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Biomedical subjects

Michael J Collins

Publications and source records attributed to Michael J Collins.

At least 37 records · Page 2Linked to original sources
PubMed2005

Regression of lid-induced corneal topography changes after reading.

PURPOSE: The purpose of this article is to investigate the magnitude of lid-induced corneal topography changes as a function of time spent reading and the subsequent time course of regression of these changes. METHODS: Six young subjects, five myopes and one emmetrope with normal ocular health, participated in the study. Corneal topography of one eye was measured with videokeratoscopy before four reading sessions of 10, 30, 60, and 120 min duration performed on four separate mornings. Corneal topography was again measured at fixed intervals up to 180 min after each reading session. A control trial without reading was also performed on a separate morning. RESULTS: All six subjects showed significant changes in corneal topography directly after each of the reading sessions. Longer reading periods caused larger corneal changes. Local instantaneous power changes reached up to 5.95 D (+/- 2.80 D), whereas refractive power changes in the superior cornea ranged up to 1.26 D (+/- 0.44) for the 120-min reading trial. The duration of regression of corneal changes showed slower recovery times after longer reading periods. The pattern of regression was similar for all reading times, showing a rapid recovery within the first 10 minutes followed by a slower regression period. CONCLUSIONS: The length of time spent reading has a cumulative effect on the period over which corneal topography remains altered as a result of lid forces.

Adult↗
PubMed2005

Evaluating tear film stability in the human eye with high-speed videokeratoscopy.

High-speed videokeratoscopy (HSV) is an emerging technology that has the potential to acquire information on the dynamics of corneal topography and tear-film behavior. We show that the surface regularity and asymmetry indices, which are traditionally used for characterizing the stability of precorneal tear film, have limitations in the context of HSV because they are highly sensitive to natural ocular microfluctuations. To overcome this problem, we propose a new microfluctuation-independent surface indicator. It is based on the root-mean-square of the error of the parametric model fit to the surface. Further, we develop techniques for estimating the tear film build-up and break-up times. The tear film build-up time estimator is based on the proposed RMS fit surface indicator while the tear film break-up time estimator is derived directly from a set of consecutive HSV digital images, without the need for estimating the resulting corneal surface.

Computer Simulation↗
PubMed2005

Applications of high-speed videokeratoscopy.

High-speed videokeratoscopy is an emerging technology that has the potential to provide new information on dynamic changes of corneal topography and tear film behaviour. We have developed a high-speed videokeratoscope that has the ability to acquire data at the rate of 50 Hz. Two major applications of the technology are considered in this paper. First, the analysis of tear film stability in the inter-blink interval is evaluated and techniques for estimating the tear film build-up and break-up times are considered. The second application involves the study of the dynamic response of the corneal anterior surface to mechanical forces exerted by the eyelids during horizontal eye movements in downward gaze. The limitations and potential opportunities for the use of this new technology are discussed.

Cornea↗
PubMed2005

Morphological and clinical findings in 247 surgically excised native aortic valves.

Aortic valve disease is an increasingly common medical problem in the 20th century; however, its pathogenesis is poorly understood. The present paper reviews 247 cases of aortic valves surgically excised over a one-year period, with special attention to the morphological and histological changes observed in surgically excised aortic valves, and to how those changes relate to the mechanisms of valve failure. Patient age ranged from 17 to 86 years, with a mean of 64.1 +/-13.4 years, and there were 156 men (63.2%) and 91 women (36.8%). A total of 172 valves (69.6%) were tricuspid and 75 valves (30.4%) were congenitally abnormal, of which 67 valves (27.1%) were congenitally bicuspid, six valves (2.4%) were congenitally unicommissural and one valve (0.4%) was quadricuspid; the status of one valve could not be determined. Almost 70% of patients had features of aortic stenosis. Calcification was seen in the large majority of cases, with almost 70% of the valves showing either grade 3 or grade 4 calcification. Ulceration of calcific nodules was present in 42.0% of cases and in the majority of the valves with grade 4 calcification. A total of 15% of the valves showed evidence of postinflammatory changes consistent with rheumatic disease. Neovascularization was present in two-thirds of all valves, while one-half showed inflammatory cell infiltrates. Inflammatory cells were composed mainly of lymphocytes and macrophages, and were located primarily near areas of neovascularization. There was no significant difference in the pathological findings between men and women.

Adolescent↗
PubMed2004

Near work induced wavefront aberrations in myopia.

