Imaging the eye more perfectly
Adaptive optics aids ophthalmology imaging
Beauty may lie in the eye of the beholder but perfection doesn’t — especially
in the optical sense. That imperfection makes imaging inside the eye for diagnostic
and other purposes a challenge. Now a team from Cardiff University in the UK, the
University of Murcia in Spain and the Paris-Meudon Observatory in Meudon, France,
has demonstrated a new implementation of adaptive optics that can be applied to
imaging of the eye.
Team leader Wolfgang Drexler, a Cardiff biomedical
imaging professor, noted that key to the approach is the use of a magnetic deformable
mirror. The team, with a significant contribution by postdoctoral researcher Enrique
J. Fernández, used the mirror to create a correcting device that can cancel
out aberrations within a few seconds, potentially enhancing almost any imaging in
the eye. “I do see this correcting device applied in any kind of retinal imaging
modality — fundus camera, scanning laser ophthalmoscope, optical coherence
tomography — to significantly improve image contrast and resolution,”
The left column shows a color-coded
map of the normal aberrations found in a young eye; red depicts negative values
and dark blue, positive ones. The middle left image shows the effect of aberrations
on vision, and a retinal image obtained with ultrahigh-resolution optical coherence
tomography from the same eye is on the bottom left. In the right column are the
effects of correction of monochromatic and chromatic aberration (pancorrection)
on the images from the left column. Courtesy of Wolfgang Drexler, University of
Cardiff, and Enrique Joshua Fernández, University of Murcia.
In the device, the researchers made
use of adaptive optics. Originally developed for ground-based astronomy so that
stars can be seen clearly through wavering air, adaptive optics employs adjustable
optical surfaces to correct for changing optical aberrations. Such deviations from
the ideal can be described by Zernike polynomials, and the ability to correct for
aberrations can be referred to by the order of the polynomial.
By moving parts of a segmented mirror
appropriately, astronomers can cancel out the twinkling of stars brought about by
the atmosphere. A figure of merit for such efforts is the Strehl ratio, which compares
the actual optical performance to that of the ideal. A ratio of 1.00 indicates perfect
In ophthalmic applications, the adaptive
optics device of choice has been a deformable membrane mirror, but there are problems
with the current technology. For one thing, the mirrors have trouble correcting
higher order aberrations because of the limited stroke of existing devices. Another
problem has been cost. Adaptive optics technology has been too expensive for mass
use in commercial ophthalmoscopes.
Two years ago Drexler became aware
of a magnetic deformable mirror from Imagine Eyes of Orsay, France. The mirror consists
of a flexible, silver-coated membrane driven by 52 individually addressable magnetic
actuators. When a voltage is applied to the actuators, they attract or repel magnets
attached to the back of the membrane, thereby pushing or pulling the membrane over
a 17-mm-diameter area. The mirror offers up to sixth-order Zernike correction with
no hysteresis and a highly linear response to a driving voltage.
The mirror’s features made it
attractive for the research in which Drexler and the others were engaged. “We
are combining adaptive optics with optical coherence tomography [OCT],” he
He noted that OCT uses broadband light
sources. For that reason, chromatic aberration can present a problem when light
of different wavelengths separates in the process of traversing an optical path.
In the case of retinal imaging, chromatic aberration of the cornea and lens limits
the axial resolution. It also reduces the contrast of the point spread function
in the retina, adversely affecting transverse resolution.
To minimize chromatic aberration and
maximize resolution, the researchers designed an achromatizing lens. They constructed
it from two different glasses, with one sandwiching the other in a flint glass-crown
glass-flint glass configuration specifically intended for the 700- to 900-nm near-infrared
spectral range. Combined with the magnetic deformable mirror, this allowed them
to achieve a more complete correction of monochromatic and chromatic aberrations.
As detailed in the Oct. 2 issue of
Optics Express, they used a helium-neon laser from Melles Griot operating
at 632.8 nm for alignment in their demonstration setup. They used a Ti:sapphire
laser from Femtolaser Productions GmbH of Vienna, Austria, operating at 800 nm with
a 140-nm optical bandwidth to illuminate a subject’s retina after sending
the beam through 100 m of fiber for safety reasons. They collected the light exiting
the eye from this illumination and sent it into a Hartmann-Shack wavefront sensor
from Imagine Eyes. The sensor measured the aberrations of the subject’s eyes
and controlled the deformable mirror to correct for those imperfections under the
direction of software supplied by the company.
With this setup, the researchers attempted
to correct the aberrations in the eyes of five subjects. In the case of four, the
Strehl ratio was 0.01 to 0.03 at the beginning of the procedure for the 800-nm center
of the spectral range of interest. In less than two seconds of device operation,
the Strehl ratio was above 0.86 in all cases and above 0.94 in three of them.
The fifth subject was quite nearsighted
and also had an irregular cornea that introduced extreme high-order aberrations
out well past the sixth order. In this case, the system corrected the Strehl ratio
from 0.05 to 0.7, a performance that exceeded Drexler’s expectations. “I
was extremely surprised that this correcting device can correct extremely high order
aberrations in addition to high myopia,” he said.
Being able to handle higher-order aberrations
could be important in elderly patients because their eyes are more likely to exhibit
such characteristics. The capability also means better imaging spot size and contrast,
Drexler noted. He added that this capability means that there is no need for the
addition of lenses for this task, as is the case with other adaptive optics implementations.
That, in turn, could mean a lower-cost solution.
It might be possible to use this adaptive
optics technology someday for vision correction, but Drexler sees it being used
primarily in ophthalmic imaging and related diagnostics applications. A key step
will be proving the clinical significance of an adaptive optics-based imaging system,
which will require clinical studies and gathering data on a wider sample of eyes.
As for turning the setup into a product, Drexler said that it would require some
careful consideration of the various options for the first ophthalmic diagnostic
device based on this adaptive optics implementation.
“It is critical to choose the
proper imaging technique to combine with AO in terms of commercialization,”
MORE FROM PHOTONICS MEDIA