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Biomedical subjects

J C Barry

Publications and source records attributed to J C Barry.

At least 37 records · Page 2Linked to original sources
PubMed1998

[Construction of a model eye for simulation of Purkinje reflections for determining the radii of curvature and the position of the crystalline lens].

UNLABELLED: Specular reflections at the optical interfaces of the eye, the Purkinje reflections, are used in physiologic optics and ophthalmology for biometric measurements. To date, there is no standard in the measurement of crystalline lens radii of curvature and position. To that end model eyes were designed. MATERIALS AND METHODS: The most important goal was to simulate the Purkinje reflections I, III, and IV, in the human eye as realistically as possible. The casing of the model eye was made of brass; the optical components were made from contact lens material. RESULTS: Several variants of components were manufactured to reproduce the range of variations in human eyes. By combining different components it was possible to vary the radii of curvature of the cornea and of the crystalline lens, axial separations, crystalline lens rotations and decentrations independently and reproducibly. First applications included devices for biometry of the crystalline lens (phacometry), validation of methods for eye alignment measurement, and refractometry. CONCLUSIONS: The properties of the specular reflections of the model eye are very close to the target values in human eyes. One of the advantages of the model eye is the compact design. This model eye is the first measuring standard for phakometry.

Anthropometry↗
PubMed1997

Algorithm for Purkinje images I and IV and limbus centre localization.

PURPOSE: To develop a fast and reliable image processing algorithm for the extraction of Purkinje reflection data for measurement of ocular alignment with standard PC hardware. METHODS: Purkinje images I and IV are specular reflections of light sources, the positions of which are proportional to eye rotation. Fast histogram-based thresholding and gradient techniques were used to localize Purkinje images I and IV, the pupil centre and the limbus centre in images obtained with near infrared light. RESULTS AND CONCLUSIONS: Execution time was 500 ms per eye. A set of 540 images was evaluated of which 94% were automatically analysed.

Accommodation, Ocular↗
PubMed1997

Catoptric properties of eyes with misaligned surfaces studied by exact ray tracing.

PURPOSE: Historically, Purkinje images have been used to calculate ocular surface curvature and misalignment. The purpose of this report is to introduce an exact ray-tracing program that can be used to examine the influence of ocular component variations on the size and position of Purkinje images I, III, and IV. METHODS: Ray tracing was carried out on Le Grand's four-surfaced schematic eye to demonstrate the main features of the program. Any location may be chosen for the point light source and the observer. Ocular component dimensions, eye rotation, and crystalline lens decentering and rotation are fully adjustable. The program computes the coordinates of the Purkinje images in three-dimensional space from the point of view of the observer. It also offers the option of exhibiting Purkinje images seen through standard and telecentric imaging devices. Both options yield Purkinje image positions in relation to the rotating center of the limbus, observed clinically. RESULTS: The resulting program compared favorably with current ray-tracing software and with data from the literature. CONCLUSIONS: The program has many research and teaching applications. For example, our requirement was to calculate a series of linear coefficients that closely approximate Purkinje-image behavior in any given eye. These coefficients may be used to measure misalignment of intraocular components in phakic and pseudophakic eyes.

Humans↗
PubMed1997

Limbus versus pupil center for ocular alignment measurement with corneal reflexes.