We undertook a detailed analysis of the wavefront aberrations of the eyes of 20 young progressing myopes (mean age=22 years; mean spherical equivalent=-3.84 D, range -1.00 to -7.5 D) and twenty young age matched emmetropes (mean age=23 years; mean spherical equivalent=-0.00 D, range +0.25 to -0.25 D). A wavefront sensor was used to measure the ocular wavefront and a videokeratoscope was used to measure corneal topography. The corneal wavefront was subsequently calculated and the difference between the corneal and ocular wavefront was derived to give the internal wavefront component of the eye. Ocular and corneal wavefronts were measured before and after a 2-h reading task. At the baseline measurements, the myopes showed greater levels of some high order ocular wavefronts than the emmetropes. These differences between the groups became larger following 2 h of reading. Ocular higher order wavefront RMS was (baseline RMS: myopes=0.21 microm, emmetropes=0.16 microm, difference p=0.05 and after 2 h reading was RMS: myopes=0.27 microm, emmetropes=0.17 microm, difference p=0.02). The differences between the groups are primarily due to changes in the corneal wavefront associated with a narrower lid aperture during reading for the myopes. These differences are enhanced by longer periods spent reading, larger pupils and consequently low light levels. We suggest lid induced corneal changes caused by reading in downgaze provides a theoretical framework that could explain the known features of myopia development. The inherited characteristics of facial and lid anatomy would provide a mechanism for a genetic component in the genesis of myopia.

Adult↗
PubMed2004

Automatic pupillometry from digital images.

Determination of two-dimensional characteristics of the anterior surface of the eye is becoming increasingly important in modern optometry and ophthalmology practice. In particular, accurate estimation of the pupil size and centration is crucial in customized refractive surgery, corneal transplantation, and advanced contact lens fitting. The pupil parameters change under different lighting conditions so they often need to be related to some fixed reference such as the limbus outline. However, current commercial pupillometers do not estimate limbus position. We present a novel algorithm for automatic extraction of pupil parameters from digital images that takes the relative limbus information into account. The algorithm utilizes several customized image processing techniques that form a robust procedure which performs well for a wide range of clinical images. We apply the developed algorithm to images obtained by a standard digital camera, and specialized ophthalmic instruments such as a wavefront sensor and a high-speed imaging system.

Algorithms↗
PubMed2004

A refined bootstrap method for estimating the Zernike polynomial model order for corneal surfaces.

Following our previous work on optimal modeling of corneal surfaces with Zernike polynomials, we have developed a refined bootstrap-based procedure which improves the accuracy of the previous method. We show that for normal corneas, the optimal number of Zernike terms usually corresponds to the fourth or fifth radial order expansion of Zernike polynomials. On the other hand, for distorted corneas such as those encountered in keratoconus or in surgically altered cases, the estimated model was found to be up to three radial orders higher than for normal corneas.

Algorithms↗
PubMed2004

Analyzing the dynamic wavefront aberrations in the human eye.

The optics of the human eye are not static in steady viewing conditions and exhibit microfluctuations. Previous methods used for analyzing dynamic changes in the eye's optics include simple Fourier-transform-based methods, which have been used in studies of the eye's accommodation response. However, dedicated tools for the analysis of dynamic wavefront aberrations have not been reported. We propose a set of signal processing tools, the combination of which uncovers aspects of the dynamics of eye's optical aberrations which were hidden from conventional analysis techniques. The methodology includes extraction of artifacts from potentially significant eye movements, filtering, optimal parametric signal modeling, and frequency and time-frequency representations. The exposition of the techniques and their advantages over traditional techniques is illustrated for real dynamic eye wavefront aberration measurements.

Algorithms↗
PubMed2004

Microfluctuations of wavefront aberrations of the eye.

To investigate fluctuations in the wavefront aberrations of the eye and their relation to pulse and respiration frequencies we used a wavefront sensor to measure the dynamics of the Zernike aberrations up to the polynomial fourth radial order. Simultaneously, the subject's pulse rate was measured, from which the instantaneous heart rate was derived. We used an auto-regressive process to derive the power spectra of the Zernike aberration signals, as well as pulse and instantaneous heart rate signals. Linear regression analysis was performed between the frequency components of Zernike aberrations and the pulse and instantaneous heart rate frequencies. Cross-spectrum density and coherence analyses were also applied to investigate the relation between fluctuations of wavefront aberrations, and pulse and instantaneous heart rate. The correlations between fluctuations of individual Zernike aberrations were also determined. A frequency component of all Zernike aberrations up to the fourth radial order was found to be significantly correlated with the pulse frequency (all R(2) >/= 0.51, p < 0.02), and a frequency component of nine out of 12 Zernike aberrations was also significantly correlated with instantaneous heart rate frequency (all R(2) >/= 0.46, p < 0.05). The major correlations among Zernike aberrations occurred between second-order and fourth-order aberrations with the same angular frequencies. Higher order aberrations appear to be related to the cardiopulmonary system in a similar way to that reported for the accommodation signal and pupil fluctuations.

