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Medical Physics Web | January 22, 2007
Deformable mirror lights up eye disease

Boston Micromachines (Watertown, MA) has unveiled a microelectromechanical systems (MEMS)-based deformable mirror for use in adaptive optics applications. The new mirror is said to meet all necessary criteria for ultrahigh-resolution retinal imaging.

Retinal imaging, which could enable the early and accurate diagnosis of ocular diseases, is limited in resolution and contrast by imperfections in the cornea and lens, and by the non-uniform nature of the vitreous humour. As light passes through these structures to reach the retina, the tissue induces wavefront aberrations. Now, however, adaptive optics systems that use deformable mirrors to correct these aberrations are enabling high-resolution imaging through thick media.

"Until now, doctors were limited in their ability to gain a clear view of the human retina due to image distortion caused by tissue-induced wavefront aberration," said Paul Bierden, president of Boston Micromachines. "Our deformable mirror corrects for that wavefront aberration. This marked improvement in retinal imaging will provide doctors with the technology necessary to detect the leading diseases of the eye – glaucoma, diabetic retinopathy and age-related macular degeneration – years earlier than previously possible."

The mirror is an enhanced version of Boston Micromachines' flagship product, the Multi-DM. It offers an increased stroke (the maximum actuator displacement) and maintains the high resolution afforded by 140 independently controlled MEMS actuators. Its 3 kHz frequency capability enables real-time imaging while its 6 mm aperture is ideally matched to the diameter of a dilated pupil.

By offering 6 µm of stroke, the new Multi-DM provides the necessary wavefront amplitude correction for older eyes. This corresponds to 12 µm of wavefront correction, which the company claims is the largest demonstrated by any MEMS-based deformable mirror on the market today.

"The ever-increasing strokes in deformable mirrors, such as the 6 µm achieved with the new Multi-DM, will allow for deeper adaptive-optics-corrected imaging in biological specimens, more effective correction at longer wavelengths and improved performance specifications in adaptive-optics-based imaging systems," commented Ben Potsaid, research scientist at the Center for Automation Technologies and Systems at Rensselaer Polytechnic Institute (Troy, NY).

"Never before has there been a compact, affordable deformable mirror available with this magnitude of resolution," claimed Bierden. "This will enable adaptive optics to become a reality for commercial instruments." The new Multi-DM is available immediately and is being demonstrated at Photonics West 2007 this week in San Jose, California.

Briefing: adaptive optics
The power of adaptive optics lies in its ability to correct the optical distortions that affect light waves as they travel from the source to the imaging system. It works by measuring the wavefront shape and compensating for any distortion, most often by using deformable mirrors that can modify the output wavefront in real time.

The human eye, for example, is a near-perfect imaging system, but imperfections in the cornea and lens distort light entering the eye and so limit its practical resolution. In the same way the performance of even the largest ground-based telescopes is restricted by the optical effects caused by turbulence in the Earth's atmosphere.

Adaptive optic systems are generally exploited for two main purposes:

  • To provide real-time compensation of optical distortions, delivering dramatic improvements in the spatial resolution and contrast of an optical system. Such dynamic correction is most widely exploited in imaging, but it can also play an important role in optical systems such as confocal microscopes, laser-beam delivery systems and instruments for ophthalmic diagnosis.
  • To alter the characteristics of an optical system to generate a particular optical output. For example, adaptive optics can be used to produce a shaped laser-beam profile instead of the usual Gaussian profile, a capability that has wide-ranging applications in laser materials processing.

Until recently, most development of adaptive optics technology has been driven by the astronomy community. Lately, however, there has been a renewed effort to develop low-cost technologies and systems that will extend its use to a range of industrial and medical applications.

Ophthalmology is one area that's seeing significant commercial activity. Several companies are now marketing adaptive-optics-based systems for measuring optical aberrations in the eye. Another key growth area is 3D imaging: adaptive optics has been shown to deliver significant improvements in the image quality obtained with confocal microscopy, and the technology is also set to play a crucial role in enhancing 3D images of living cells.

About the author
Tami Freeman is industry editor on medicalphysicsweb.

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