spotlight on technology & technique
Visiometrics' Optical Quality Analysis System
The OQAS is a diagnostic tool that objectively analyzes total quality of vision.
By Leslie Goldberg, Assistant Editor

It is known throughout the eyecare industry that the 20/20 vision standard needs to be supplemented, as it refers to only one component of vision, specifically visual acuity (VA). Visiometrics' Optical Quality Analysis System, or OQAS (rhymes with focus), provides objective measurements of true vision quality, which is increasingly important to eyecare professionals.

"The OQAS is the only available objective technology that can completely and objectively measure a patient's total optical quality," says Steve Elfrink, Visiometrics' head of business development in the United States.

The OQAS creates two- and three-dimensional retinal images (or maps) that describe a patient's total optical system. These images, displayed on a computer screen, include the effects of light scatter and all higher order aberrations (HOA). These are now seen as essential factors in assessing vision quality.

How OQAS Works

Developed by Pablo Artal, Ph.D., and Jaume Pujol, Ph.D., of Barcelona, Spain, the OQAS system consists of laser diagnostic sensory equipment, a computer workstation and custom designed software. It works by sending in an infrared light source and directly measuring the point spread function (PSF) of the optical system of the eye.

"The OQAS is looking at the total quality of the image coming through the whole optical system," says Daniel Durrie M.D., F.A.C.S., director of refractive surgery at Durrie Vision, Overland Park, Kan.

During the first pass, OQAS shines a low-power beam of diode laser light (wavelength = 780 nm) into the eye, producing an image of a punctual circular spot on the retina. During the second pass, the reflection of this spot on the retina is measured by a highly sensitive low- light camera. OQAS measures the ability of the eyes' optical system to project a beam of light onto the retina. Because OQAS measures all light reflected back off the retina, the measurement contains the effects of light scatter and all HOA (up to the nth order). Studies conducted by Drs. Artal and Pujol show that this is the area where Hartmann-Shack or ray-tracing aberrometers fall short. They are unable to measure light scatter or HOA (nth order aberrations to scatter).

"Knowing the PSF of the optical system makes it possible to identify all the properties of this system. In this sense, OQAS is the only instrument that provides all the information about the optical vision quality including the effects of aberration and scattering," says Dr. Pujol. Using Fourier analysis, this PSF data is translated to an MTF (modulation transfer function) value.

MTF Values

According to Jan Bonel, CEO of Barcelona-based Visiometrics, "The challenge with current aberrometers is that they need to mathematically interpolate MTF values. The OQAS obtains MTF values through direct measurement. This gives an accurate measurement of the optical quality of the eye."

Figure 1. 2D and 3D images of a normal patient demonstrating a tight PSF map.

The MTF correlates to the loss of contrast in the retinal image for each spatial frequency. MTF values are related to the contrast sensitivity function (CSF). The MTF considers the optical system of the eye and CSF objectively considers the entire visual system. The MTF value generated by the OQAS allows surgeons to characterize quantitatively, the performance of the optical system of the eye.

For years, the photography industry has used MTF values to measure the optical quality of lenses. The higher the MTF value, the higher the quality of the image after it passes through a lens.

"Simply put, MTF is the measurement of different spatial frequencies all stacked up from low to high with the contrast sensitivity of the image at each of those frequencies. This produces a bell curve," explains Trevor Woodhams, M.D., founder of the Woodhams Eye Clinic in Atlanta and clinical associate professor of Ophthalmology at Emory University. "Comparing these curves from pre- to post-op tells you what is normal: whether they have a focus issue or if there is an HOA with light scatter/veiling glare as a complicating factor."

"In the past," says Dr. Woodhams, "we have had patients who came back after having refractive surgery with 20/20 vision but with often vague and even disabling visual results. While wavefront aberrometry taught us that there is more to vision than de-focus, this alone does not explain all visual complaints, due to the inability to assess light scatter."

Dr. Woodhams says that in the future, machines will be combined to assess both HOA and MTF. OQAS could then allow measurement of MTF as well as wavefront and then separate them out to assess their visual impact. "The benefit is that you will be able to evaluate glare and contrast as distinct from HOA," says Dr. Woodhams, "This should prove invaluable in diagnosing the
often confusing issues we face in patients that have had previous refractive surgery."

Applications of the OQAS

Figure 2. 2D and 3D images of a Post-LASIK patient. Here we see the PSF image with increased scatter.

OQAS measurements are useful in a wide range of clinical applications. It excels at diagnosing pathologies such as cataracts, evaluating patients pre- and post- operatively and measuring visual accommodation. A database facilitates tracking patient results, giving the researcher or physician the tools to manage and analyze measurements for many patients over a period of time.

The OQAS may be used as a screening tool that measures the impact of all the factors in the retinal image of LASIK candidates. "This double-pass procedure benefits patients by providing them with a clearer understanding of what to expect post-surgery," says Dr. Artal. "A surgeon may want to heavily counsel a patient with a very high MTF value on the expected reduced optical quality. A patient may obtain 20/20 vision post-LASIK but their retinal image may be similar to that of a 70 year old."

In an article published in the July 2003 Eurotimes, Dr. Artal noted that he and his associates were able to demonstrate that scatter increases with age and that LASIK can add years to a patient's scatter quotient.

They also determined that while those undergoing LASIK for less than -2.5 D of myopia had a moderate increase in scatter, those who underwent the procedure for -4.5 D or more of myopia had the same amount of scatter as in the untreated normal eyes of people 60 to 70 years old. Elfrink adds,"We know that LASIK induces HOA but our studies now reveal that LASIK also induces scatter that is real, measurable and can have an impact in the quality of vision."

"The whole concept of quality vision is going to be the next wave for both cataract and refractive surgery," says Dr. Durrie. "As surgeons, we need to be aware of the different surgical procedures that we are performing on patients. Optimal patient outcomes are our highest priority. I feel that instruments that objectively measure optical quality, like OQAS, are going to become essential tools for practicing ophthalmologists."

"The OQAS has very important research applications," says Cynthia Roberts, Ph.D., associate director at Ohio State's biomedical engineering center.

"It is so promising because it is the only system on the market right now that can quantify ocular scatter," says Dr. Roberts.

She also says that OQAS can be used as a diagnostic tool. According to Dr. Roberts, OQAS can be used in the following areas: in transplants, corneal response to both lamellar and penetrating procedures can be quantified; in cataracts, OQAS can help to develop new guidelines for cataracts; in refractive surgery, OQAS can help to evaluate new procedures and monitor current procedures in corneal wound healing; and in surface procedures, OQAS can measure improvements in reduction of haze.

For Interested Parties

For more information on the OQAS, please visit www.visiometrics.com or contact Steve Elfrink at steve.elfrink@visiometrics.com.