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.

He 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.

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, he commented.

"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 were aberration-free.
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 image information.

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.

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