Accommodation, Ocular↗
PubMed2004

Constrictive pericarditis: case presentation and a review of the literature.

Constrictive pericarditis (CP) is a relatively rare entity, the result of a thickened, scarred and often calcified pericardium, adherent to the heart and limiting diastolic ventricular filling. Some reported etiologies for CP include pericardial involvement from cardiac trauma (including surgery), mediastinal irradiation, tuberculosis and other infectious diseases, neoplasms and myocardial infarction. However, the majority of CP cases are idiopathic. This report presents three cases of postoperative, tuberculous and idiopathic CP, highlighting the clinical and pathological changes seen in CP, and a review of the literature.

Adult↗
PubMed2003

Corneal topography and accommodation.

PURPOSE: To investigate whether there are significant changes in corneal topography during accommodation in normal corneas and corneas that are pathologically thinner due to keratoconus. METHODS: A videokeratoscope was modified to present an accommodation stimulus that was coaxial with the instrument's measurement axis. Six subjects with normal corneas and four subjects with keratoconus were studied. Eighteen videokeratoscope measurements of one eye of each subject were taken at 0 diopter (D) accommodation demand and six for both of 4 D and 9 D accommodation demand. The effects of ocular micromovements on multiple topography maps were minimized using software algorithms. Average maps for the 4 D and 9 D accommodation demands were calculated and subtracted from the average map of the 0-D accommodation demand. A t test was applied at each point location within the topography maps to analyze the statistical significance of change (p < 0.001) within the difference maps. RESULTS: In the initial analysis, we found that a number of the subjects showed significant changes in corneal topography as accommodation changed. However, further analysis showed a significant group mean excyclotorsion of the topography maps of 1.6 +/- 1.1 degrees (p < 0.03) for the 4-D stimulus and 2.0 +/- 1.3 degrees (p < 0.01) for the 9-D stimulus compared with the 0-D stimulus. When we accounted for the excyclotorsion, we did not find clear evidence of statistically significant changes in corneal topography as a result of accommodation, either for the normal corneas or the keratoconic corneas. CONCLUSIONS: It appears unlikely that changes occur in central corneal shape during accommodation up to a level of 9 D in normal or keratoconic corneas. A small ocular excyclotorsion typically accompanies accommodation, and this changes the relative orientation of the topography of the cornea. This has significant implications for the interpretation of the optical characteristics of eyes during near viewing conditions.

Accommodation, Ocular↗
PubMed2003

Corneal aberrations and reading.

PURPOSE: To investigate the effects of eyelid pressure on corneal shape and corneal aberrations during reading. METHODS: Twenty young subjects with normal ocular health were recruited for the study. The experiment was conducted early in the morning, with subjects instructed not to perform any prolonged reading before the experiment. Corneal topography of one eye was measured with a videokeratoscope before reading and then again after a 60-min reading task. The natural position of the eyelids was photographed in primary gaze and during the reading task. RESULTS: Twelve of the 20 corneas showed significant changes in central topography immediately after reading. The location of the changes corresponded closely to the position and angle of the subject's eyelids during reading. The change in shape was best described as a wave-like distortion that significantly altered some corneal wavefront Zernike coefficients. There was a significant correlation between the changes in primary vertical coma and trefoil (along 30 degrees). Within the central 6 mm of the cornea, there were significant changes in the root mean square error, overall refractive power, and astigmatism. CONCLUSIONS: The changes we observed in corneal topography appear to be directly related to the force exerted by the eyelids during reading. Because the cornea is the major light-refracting surface of the eye, the optical characteristics of some eyes can be significantly changed during reading by the force of the eyelids. These findings may have important implications for the definition of refractive status and may also aid in the understanding of the relationship between reading and the development of refractive errors.

Adult↗
PubMed2002

Accuracy of Orbscan II slit-scanning elevation topography.

