Some of the technology derived from adaptive optics (AO) research is already being used by ophthalmologists to measure aberrations in the eye with unprecedented accuracy, and it may not be long before AO-based devices replace the conventional phoropter used to calculate prescriptions for eyeglasses and contact lenses. In addition, researchers are using adaptive optics technology to obtain extraordinary views of microscopic structures in the eyes of human subjects.
"Our goal is to develop low-cost AO systems for clinical use by ophthalmologists. That's definitely on the horizon, but first we need to reduce the cost and the size of the instruments," said David Williams, director of the Center for Visual Science at the University of Rochester.
Williams is an associate director of the national Center for Adaptive Optics, which plays a key role in transferring information about developments in adaptive optics from the research labs to industry and the broader scientific community. Funded by the National Science Foundation, the center is based at the University of California, Santa Cruz, and has a network of partners that includes academic institutions, national laboratories, and companies in related industries.
In 1998, Williams and Austin Roorda, then a postdoctoral researcher at Rochester, used an adaptive optics ophthalmoscope to obtain the first images showing the arrangement of all three kinds of cones--the light receptors responsible for color vision--in the living human retina (the back surface inside the eye). Roorda, now at the University of Houston, has since combined adaptive optics with another technique called confocal imaging to construct an even more powerful instrument: the adaptive optics scanning laser ophthalmoscope (AOSLO).
Using this new instrument, the first of its kind, Roorda is able to view internal structures in the eye in real time and with such high resolution that he can watch individual blood cells moving through tiny blood vessels in the retina. Confocal imaging enables the instrument to focus separately on the different layers of cells within the retina, revealing nerve fibers, blood vessels, and light receptors with a clarity that is greatly enhanced by adaptive optics.
"Using adaptive optics in a scanning laser ophthalmoscope increases the resolution in three dimensions by an order of magnitude, so it's about 10 times better than a conventional scanning laser ophthalmoscope," Roorda said.
Such extraordinary advances in the ability of doctors to study a patient's retina will greatly improve their ability to diagnose and monitor retinal diseases, Roorda said. In addition, these instruments will make it much easier for clinical researchers to evaluate the effectiveness of experimental treatments. Retinal diseases include macular degeneration, the leading cause of blindness in the United States.
An adaptive optics ophthalmoscope could also be very helpful for studying and treating glaucoma, said Michael Drake, vice president for health affairs at the University of California and a professor of ophthalmology at UC San Francisco.
"An important component of glaucoma management is the ability to evaluate the quality of the retinal nerve-fiber layer, so if we could see structures in the retina 10 times better than we can now, that would be outstanding," Drake said.
One spin-off from adaptive optics research, the "wavefront sensor," is already entering clinical use in the form of diagnostic instruments for measuring optical aberrations of the eye. A wavefront sensor is an essential component of any AO system, measuring optical distortions that are then corrected by other components of the system. Stand-alone devices that incorporate wavefront sensors can provide the most precise measurements available of the optical aberrations of the eye.
These devices use a laser beam reflected off the retina so that the light passes out through the eye's optics and onto an array of tiny lenses called lenslets. The lenslet array focuses the light into an array of spots on a detector, producing a wavefront pattern that is like a fingerprint of the eye's optical aberrations.
Conventional devices only measure the two basic refractive errors of the eye--defocus and astigmatism. Eyeglasses, contact lenses, and even laser refractive surgery only correct for these two basic errors, and clinicians have generally ignored the so-called higher-order aberrations. With wavefront sensors, however, a complete analysis of the eye's optical errors is now possible.
The results of wavefront analysis can be used to design customized contact lenses or to guide laser surgery for permanent vision correction. These applications are being developed and tested by several companies. Laser surgery devices that incorporate wavefront analysis are being used in Europe and have been submitted for Food and Drug Administration (FDA) approval for use in the United States. For example, Bausch & Lomb, one of the industry partners of the Center for Adaptive Optics, has conducted clinical trials using a wavefront diagnostic instrument linked to the company's laser refractive surgery device, and has applied for FDA approval of the system.
One of the complicating factors in laser refractive surgery is that the living tissue of the cornea, after being reshaped by the laser, often responds in ways that introduce higher-order aberrations, said Ian Cox, a research fellow at Bausch & Lomb.
"We've found that the wavefront-driven custom surgery significantly reduces the postoperative higher-order aberrations compared to the conventional technique," Cox said. "We haven't completely eliminated them, however, so the next step is to understand what controls those induced aberrations so we can use the wavefront technology more effectively."
Companies are also developing the technology for making customized contact lenses based on wavefront analysis. This is actually a more challenging application than wavefront-driven laser surgery, Cox said. Bausch & Lomb has developed a process for taking wavefront measurements and sending them electronically to a manufacturing site, where the wavefront analysis guides the production of customized lenses. The challenge, according to Cox, is to make that process practical and affordable in comparison with current manufacturing processes, which produce high volumes of low-cost disposable contact lenses based on a limited set of standard corrections.
In an adaptive optics system, the output from a wavefront sensor is used to calculate corrections that actively remove distortions from an image. In an AO ophthalmoscope, for example, a deformable mirror is used to apply a distortion that is the exact reverse of that measured by the wavefront sensor, so that the resulting image is error-free.
CfAO researchers are working to develop a small, low-cost mirror for use in clinical instruments. Micro-electro-mechanical systems (MEMS) technology is one promising approach. MEMS devices integrate tiny mechanical elements, sensors, actuators, and electronics on a silicon chip substrate. Williams and other researchers at the University of Rochester, using a device made by Boston MicroMachines, recently demonstrated the first use of a MEMS mirror in an AO system for vision science.
"It is much cheaper and 25 times smaller than previous devices, so it would be compatible with an instrument designed for use in a doctor's office," Williams said.
The applications of adaptive optics in vision science have received considerable impetus from the knowledge developed by astronomers about how to correct images using AO technology. Although many applications, such as AO ophthalmoscopes, are still in the early developmental stages, adaptive optics is an exciting new technology for vision science, said UC's Drake.
"It's a great concept that offers promise for many new areas of investigation," he said.