PURPOSE: To investigate the accuracy of ocular misalignment measurement, using corneal reflections. METHODS: Corneal reflex positions were measured relative to two landmarks, the limbus center and the entrance pupil center, using high-resolution digital images for cyclopean gaze angles from 0 degree to 18.8 degrees (34.04 prism diopters; PD) to the right and to the left in 10 subjects. Distance h from the center of the corneal curvature to each landmark was determined from linear regressions and showed significant differences between both conditions: mean hlimbus was 5.243 mm, and mean hpupil was 4.884 mm. From these, limbus- and entrance-pupil-center-related Hirschberg ratios were determined as 11 degrees/mm (19.43 PD/mm) and 11.82 degrees/mm (20.92 PD/ mm), respectively; and ocular alignment was calculated for both conditions. Simulated angles of strabismus were calculated as the binocular difference between ocular alignment of the right and left eyes (condition 1), and as the monocular difference between the ocular alignment of right eyes and left eyes separately (condition 2). RESULTS: Condition 1: Errors in simulated angles of strabismus were approximately twice as large for entrance pupil center compared with those in limbus-center-based evaluation; in the primary position, the 95% pupil-related confidence interval of the binocular difference was +/- 5.217 degrees (9.1 PD), compared with +/- 3.174 degrees (5.5 PD) for the limbus-related option. Condition 2: Errors were approximately equal. CONCLUSIONS: The entrance pupil center is a less reliable landmark than is the limbus center for measuring ocular alignment by using corneal reflections, because of unreliable positions of the entrance pupil center; different mean Hirschberg ratios should be used in both conditions.

Blinking↗
PubMed1997

Improved computing scheme for measuring eye alignment with Purkinje images I and IV.

This study introduces an improved computing scheme for determining eye rotation from Purkinje images I and IV. The original computing scheme systematically underestimated eye rotation. Paraxial raytracing calculations revealed that this error resulted from failure to account for the fact that Purkinje images I and IV fall at different distances behind the cornea. The error could be overcome with a correction factor derived from paraxial raytracing calculations. A series of experiments were carried out to test the validity of this correlation factor, involving exact raytracing calculations as well as measurements on physical model eyes and human eyes. The influence on the correction factor of ocular surface asphericity, accommodation, age and ocular component variations were examined. The new method was also compared to Hirschberg's technique, which makes use of Purkinje image I alone, as a means of screening for strabismus.

Child↗
PubMed1996

[Detection and diagnosis of small ocular misalignment with the Purkinje reflex pattern method].

BACKGROUND: Application test for an automatic classification strategy for ocular alignment data for the detection of ocular misalignment in strabismic patients. METHODS: Photographic Purkinje Reflection Pattern Evaluation was used a) with a handheld device for the detection and measurement of ocular misalignments in near fixation (group 1, n = 64 strabismic patients) and b) with a stationary device for the detection and measurement of ocular misalignments in near fixation (group 2, n = 38 patients) and in distance fixation (group 3, n = 36 patients). The orthoptic diagnoses were mostly primary and secondary microtropia with manifest angles of strabismus from naught or 0.25 degrees to 3-4 degrees, with maximum angles up to 6-9 degrees. The ocular alignment data were classified using the computer based strabismus index procedure. This strategy relies on thresholds derived from means and standard deviations in orthotropic control populations. In this way the data sets were classified automatically as "no referral" or "referral". In addition, an automatic diagnosis of the type of misalignment was given and the results were compared to the orthoptic gold standard. RESULTS: The sensitivity for the detection of a manifest ocular misalignment was a ca. 80% in group 1 and 2, and 90% in group 3, with specificities from 90% to 100%. All manifest angles of strabismus larger than 1 degree were correctly classified as "referral". There was good agreement between the diagnoses of the type of misalignment in most cases. Discrepancies were observed with very small ocular misalignments or with incomplete data sets, or they could be explained by a switch of fixation. The amount of the misalignment varied markedly as compared to the orthoptic measurement in a number of cases. CONCLUSIONS: The examination allows for a detection of small manifest ocular misalignments with a very high sensitivity. The deviated eye and the type of the misalignment in the primary position are evaluated automatically by a data base computer algorithm. The differences between the measured angles of strabismus indicate that the photographic examination conditions and the orthoptic simultaneous prism and cover test conditions are not exactly alike. Purkinje Reflection Pattern Evaluation represents a step towards an examiner-independent measurement of the angle of strabismus.

Adolescent↗
PubMed1995

Self-assessment of angles of strabismus with photographic Purkinje I and IV reflection pattern evaluation.

