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Biomedical subjects

Stephen A Burns

Publications and source records attributed to Stephen A Burns.

11 recordsLinked to original sources
PubMed2005

Investigating the light absorption in a single pass through the photoreceptor layer by means of the lipofuscin fluorescence.

Reflection densitometry has been widely used to measure the density difference of the bleachable cone photopigments in human eyes. Most such measurements make a series of assumptions concerning the amount of scattered light to derive an estimate of the true cone photopigment density from the density difference measurements. The current study made three types of measurements of the light returning from the eye before and after bleaching: the amount of light returning in the "directed" reflection, which is a double-pass estimate of the cone photopigment density; the amount of light in undirected or diffuse reflection; and the amount of fluorescence from lipofuscin in the RPE, which provides a single-pass measurement of optical density difference. For a 1 deg foveally fixated field, the density difference estimates for the three measurements were 0.68, 0.21, and 0.22 respectively. The lipofuscin fluorescence was found to be unguided. The background density difference was non-negligible and very close to the single pass estimate from fluorescence. These measurements each involve potentially different pathways of light through the retina, and therefore place different constraints on models of these pathways. A simple model comparing the directional and the fluorescence optical densities produced retinal coverage estimates around 70-75%. Estimates of the shape factor of the single pass optical Stiles-Crawford effect were evaluated from the dark-adapted and bleached fluorescence measurements. The values were closer to those obtained from psychophysical methods than to the double pass optical Stiles-Crawford shape factors obtained directly from retinal reflectometry.

Adult↗
PubMed2005

Improved contrast of peripapillary hyperpigmentation using polarization analysis.

PURPOSE: To improve detection and quantification of peripapillary hyperpigmentation, associated with aging, open-angle glaucoma, and age-related macular degeneration. METHODS: A computational approach was implemented with a readily available polarimeter used in glaucoma diagnosis, a nerve fiber analyzer (GDx; Laser Diagnostic Technologies, San Diego, CA). Using near-infrared illumination at each of 20 input polarizations, a series of image pairs was digitized. One image is made from the light returning from the eye that is polarized parallel to the input light, and the other image is made from the light that is rotated by 90 degrees from the input polarization. Using raw data from these 40 images, and a simplified model of ocular polarization properties, images were computed based on their polarization content. Regions of hyperpigmentation, selected using stereo color fundus photographs, were quantified in three types of polarimetry images: (1) a depolarized light image resulting mainly from multiply scattered light; (2) an average image that is typical of confocal images; and (3) a birefringence image. Measurements on versus off hyperpigmentation were made in nine persons with suspected glaucoma or patients with primary open-angle glaucoma, selected to have clinically visible hyperpigmentation. RESULTS: In the depolarized light images, hyperpigmented regions were significantly brighter than comparison areas (P < 0.0425)-that is, had more scattered light and therefore more contrast (P < 0.037) than did color or other polarimetric images. CONCLUSIONS: With this polarimetry imaging method, subretinal tissues such as those with hyperpigmentation can be visualized with increased contrast.

Cross-Sectional Studies↗
PubMed2004

Stroke amplifier for deformable mirrors.

We demonstrate a simple optical configuration that amplifies the usable stroke of a deformable mirror. By arranging for the wavefront to traverse the deformable mirror more than once, we correct it more than once. The experimental implementation of the idea demonstrates a doubling of 2.0 and 2.04 by two different means.

Journal Article↗
PubMed2004

Using InterWave aberrometry to measure and improve the quality of vision in LASIK surgery.

OBJECTIVE: To compare visual outcomes in eyes undergoing aberrometry-guided (InterWave) LASIK with those in eyes undergoing standard LASIK treatment based upon refractive measures. DESIGN: Single-center, comparative, interventional, consecutive case series. PARTICIPANTS: Four hundred two consecutive eyes undergoing LASIK were analyzed retrospectively. One group, 106 eyes undergoing primary LASIK and 224 eyes undergoing LASIK enhancement, was treated with standard LASIK treatment using a 5.5-mm optical zone, 1.5-mm transition zone laser with the settings determined by manifest refraction. The second group, 44 untreated (primary) eyes and 28 previously treated (enhancement) eyes, received a multipass, multistage treatment in which the laser settings for each stage were determined by aberrometry measurements. Eyes with desired monovision (undercorrected) outcome and preoperative hyperopia were excluded from the study. INTERVENTION: An aberrometry-guided laser treatment (InterWave LASIK) was compared with the standard LASIK treatment based upon the manifest refraction. MAIN OUTCOME MEASURES: Uncorrected visual acuity (VA), manifest refraction, best spectacle-corrected VA (BSCVA), severity of halos, and root mean square (RMS) retinal blur area measured at 3 months postoperatively. RESULTS: Three months postoperatively there was no difference in uncorrected VA, BSCVA, refraction, or RMS retinal blur areas for pupil sizes of 3.5 mm between eyes treated by InterWave and those treated by standard LASIK. However, InterWave LASIK reduced the retinal blur area by 48% (P<0.0103) and 58% (P<0.0004) in primary cases and 43% (P<0.0430) and 74% (P<0.0271) in enhancement cases, respectively, for pupil sizes of 4.5 and 6.5 mm relative to standard LASIK treatments. Patients undergoing InterWave-guided treatment reported less severity of halo (0.37 vs. 0.98 [P<0.016] for primary cases and 0.35 vs. 0.73 [P<0.04] for enhancement cases). CONCLUSION: InterWave LASIK achieved acuity and refractive results equivalent to those of standard LASIK treatment based upon refraction, but resulted in superior quality mesopic vision.