PURPOSE: To establish the accuracy of Orbscan II (Orbtek Inc.) slit-scanning elevation topography in analyzing the anterior surface of complex test objects. SETTING: Discipline of Ophthalmology, University of Auckland, Faculty of Medical and Health Sciences, Auckland, New Zealand. METHODS: Six test objects were created from 2 materials: standard calibration poly(methyl methacrylate) (PMMA) (Orbtek Inc.) and a research PMMA material. The test objects were produced with spherical (radii of curvature 6.00 mm, 7.67 mm, and 8.88 mm), aspherical (apical radius 7.67, eccentricity 0.5, Q -0.25), and toroidal (7.67/7.92 mm radii of curvature) surfaces. The accuracy of the test surfaces was established by Form Talysurf Analysis. A single calibrated Orbscan II device was used to obtain 20 separate anterior elevation maps of each test object. The data obtained from Orbscan II, at 0.2 mm intervals along the chosen meridian, were directly compared with the Talysurf values for each test surface. RESULTS: Orbscan II measurements of all test objects were statistically significantly different from the Talysurf values (P <.001). The test objects produced from standard calibration material were more accurately measured by Orbscan II than the objects produced from the research material. Data obtained by Orbscan II from the central 3.5 mm of all test objects were more accurate than peripheral data when compared with the Talysurf values (P =.001). CONCLUSIONS: Orbscan II anterior surface elevation measurements differed significantly from Form Talysurf Analysis of complex test surfaces. However, the magnitude of the errors in the measurement of standard test objects was small, less than 0.20 microm centrally and 0.70 microm peripherally. Clinically, if similar accuracy of measurement is confirmed in the human eye, anterior surface elevation maps can be considered accurate representations of corneal shape.

Contact Lenses↗
PubMed2002

Ocular microfluctuations and videokeratoscopy.

PURPOSE: We aimed to minimize the effect of ocular microfluctuations during videokeratoscopy to improve the qualitative and quantitative analysis of corneal topography. METHODS: A technique was developed to measure the tilt, displacement, and cyclotorsion in multiple videokeratographs from the same cornea. This information was used to reposition each videokeratograph according to the average position of a sample of multiple measurements. The corneal topography of 10 subjects was measured 20 times each, using videokeratoscopy. RESULTS: The root mean square error calculated from difference between single videokeratographs and the average videokeratograph decreased by an average of 24.6% for the 10 subjects' data. The standard deviation of some terms of Zernike polynomials fitted to the cornea was significantly reduced after the procedure. CONCLUSION: The method that we propose can improve the precision performance of videokeratoscopy in multiple measurements of corneal topography.

Adult↗
PubMed2002

Optical considerations in the contact lens correction of infant aphakia.

BACKGROUND: Correction of infant aphakia with conventional soft and rigid designs induces significant amounts of positive spherical aberration. METHODS: Different levels of positive spherical aberration were induced by rigid lenses positioned close to the eye. Visual acuity and depth of focus were measured for twelve young adult subjects. RESULTS: Positive spherical aberration acts to produce an increase in relative depth of focus, but because of the reduction of best visual acuity, there is a concurrent decrease in absolute depth of focus. As the spherical aberration increases, there is a progressive loss of best-corrected visual acuity. CONCLUSIONS: The correction of spherical aberration in contact lenses for infant aphakia provides the best theoretical optical conditions for normal ocular growth and visual development. We provide examples of the optimal aspheric surfaces to minimize spherical aberration in contact lenses for infant aphakes.

Adult↗
PubMed2002

Modeling of corneal surfaces with radial polynomials.

We consider analytical modeling of the anterior corneal surface with a set of orthogonal basis functions that are a product of radial polynomials and angular functions. Several candidate basis functions were chosen from the repertoire of functions that are orthogonal in the unit circle and invariant in form with respect to rotation about the origin. In particular, it is shown that a set of functions that is referred herein as Bhatia-Wolf polynomials, represents a better and more robust alternative for modeling corneal elevation data than traditionally used Zernike polynomials. Examples of modeling corneal elevation are given for normal corneas and for abnormal corneas with significant distortion.

Computer Simulation↗
PubMed2002

Automatic estimation of the corneal limbus in videokeratoscopy.

An algorithm for estimating the corneal limbus from videokeratoscopic images is proposed. After the image is transformed to a polar grid, a novel edge-detection procedure, suitable for the detection of the soft edge produced by the limbus, is used to locate the limbus. Outliers due to the eyelids, eyelashes, and videokeratoscopic rings are removed by taking advantage of the approximate circularity of the cornea. An ellipse which minimizes the sum of the squared algebraic errors is fitted to the remaining edge points. Comparisons between the proposed algorithm, a manual computer-based technique and an algorithm which uses conventional edge-detection techniques demonstrate the accuracy of the proposed algorithm.

Algorithms↗