BACKGROUND: Accurate assessment of the angle of strabismus, e.g. of variable angles of strabismus, is crucial in preoperative patient management and is usually performed in a clinical environment. Objective assessment by patients themselves, under everyday conditions, could contribute to a better preoperative work-up. A new objective evaluation procedure for the measurement of manifest angles of strabismus for near and distance fixation by the patient himself is presented. METHODS: To account for the modified experimental setup used for the self-assessment, an amended computation procedure of Purkinje reflection pattern evaluation was developed. For measurement, patients and controls placed their head on a head/chin rest and fixated at 33 cm or 4 m distance in primary position. A reflex camera and three photo flash units were positioned on a special frame underneath the visual axis and in front of the subject so that both eyes could be photographed simultaneously. The camera's remote shutter control was released together with the photo flash units by the properly fixating subject. The angles of strabismus were obtained from the series of pictures through later evaluation of the Purkinje I and IV reflection patterns recorded in the photographs of the eyes. RESULTS: Measurements of the ocular alignment in two control groups and in a group of strabismic subjects showed satisfactory accuracy of the "self-assessment" method compared to "standard" Purkinje reflection pattern evaluation and orthoptic measurements of the angle of strabismus. CONCLUSION: The modified "self-assessment" method can be used for the objective recording of angles of strabismus as needed in the preoperative work-up of patients with variable angles of strabismus, over prolonged periods of time, and outside a clinical setting.

Adolescent↗
PubMed1994

Computational principles in Purkinje I and IV reflection pattern evaluation for the assessment of ocular alignment.

PURPOSE: To develop a standardized reasoning for the evaluation of Purkinje I and IV Reflection Pattern data in primary, secondary, and tertiary positions of gaze in the diagnosis of strabismus with a mathematical approach. To demonstrate the applicability of certain mathematical relations and the appropriate graphic representation of computed ocular alignment data. METHODS: Starting from the known Reflection Pattern Evaluation formulae, equations were derived that allowed for the computation of the relative and absolute positions of the optical and visual axes of both eyes from original data in binocular and monocular fixation. These equations were simplified for clinical use. RESULTS: The authors obtained a set of equations that could be applied to the objective, quantitative analysis of eye alignment in screening for microtropia, in concomitant and incomitant diagnoses of strabismus in primary and nonprimary positions. CONCLUSIONS: Purkinje I and IV Reflection Pattern Evaluation can be extended to the diagnosis of strabismus in nonprimary positions with sufficient clinical accuracy. The newly presented principles and equations serve as a basis for a convenient graphic representation of Purkinje I and IV Reflection Pattern data. These principles of evaluation may be applied to any data dealing with ocular alignment, independently of the method.

Afterimage↗
PubMed1994

Measurement of ocular alignment with photographic Purkinje I and IV reflection pattern evaluation.

PURPOSE: To provide reference data for the measurement of ocular alignment and of angles of strabismus with a new stationary photographic apparatus at near and at distance fixation; to verify quantitative relations between the data by comparing experimental data with theoretical predictions. METHODS: Use of Purkinje I and IV Reflection Pattern Evaluation in conjunction with a new stationary photographic apparatus; application of previously derived equations; simulation of angles of strabismus of 5 degrees and 10 degrees in the primary position. RESULTS: Data from 62 subjects with orthotropia show good linearity of measured angles in different directions of gaze; a 95% confidence interval for errors up to 18.6% in the simulated angles of strabismus; no need for individual calibration of the apparatus; no bias due to wearing of spherical corrections, and a detection threshold for microstrabismus of +/- 0.35 degrees. CONCLUSIONS: Data concerning orthotropic and simulated strabismus prove the accuracy of Reflection Pattern Evaluation to assess ocular alignment, for instance, for the diagnosis of strabismus in primary and secondary positions, at near and distance fixation. The measuring range can easily be extended to larger angles of strabismus.

Adult↗
PubMed1994

[Computer-assisted measurement of ocular misalignment in infants and young children using the digital Purkinje reflection pattern procedure].

