During the Macula of Paris event, sponsored in part by Imagine Eyes, Pr. Wolfgang Drexler offered an outstanding presentation highlighting the results of his work in retinal imaging using OCT enhanced with adaptive optics and the company's founder Nicolas Chateau presented his views on the future of adaptive optics in retinal imaging to a distinguished group researchers and clinicians in the field of Age-Related Macular Degeneration (AMD).  Click the "read more" link to read the abstract.

Adaptive optics in retinal imaging

Nicolas Chateau, PhD, Imagine Eyes, Orsay, France

OCT technologies have enabled doctors to image the eye fundus with high longitudinal resolution and observe the multiple layer structure of the retina. However, current imaging instruments, including OCT and SLO systems, are limited by their lateral resolution (in the range of 15-20μm) and therefore cannot effectively image retinal cells. This limitation has been attributed to optical imperfections, also known as ocular wavefront aberrations (OWAs), which are present in every human eye. OWAs blur retinal images in sometimes subtle, yet never negligible amounts. This effect has been the main obstacle impeding imaging the retina in-vivo at the cellular scale.

In the late 1990s, adaptive optics (AO) technology demonstrated the capacity to enhance lateral resolution in retinal images up to 3-4μm, enabling the visualization of individual photoreceptors and microscopic vessels. The AO technique was first implemented in the 1970s by astrophysicists in order to cancel the blur and twinkling effects caused by atmospheric turbulence when viewing stellar phenomena with ground-based telescopes. In ophthalmic instruments, AO can virtually transform the eye into a nearly perfect optical system, through which blur-free images can be obtained.

There are three main elements in an AO system: (i) a deformable mirror whose shape is continuously adjusted to compensate for OWAs; (ii) a wavefront sensor that detects changes in OWAs; (iii) a closed-loop calculator that feeds the sensor information back to the deformable mirror. One key advantage of AO is its ability to be combined with almost any other imaging technology including CCD fundus cameras, SLO, OCT and other existing systems. Recently, Drexler et al. obtained in-vivo cone images with a three-dimensional resolution of 4x4x4μm using combined AO and OCT technologies.

Several laboratory experiments using AO have been successful in imaging photoreceptor cells in living human eyes. Automated cell counting techniques applied to such images have produced quantitative information in the form of cone density maps. Researchers have also even been able to identify the three types of cones and retrieve the trichromatic cone mosaic. Recent experiments have employed AO to image retinal pigment epithelium cells in living human eyes and marked ganglion cells in mammalian eyes. Beyond anatomical analysis, AO has also provided functional information: blood velocity measured in single capillaries, scintillation that accompanies phototransduction in individual cones, and microperimetry based on cone-targeted light stimulus delivery.

Intensive clinical research is now needed to explore the possible relations between findings obtained using AO, genotype data and visual function in a number of visual disorders and retinal diseases. In order to render these investigations feasible, the technology must continue to evolve: AO systems must be smaller, easier to operate, compatible with a wider variety of eyes, more resistant to the effects of eye movements, and have the ability to image wider retinal areas. Several research projects currently underway are competing to achieve these goals. The INOVEO project, carried out by a French consortium of 11 partners, is poised to reach this goal and will deliver several AO fundus cameras for clinical research in early 2008. These devices will be based on a new electromagnetic deformable mirror technology that is able to correct OWAs in almost any human eye.

The potential applications for AO in the area of ophthalmology are vast. The medical and clinical investigations that will be carried out in the next 2-3 years will be key to identifying the actual clinical use and benefits of this exciting new technology.