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At least 1,585 records · Page 88Linked to original sources
PubMed1995

Ultrastructure in anterior and posterior stroma of perfused human and rabbit corneas. Relation to transparency.

PURPOSE: The authors sought to discover whether there are differences in the degree of spatial order in the fibrillar ultrastructure between anterior and posterior stroma. METHODS: Human corneas were obtained from eye bank eyes. Although they had been classified as normal, some swelling remained after 3 hours of deturgescence. Freshly excised, unswollen rabbit corneas also were used. Image analysis methods were applied to transmission electron micrographs of the anterior, middle, and posterior stroma of these corneas to determine the positions and radii of fibrils, the fraction of total area occupied by fibrils, and the fibril number density. Results were used to calculate the interference factor that appears in the direct summation of the fields for light scattering theory and to estimate the total scattering cross-section per fibril. The interference factor is a measure of the spatial order in the positions and sizes of the fibrils. RESULTS: Electron micrographs showed anterior-posterior variations in size and number density of fibrils. The interference factor at wavelengths of visible light was lower in posterior stroma than in anterior stroma for humans and rabbits. In some instances in humans, the anterior interference factor was characteristic of mildly swollen cornea. When averaged for the electron micrographs analyzed, the anterior stroma was predicted to scatter approximately twice as much light per unit depth as the posterior stroma in humans (at any given wavelength) and approximately three times as much in rabbits. CONCLUSIONS: Calculations of the interference factor showed that there were differences in the anterior-posterior spatial ordering of fibrils. In human corneas, the differences could have been caused by intrinsic in vivo differences between anterior and posterior stroma; however, possible anterior-posterior variations in swelling between the two regions in vitro also could have affected the results.

Aged↗
PubMed1995

Change with age of the refractive index gradient of the human ocular lens.

PURPOSE: To study age-related changes in the refractive index distribution of the human ocular lens. METHODS: Biometric data collected on 48 eyes in subjects ranging in age from 19 to 31 years and 48 eyes in subjects ranging in age from 49 to 61 years allowed estimation of a single parameter related to the refractive index distribution of the crystalline lens. The authors selected a gradient index model of the lens characterized by a fixed index at the lens center, a somewhat lower fixed index at the surface, and a continuum of index values between center and surface depending on a single parameter, beta. This parameter was evaluated for each of the two age groups. RESULTS: The distributions of the gradient index parameter beta for the two age groups were found to be statistically well separated. On average, the older group was found to have an index gradient that was flatter near the lens center and steeper near the surface, implying a lower refractive power of about 2 D for representative lens surface curvatures. CONCLUSIONS: It has been observed that surface curvatures and thicknesses of the ocular lens increase with age, whereas other ocular dimensions apparently do not change, implying a trend toward myopia. This trend has not been observed. The authors' results are consistent with and strongly in support of the hypothesis that subtle index changes in the aging lens compensate to a large extent for changes in surface curvatures.

Adult↗
PubMed1994

Ultrashort laser pulse effects in ocular and related media.

Relatively little experimental and theoretical data exist on the retinal hazards of ultrashort laser pulses operating in the visible and near infrared spectral regions. Because of potential nonlinear effects that can occur from high-peak irradiance, ultrashort laser pulses propagate from the cornea to the retina, we have developed four projects within our Ultrashort Pulse Effects program. First, we discuss preliminary ED50 threshold values for nanosecond (ns), picosecond (ps), and femtosecond (fs) single pulses for in-vivo ocular exposures in Dutch Belted Rabbits using pulses in the visible spectral region. Then we examine two experiments that study nonlinear absorption using water tubes and measure the nonlinear refractive index of ocular tissue using the Z-Scan technique. Finally, we determine laser-induced breakdown thresholds in ultrahigh purity water. These studies give reasonable estimates of the damage thresholds and insight into the biophysics of how ultrashort pulses interact with ocular media.

Animals↗
PubMedDate not supplied

Noninvasive imaging and analysis of skin lesions for early detection of cutaneous malignant melanoma.