Adult↗
PubMed2003

SSR (spatially resolved refractometer): a null-seeking aberrometer.

The spatially resolved refractometer is an aberrometer used to measure the wave-front aberrations of the human eye. In its original form and the new configuration that we report, it uses the patient's perception in a psychophysical task to evaluate the wave-front errors at a variable number of loci (typically 40 or 160) across the cornea. This configuration includes pupil tracking and the ability to choose the measurement loci in software. An automated configuration that does not require patient input is also described.

Calibration↗
PubMed2003

Improved contrast of subretinal structures using polarization analysis.

PURPOSE: To improve the ability to detect and quantify the early retinal changes associated with aging, age-related maculopathy, and age-related macular degeneration. METHODS: A computational approach was implemented for analyzing images using a readily available polarimeter that is used for glaucoma diagnosis. This device, the GDx Nerve Fiber Analyzer (Laser Diagnostic Technologies, Inc., San Diego, CA), takes a series of images as a function of the polarization angle of the illuminating light. For each of 20 input polarizations, pairs of retinal images are digitized. One image is made of the light returning from the eye that is polarized parallel to the input light, and the other image is made of the light that is rotated by 90 degrees from the input polarization. Using the raw data from these 40 images, and a simplified model of the polarization properties of the eye, we calculated the amount of light that returns in a parallel polarized state, and the amount of light that is depolarized by multiple scattering. Measurements were made in seven subjects with small drusen. RESULTS: The depolarized light image produced a 3.4 times higher contrast of drusen and subretinal changes than the parallel polarized light images. CONCLUSIONS: Polarization-sensitive imaging combined with a simple computational approach allows the measurement of the retinal distribution of multiply scattered light. With this technique, retinal imaging of age-related changes in retinal and subretinal tissue can be improved.

Contrast Sensitivity↗
PubMed2002

Imperfect optics may be the eye's defence against chromatic blur.

The optics of the eye cause different wavelengths of light to be differentially focused at the retina. This phenomenon is due to longitudinal chromatic aberration, a wavelength-dependent change in refractive power. Retinal image quality may consequently vary for the different classes of cone photoreceptors, cells tuned to absorb bands of different wavelengths. For instance, it has been assumed that when the eye is focused for mid-spectral wavelengths near the peak sensitivities of long- (L) and middle- (M) wavelength-sensitive cones, short-wavelength (bluish) light is so blurred that it cannot contribute to and may even impair spatial vision. These optical effects have been proposed to explain the function of the macular pigment, which selectively absorbs short-wavelength light, and the sparsity of short-wavelength-sensitive (S) cones. However, such explanations have ignored the effect of monochromatic wave aberrations present in real eyes. Here we show that, when these effects are taken into account, short wavelengths are not as blurred as previously thought, that the potential image quality for S cones is comparable to that for L and M cones, and that macular pigment has no significant function in improving the retinal image.

Adult↗
PubMed2002

Cone photopigment in older subjects: decreased optical density in early age-related macular degeneration.

We measured changes to cone photoreceptors in patients with early age-related macular degeneration. The data of 53 patients were compared with normative data for color matching measurements of long- and middle-wavelength-sensitive cones in the central macula. A four-parameter model quantified cone photopigment optical density and kinetics. Cone photopigment optical density was on average less for the patients than for normal subjects and was uncorrelated with visual acuity. More light was needed to reduce the photopigment density by 50% in the steady state for patients. These results imply that cone photopigment optical density is reduced by factors other than slowed kinetics.

Aged↗
PubMed2002

Effect of the polarization on ocular wave aberration measurements.

Measurement of the eye's wave aberrations has become fairly standard in recent years. However, most studies have not taken into account the possible influence of the polarization state of light on the wave aberration measurements. The birefringence properties of the eye's optical components, in particular corneal birefringence, can be expected to have an effect on the wave aberration estimates obtained under different states of polarization for the measurement light. In the work described, we used a psychophysical aberrometer (the spatially resolved refractometer) to measure the effect of changes in the polarization state of the illumination light on the eye's wave aberration estimates obtained in a single pass. We find, contrary to our initial expectation, that the polarization state of the measurement light has little influence on the measured wave aberration. For each subject, the differences in wave aberrations across polarization states were of the same order as the variability in aberrations across consecutive estimates of the wave front for the same polarization conditions.

Adult↗
PubMed2002

Pupil location under mesopic, photopic, and pharmacologically dilated conditions.

PURPOSE: To determine whether there are systematic changes in pupil location with changes in the state of pupil size and with other ocular variables. METHODS: High-resolution images of the pupil of the eyes of 70 subjects were taken using an infrared-sensitive camera. Images were obtained under mesopic, photopic, and pharmacologically dilated conditions. From the images, the center and diameter of the corneal limbus and the pupil were computed. In addition, the location of the first Purkinje image was calculated. RESULTS: The pupil center shifted consistently temporally as the pupil dilated. The total motion was relatively small, with a mean distance of 0.133 mm motion between the mesopic and photopic conditions, with the pupil diameter changing from 6.3 to 4.1 mm. Ninety percent of the subjects had a motion of less than 0.3 mm. One patient showed a motion of almost 0.6 mm. The change in location of the pupil center was not significantly related to refractive error, age, or the change of pupil diameter. CONCLUSIONS: Changes in the location of the pupil center with changes in the dilation of the pupil are typically slight, but can be significant in a few subjects, especially in pharmacologically dilated pupils.

Adaptation, Ocular↗