A digital image recording and processing system is presented that allows a quick diagnosis of microstrabismus in non-cooperative children. It is thus particularly suited for screening purposes. METHOD: The Purkinje Reflection Pattern Evaluation (RPE) method is used: three small flashes are used to produce the desired Purkinje images. Two horizontal rows of the three 1st Purkinje images (anterior corneal reflections) and of the three 4th Purkinje images (posterior crystalline lens reflections) stemming from the three light sources form the characteristic Purkinje image reflection pattern. Each eye's position is calculated from the shift between the upper and lower rows of reflections by means of two simple formulae. From the angles obtained in binocular fixation and monocular fixation the manifest angle of strabismus corresponding to the angle measured in the simultaneous prism-and-cover test is computed. The measurement is performed at a fixation distance of 50 cm under natural viewing conditions. To obtain a picture one only has to get the child's attention for a short moment. The primary position is triggered with the fixation light, which is operated by a switch. APPARATUS: The digital image recording is done with a hand-held device comprising two miniaturized video cameras, three photo flashes and a fixation light that is operated manually. An IBM-compatible PC equipped with a hard disk and two frame grabbers was adapted for the storage and processing of the pictures. The pictures are evaluated interactively in a few minutes on the workstation's monitor immediately after the measurement. To this end specially designed menu-driven software was implemented. RESULTS: Examples of the measuring procedure and clinical results in infants with microtropic highlight the potential of the system as a screening apparatus and for the exact measurement of small and large squint angles. Usually even 1-year-old children can cooperate well enough to get good-quality pictures in binocular fixation. The new digital system allows easy and rapid application of the Purkinje Reflection Pattern Evaluation method since the time-consuming photographic film processing and evaluation are no longer necessary. For the first time small angles of strabismus under 5 degrees (10 PD) can be measured with a precision of less than 1 degree (2 PD) under clinical conditions in non-cooperative children.

Adult↗
PubMed1992

Objective measurement of small angles of strabismus in infants and children with photographic reflection pattern evaluation.

A new photographic method for the precise measurement of the angle of squint in children and infants is presented. The apparatus consists of a reflex camera with three horizontally aligned flashes and a small fixation light. The subjects are photographed while fixating binocularly or monocularly in the primary or secondary position. Six reflections can be seen on the photograph of each pupil. These are the first and fourth Purkinje images of each light source. From the reflection patterns, the squint angle or the angle kappa or alpha can be computed using a simple formula. A vertical angle of strabismus may be calculated from the same photograph using the same principle. Results from orthotropic and strabismic adults and from children are evaluated to establish reference values. Cooperation from the children is generally very good. The accuracy of the new method is between between 2 and 4.5 prism diopters (between 1 degree and 2.5 degrees), depending on the measuring strategy.

Adult↗
PubMed1991

[A new photographic method for measuring squint angles in infants and small children].

A new photographic method for measuring squint angles in children and infants is presented. A photographic picture is taken from the subject, using a camera with the three flashes bulle. One flashbulle is placed vertically over the lens, two other flashbulles are placed symmetrically in an angle of 10 degrees beside the middle flash. Six reflections can be seen on the photographic picture in each pupil. (1. and 4. Purkinje Sanson Images of each light source.) The horizontal distance of two reflections is determined by the distance of the flashguns. If the reflection lines in both eyes are symmetrical, there is no squint. If there is a shift, it can be measured on the slide and by using a simple formula the squint angle can be calculated. The accuracy of the method is between 2 and 3 degrees.

Child, Preschool↗
PubMed1990

New Sivapithecus humeri from Pakistan and the relationship of Sivapithecus and Pongo.

New humeri of two species of the Miocene hominoid Sivapithecus are described from near Chinji in Pakistan from between approximately 9 and 11 Myr ago. Sivapithecus, a middle and late Miocene hominoid from Turkey and Indo-Pakistan, is overall unlike any living hominoid, although facial-palatal similarities to the extant orangoutan, Pongo, have been used to support a hypothesis of close relationship. Living hominoids have postcranial similarities assumed to be shared derived, among them features of the proximal humerus. However, the new Sivapithecus proximal humeri differ from those of living hominoids, supporting an alternative hypothesis in which Sivapithecus and Pongo are not closely related. It is not clear how to choose between these incompatible hypotheses.

