Hartmann-Shack aberrometer finds new
application in evaluation of nuclear cataract
By Laszlo Dosa In Ft Lauderdale
HARTMANN-SHACK wavefront sensing appears to be a viable, non-invasive
method for measuring early progression of nuclear opalescence due
to cataract, say researchers at the Visual Optics Institute at the
University of Houston College of Optometry.
William J. Donnelly III, M.S. and colleagues conducted a study to
explore a wavefront image processing technique that identifies object
borders of like intensities as a measure of forward light scattering
associated with cataract.
presented the initial results of the study at ARVO this May. The
85 patients in the study were between 20 and 80 years of age, with
Lens Opacities Classification System (LOCS III) nuclear opalescence
score (NO) scores ranging from 0.1 to 4.9. The initial analysis
of Hartmann-Shack images from the study participants suggests that
such images can be used to measure forward scatter, he reported.
J. Donnelly III, M.S.
The study initially revealed a negative correlation between the
number of objects found and the Lens Opacities Classification System
(LOCS III) nuclear opalescence score (NO). Mr. Donnelly had anticipated
that the number of objects found would decrease with increasing
scattering and indeed, the initial results have demonstrated this.
He noted that counting objects is a rather coarse measurement. The
researchers are attempting to make the process more robust. A modified
system is now being used to investigate object areas at a finer
localised level of analysis.
"We are using the Hartmann-Shack image to figure out how much
scattering of the light can be caused by cataract, nuclear cataract
in particular. A look at this image that the machine takes can give
a better understanding of how a patient's cataract might be progressing,"
Mr. Donnelly explained.
Link between scatter and aberration
Since the initial presentation of results at ARVO, 79 more patients
have been analysed. This led to further adjustments in the analysis
and spawned some additional detailed image analysis. The method
is not yet foolproof and refinements to the evaluation process continue,
"You can tell what types of optical errors the eye has by using
the Hartmann-Shack machine. It can be used as a diagnostic tool.
We are also using it to assess how light is scattered in the eye.
We are investigating if light scatter is or is not associated with
aberrations. Currently the scatter is believed to be high frequency
noise that broadens a lenslet's point spread function, not displace
it as an aberration does."
However, this may not hold true in all instances. When aberrations
are large enough, such as erratic index changes in the lens from
a cataract, there may be some more direct link between the scatter
and the localised aberrated wavefront area. Also, in using this
method, complications in the assessment of severe cataract arise
from the absorption of scattered light by cataractous occlusions.
The light will just not make it to the CCD array for assessment.
However, techniques in exploiting this complication may be forthcoming.
The Hartmann-Shack instrument has many optical and electronic elements,
including a charge-coupled device (CCD) detector array. The patient
looks into the instrument while a low-power laser beam is projected
as a point onto the retina. This point is reflected as an optical
wavefront traversing back through the eye's optical system and through
a lenslet array that produces multiple point images.
Reflecting from the laser spot appearing on the retina, as the total
wavefront of the eye passes through the lenslet array, it projects
multiple images of the retinal spot onto the CCD detector. If these
spot images are not regular, i.e., if some are displaced, the aberrations
of the eye can be calculated from the displacements of each lenslet
on the CCD array from the point where they would be if the wavefront
Noise on the wavefront
Forward light scattering can be thought of as high frequency noise
on the optical wavefront. If an area of the eye lens causes forward
scattering like a cataract does, this forward scattering is carried
through the local lenslets (corresponding to that cataractous area)
and manifests as a broadened 'blob' in the Hartmann-Shack image.
Effects from other intraocular scatter are visible as a grey wash
in the image. A level called the "histogram second peak threshold"
is set to separate this wash from the remaining forward scatter
According to electrical engineer Edwin Sarver, Ph.D., who has been
involved in the design of wavefront sensing instruments, it is clear
from looking at the various Shack-Hartman images that everybody
has a different refraction coming back from the retina. Some have
a lot of light, some that don't have very much light coming through.
He told EuroTimes that the Houston researchers have normalised that.
"That's why they talk about finding the second peak in the
intensity profile. They use that to normalise their calculation
of what it means to have a particular blob in one of these lenslet
images. The normalisation is, I think, very important. It is a good,
robust diagnostics tool because it works from one extreme set of
images to another," Dr. Sarver said.
University of Houston College of Optometry
Edwin Sarver, Ph.D.