat-field correction. Uneven flash illumination has been reduced and retinal vessel

**Figure 14.** Montage of four DCAO images of the retinal nerve fibers and blood vessels.

fundus images (Fig. 14).

contrast has been improved.

**Figure 11.** Cone photoreceptor density profile calculated from cone distribution in Fig. 10. Color bar represents cell density in cells/mm2.

#### **5.3. Nerve fiber layer imaging**

Evaluation of the retinal nerve fiber layer (RNFL) is of particular interest for detecting and managing glaucoma, an eye disease that results in nerve fiber loss. Changes in the RNFL are often not detectable using red-free fundus photography until there is more than 50% nerve fiber loss [41]. Although DCAO imaging does not yet provide information about RNFL thickness it can be used to obtain images with higher resolution and contrast than red-free fundus images (Fig. 14).

**Figure 11.** Cone photoreceptor density profile calculated from cone distribution in Fig. 10. Color bar represents cell

Evaluation of the retinal nerve fiber layer (RNFL) is of particular interest for detecting and managing glaucoma, an eye disease that results in nerve fiber loss. Changes in the RNFL are

density in cells/mm2.

16 Adaptive Optics Progress

**Figure 12.** Camera raw DCAO image of foveal capillaries.

**5.3. Nerve fiber layer imaging**

**Figure 13.** Image in Fig. 12 after flat-field correction. Uneven flash illumination has been reduced and retinal vessel contrast has been improved.

**Figure 14.** Montage of four DCAO images of the retinal nerve fibers and blood vessels.