Knowledge of the depth of invasion and three-dimensional (3D) characteristics of skin lesions is crucial to the early diagnosis and prognosis of malignant melanoma. These parameters are currently available only after biopsy. A clinical prototype of an optical imaging instrument, called a nevoscope, has been developed to noninvasively image skin lesions. The nevoscope provides multiple views of a transilluminated skin lesion. These views provide a three-dimensional perception of the lesion. In addition, image-analysis techniques are applied to these images to quantify various diagnostic parameters, such as 3D geometric features and surface texture and pigmentation characteristics. Clinical trials are under way to evaluate the utility of the nevoscope as a diagnostic tool for early detection of melanoma. The construction of the clinical prototype and the integrated clinical imaging environment are described. The development of iterative reconstruction techniques suitable for nevoscopy data is also described. Experimental reconstructions of a physical tissue-like phantom and an in-vivo lesion are presented. The phantom is recovered effectively. Thus, nevoscopy is of potential diagnostic value.

Absorption↗
PubMedDate not supplied

Model for predicting the optical performance of the eye in refractive surgery.

BACKGROUND: Predicting the outcome of refractive surgical procedures is one of the chief goals of the refractive surgeon. Besides perfecting the change required to nullify a refractive error, the optical quality of the postsurgical eye should be maximized. METHODS: A model eye useful for predicting the likely effects on the optical performance of the eye as a result of refractive surgical intervention is presented. The model features aspheric ocular interfaces, a gradient refractive index within the lens, and a uni-index cornea. The dimensions of the model parameters are taken from the literature. Baker's method of ray tracing through aspheric surfaces is used to predict the lateral spherical aberration of the eye for specific pupil sizes. The results for various operational conditions such as number of lens layers and corneal shape were calculated. RESULTS: The model predicts that optimal optical imagery is produced when the corneal profile is represented by a flattening ellipse (shape factor = .65 to .85). Ideally, in refractive surgery involving the cornea, the postoperative corneal contour should conform to this flattening ellipse.

Cornea↗
PubMed1993

[Change in the size of particles in actomyosin suspensions during superprecipitation].

Changes in the properties of natural actomyosin suspensions, such as turbidity, light scattering, volume concentration, particle size and particle size distribution during superprecipitation (SPP) of actomyosin in media of different compositions have been investigated. Two sets of changes indicating the existence of two SPP types--the "immediate" SPP characterized by a rapid one-step rise in turbidity and the "spontaneous" SPP, when the slow increase in turbidity follows the clear phase, have been revealed. In both cases the mechanism of the increase in the suspension turbidity is assumed to be similar and is conditioned by an increase in the refraction index (density) of the particles; however, the ways of formation of dense particles are different. During the "immediate" SPP, the particle density increases due to particle striction, whereas during the "spontaneous" SPP dense particles are formed as a result of ordered lateral association of myosin and actin filaments.

Actomyosin↗
PubMed1993

The development of myopia up to the age of twenty and a comparison of refraction in parents and children.

As a part of a longitudinal birth cohort study, refraction was measured at the age of 20 years in 236 persons known to have had myopia at the age of 14 years and 266 controls (2982 refractions). The earlier the myopia had started, the more myopic the eyes were at the age of 20 years. The greatest mean refraction values, -4.94 D for males and -6.62 D for females were found when the change to myopia had started before the age of 10. The mean progression of myopia (the mean progression curves were achieved by calculating the mean refraction values at different ages) seems to continue at least to the age of 20. A female child with a myopic mother is more likely to develop myopia than a female child with a non-myopic mother and the same relation holds good between a male child and his father. The myopic refractive error seems to be greater in the children than in their parents.

Adolescent↗
PubMedDate not supplied

[Visualization of the effects of millimeter radiation on blood plasma].

Experiments on millimeter wave radiation (range 37.5-78.5 Ghz incident radiation power density 1-10 mVt/cm2) upon index refraction of 2-10% water solution of human blood plasma are presented. Investigations were carried out by holographic interferometry with sufficient sensitivity for estimating the index refraction -10(-6). Index refraction variations in 2% water solution of human blood plasma are detected to be equal to 10(-4). Such index refraction variations upon millimeter wave is one order greater than that of the temperature effect and two order greater than the sensitivity of settlement. The mechanism of these phenomena is discussed.

Blood↗
PubMed1995

The refractive index and protein distribution in the blue eye trevally lens.