Animals↗
PubMed1989

High resolution electron microscopy of nonstoichiometric apatite crystals.

The application of high resolution electron microscopy, computer image processing, and image simulation techniques to the investigation of synthetic nonstoichiometric apatites has provided new details of apatite crystal growth mechanisms. Under certain precipitation conditions, calcium-deficient apatites with distinct octacalcium phosphate (OCP)-apatite intergrowths have been observed. Apatite crystals with unit-cell thick overgrowths of OCP on their surfaces confirmed the stepwise hydrolysis crystal growth mechanism initially proposed by Brown (Nature 196:1048-1050). However, many crystals also contained a central two-dimensional OCP inclusion one to two unit cells thick, embedded in an apatite matrix. Similar planar defects have been observed in dental enamel, dentin, and bone crystals. We have developed a modified version of Brown's stepwise OCP hydrolysis apatite crystal growth mechanism to explain the formation of crystals with OCP central planar defects. The mechanism involves the nucleation of an OCP seed that grows until it reaches a critical size, rh, before OCP hydrolysis occurs. Apatite subsequently grows epitaxially on the OCP seed, thereby embedding it in the center of an apatite crystal. Apatite growth is facilitated by partial screw dislocations emanating from the planar defect.

Animals↗
PubMed1988

Oxygen ordering and twinning in YBa2Cu3O7-x.

Direct structure images of the YBa2Cu3O7-x high Tc superconducting ceramic (also called the 1-2-3 compound) at 1.7 A resolution have been obtained for the [100] and [001] orientations. It was found that for the purposes of studying oxygen ordering in this compound it is better to use lattice images of lower resolution. The oxygen ordering was studied via the measurement of the bending of (100) and (110) lattice planes on crossing the (110) twin boundaries in crystals oriented in the [001] zone. Significant variations were found in the b/a ratios, owning to a variation in oxygen ordering, between different crystal grains, and between different regions in the same grain. For the three different 1-2-3 samples studied, the average b/a ratio was 1.016, the same value as was found in neutron diffraction studies. The twin boundaries in the orthorhombic 1-2-3 phase are sharp and planar. It seems likely that the transformation from the high-temperature tetragonal phase to the lower-temperature orthorhombic phase is martensitic in nature. A new phase has been discovered on some of the twin boundaries. The new phase can be indexed as tetragonal with a = 7.5 +/- 0.2 A, and c = 6.8 +/- 0.2 A. It is possible that the new phase is stabilized by the stress which occurs at the twin boundaries.

Barium↗
PubMed1986

The structure of (100) defects in carbonated apatite crystallites: a high resolution electron microscope study.

Planar defects parallel to (100) with an approximate [1/400] displacement vector have been identified by high resolution transmission electron microscopy and by micro-electron diffraction in the center of synthetic carbonated apatite crystallites. Similar intergrowths, 0.8-1.5 nm in width, have been observed in dental enamel, dentin and bone apatite crystallites. Four possible structural models of the defect core are proposed to explain these experimental features, and computer-simulated lattice images of the models are compared with the experimental images. Typical defects were consistent with a two-dimensional octacalcium phosphate inclusion, one unit cell thick, embedded in an apatite matrix.

Apatites↗
PubMed1986

Theoretical detection of a dark contrast line in twinned apatite bicrystals and its possible correlation with the chemical properties of human dentin and enamel crystals.

Electron microscope images of twinned apatite bicrystals oriented along the [1120] crystallographic direction have been simulated for various experimental conditions, and the validity of the calculation has been checked. These images show a dark contrast line similar to the one observed experimentally in enamel and dentin crystals and therefore strongly suggest the presence of a twin plane parallel to the (1100) crystallographic planes, in these crystals. The presence of a twin boundary in teeth and bone crystals is of prime importance for the adsorption and the dissolution properties of the calcified tissues as a whole.

Apatites↗