glaucoma

The Next Generation of Imaging Technology

BY MITCHELL DUL, O.D., M.S., F.A.A.O. New York

"TWINKLE, TWINKLE LITTLE STAR, HOW I WONDER WHAT YOU ARE?" But did you ever wonder why that star is twinkling? Light from a star reaches the earth's atmosphere as parallel rays of light. As this light enters Earth's ever changing atmosphere, it encounters turbulent air of different temperatures and densities (and therefore different indices of refraction). Consequently, the light is refracted, which results in the "twinkling" image of the stars and planets. While this may add some luster to a bedtime story, it's a challenge we must overcome to obtain high-quality images for the purpose of scientific inquiry.

Eye in the sky

Just as atmospheric turbulence distorts light arriving from distant stars, imperfections in the eye's cornea and lens distort light passing through the eye, causing blurred images to form on the retina. It's not hard to imagine the technical difficulties involved in transferring large astronomical telescope technologies to the study and treatment of the human eye, but great strides have already occurred. Efforts aimed at reducing the size and cost of these devices using micromechanical (MEMS) systems have been very successful.

Adaptive optics fluorescence images of primate ganglion cells.

This technology has some very clear implications for practicing optometrists. The optical defects of the eye blur the retinal image in qualitatively the same way as atmospheric turbulence blurs images in telescopes, so adaptive optics can play a role in vision correction. We correct for second-order aberrations every time we refract a patient. We refer to these second-order aberration corrections as the spherical and astigmatic corrections.

Unfortunately, we have had no means to correct for higher-order aberrations until recently. Now, with adaptive optics, we can correct most of the optical aberrations in the eye, providing the best possible image quality.

Seeing stars

We can correct higher-order aberrations with adaptive optics in various ways. The most promising of these, and the one that will likely further enhance our current generation of commercial wavefront sensing technology, is the MEMS Deformable Mirror (MEMS DM), a novel, compact, inexpensive wavefront correction technology that researchers have already developed and tested. Wavefront sensors that incorporate a MEMS DM can both measure and correct for the aberrations of the eye and allow the patient to make a direct assessment of the subjective benefit of high-order aberration wavefront correction. This technology can also set the stage for the more routine use of adaptive optics in clinical practice in the form of adaptive-optics phoropters.

Comparison of in-vivo- (using adaptive optics) and ex-vivo images (post-mortum histologic sections) of retinal ganglion cells.

Variations of this technology have already been incorporated into refractive surgery and in the manufacturing of spectacle lenses, contact lenses and intraocular lenses. These developments have the potential to afford a degree of clarity beyond our traditional approaches. During the Annual Meeting of the Association for Research in Vision and Ophthalmology (ARVO), in May of 2007, researchers presented results from studies using commercially available products to simulate the way higher-order aberrations influence vision.¹ This may eventually assist you in choosing between various refractive treatment options to ensure optimal results.

For example, patients may choose to have customized refractive surgery or customized contact lenses if they experience a large improvement in image quality while looking though a phoropter that corrects higher-order aberrations. One study used an adaptive optics program designed to investigate a system's ability to correct for the large amounts of aberrations found in highly-aberrated eyes, including those of keratoconic subjects presenting with extreme higher-order aberrations. The authors reported that this adaptable mirror could be a powerful tool in assessing the limits of visual performance achievable after correcting for patients' aberrations, especially in eyes with abnormal corneal profiles.²

The great unknown

If this technology can enable patients to see the world more clearly, could it possibly work the other way around? What if we applied this technology in reverse? If we correct for the higher-order aberrations of the eye, could we not enhance the doctors' view of the patients' eyes? Absolutely. Work is already underway to accomplish this, and some of the results are very impressive.

Arrangement of short-, middle- and long-wavelength-sensitive cones in human foveal mosaics.

Researchers are using adaptive optics to explore the organization and function of the retina and the optical and neural limits on human vision. Using a custom microscope equipped with a very sensitive camera, researchers have been able to take pictures of the retinal cone photoreceptors at light levels low enough that the photopigment in the retina is only modestly bleached. These experiments reveal the packing arrangement and relative numbers of the three cone types and clarify how the retina encodes color information for transmission to the brain.³ Scientists at the Center for Visual Science at Rochester University have also been able to resolve features at the spatial scale of single cells for the first time in the living retina. When compared with histological sections, the similarity to the in-vivo image is striking.^4-6 Using high-resolution adaptive-optics imaging combined with retinal densitometry, another group also characterized the arrangement of short-(S), middle-(M) and long(L)-wavelength-sensitive cones in human foveal mosaics.^7,8

Researchers will continue to incorporate versions of adaptive optics technology into ocular imaging devices, and these devices will likely find their way into mainstream clinical practice within two to three years. It's even likely that the information provided by this technology will enhance the diagnosis and treatment of eye disease. For instance, in the presence of glaucoma, we could monitor ganglion cell loss at the cellular level. It's also conceivable that, by correcting for the optical aberrations of the eye and using light wavelengths that have less impact on pupillary function, we may be able to thoroughly assess the retinal periphery without a dilated fundus examination.

Although commercial variations of this technology exist at present, the future addition of adaptive optics will likely bring this technology to a whole new level. It will also continue to refine the way in which we manage refractive errors and other causes of aberrations in the eye. OM

1.Rocha KM, Vabre L, Khoa JLN, et al. Effects of Zernike wavefront aberrations on visual acuity measured using electromagnetic adaptive optics technology. Presented May 8, 2007 at the Association for Research in Vision and Ophthalmology's (ARVO) annual meeting in Fort Lauderdale, Fla.

2. Sabesan R, Yoon G. Correcting highly aberrated eyes using large-stroke adaptive optics. Presented May 8, 2007 at the Association for Research in Vision and Ophthalmology's (ARVO) annual meeting in Fort Lauderdale, Fla.

3. Packer OS, Williams DR, Bensinger, DG. Photopigment transmittance imaging of the primate photoreceptor mosaic. J. Neurosci. 1996 Apr 1;16:2251-60.

4. Miller DT, Williams DR, Morris GM, Liang J. Images of cone photoreceptors in the living human eye. Vision Res. 1996 Apr;36(8):1067-79.

5. Liang J, Williams DR. Aberrations and retinal image quality of the normal human eye. J Opt Soc Am A Opt Image Sci Vis.1997 Nov;14(11):2873-83.

6. Liang J, Williams DR, Miller DT. Supernormal vision and high-resolution retinal imaging through adaptive optics. J Opt Soc Am A Opt Image Sci Vis. 1997 Nov;14 (11): 2884-92.

7. Hofer H, Carroll J, Neitz J, et al. Organization of the human trichromatic cone mosaic. J. Neurosci. 2005 Oct 19;25(42):9669-79.

8. Roorda A, Williams DR. The arrangement of the three cone classes in the living human eye. Nature. 1999 Feb 11;397(6719):520-2.