BACKGROUND: The relationship between structure (crystallin distribution) and function (refractive index) in the lens is not understood and can be studied by comparing biochemical and optical properties. Such a comparison has been made using a blue eyed trevally lens. METHODS: The optical parameter of refractive index distribution was determined using a nondestructive ray tracing technique. The distributions of the various classes of proteins in the lens were determined by dissolving lenses in concentric layers and using biochemical protein assay. HPLC and SDS-PAGE electrophoresis were used to investigate the proportion of proteins in each layer. RESULTS: The refractive index distribution, from center to edge, follows a second order polynomial. The proteins do not vary in their proportions over most of the lens; only in the inner-most regions is there a rapid increase in insoluble protein and a concomitant decrease in the soluble protein classes. The smallest proteins (gamma crystallins) become insoluble later than the alpha- and beta-crystallins. CONCLUSIONS: There are no similarities in the distributions of any of the protein classes to that of the refractive index in the fish lens. This result indicates that a quantitative relationship cannot be derived by comparing protein to refractive index distributions. However, the findings are consistent with those made in other species: a high content of gamma-crystallins is always found in lenses which have steep refractive index gradients and high index magnitudes.

Animals↗
PubMed1995

Rapid differentiation of pathogenesis in patients with pulmonary edema by protein measurement with refractometer.

BACKGROUND: No study has focused on using refractometer to rapidly determine total protein ratio of edema fluid to blood concentration (TPe/TPb) and, to differentiate hydrostatic pulmonary edema (HPE) from adult respiratory distress syndrome (ARDS). METHODS: The reliability of refractometer was evaluated by correlating the protein concentration measured by refractometer with that by sequential multiple autoanalyzer (SMA) in 91 blood samples. In intensive care unit, thirteen patients with florid pulmonary edema were included. All had an unequivocal diagnosis of either HPE or ARDS. Edema fluid and blood were collected simultaneously for the measurement of protein concentration by refractometer. RESULTS: Protein concentration measured by refractometer correlated excellently with that by sequential multiple autoanalyzer (r = 0.991, p < 0.00001). In 7 patients with ARDS, the mean TPe/TPb was 0.75 +/- 0.13. In contrast, in 6 patients with HPE, the mean TPe/TPb was 0.36 +/- 0.14. The difference between groups was significant (p < 0.005). An inverse correlation between TPe/TPb and wedge pressure was also significant (r = -0.814, p < 0.001). CONCLUSIONS: Using refractometer to determine protein ratio of edema fluid to blood concentration can rapidly and effectively distinguish ARDS from HPE in patients with florid pulmonary edema. TPe/TPb correlates inversely with wedge pressure in these patients.

Adolescent↗
PubMed1996

[Results of photorefractive keratectomy for myopic astigmatism].

In 54 patients, 92 eyes with different degrees of compound, myopic astigmatism underwent photorefractive keratectomy with the VISX 20/20 excimer laser. The eyes were grouped by degree of refractive astigmatism into: high (-2.75 to -5.0 diopters), moderate (-1.25 to -2.50 diopters), and low (< -1.0 diopter) astigmatism. At 6 months after the procedure the mean reduction from the preoperative refractive cylinder was 80.7% in the high, 68.4% in the moderate, and 47.6% in the low astigmatism group. 89.2% of the eyes with low, 81.8% with moderate, and 85% with high cylinders had achieved a final, uncorrected visual acuity of between 6/6 and 6/10. The VISX laser is an effective tool for the correction of high and moderate astigmatism, but is less effective for low astigmatism.

Adult↗
PubMed1996

Analytical and clinical evaluation of refractive index-matched anomalous diffraction (RIMAD) for assessment of fetal lung maturation.

We have evaluated refractive index-matched anomalous defraction (RIMAD) (Dubin SB, Clin Chem 1988;34:938-43) as a potential method for assessment of fetal lung maturity (FLM). This method determines the total light scattered by the surfactant-containing lamellar bodies by subtraction of the A650 from amniotic fluid diluted in glycerol from that of amniotic fluid diluted in distilled water. It is not significantly affected by such contaminating chromogens as hemoglobin and bilirubin up to 2.0 g/L and 11.0 mg/L, respectively. However, the addition of as little as 2.5 microL of erythrocytes as whole blood resulted in significant interference. RIMADs for normal respiratory outcomes (n = 78) ranged from 0.018 to 0.471. RIMADs for respiratory distress syndrome (RDS) outcomes (n = 8) ranged from 0.004 to 0.036. Use of a RIMAD referent value of > 0.040 to indicate maturity yielded sensitivity, specificity, predictive value (PV)RDS, and PVmaturity of 100%, 96.2%, 72.2%, and 100%, respectively. The areas under the receiver-operating characteristic curves were 0.997 for the RIMAD assay, 0.993 (P = 0.3) for the TDx-FLM assay, 0.89 (P = 0.017) for the lecithin/sphingomyelin ratio, and 0.87 (P = 0.023) for the foam stability index.

Amniotic Fluid↗