Ocular Shape Research Articles

Correction of Ocular Shape in Retinal Optical Coherence Tomography and Effect on Current Clinical Measures

To address the misrepresentation of the eye in retinal optical coherence tomography (OCT) images and to examine the effect of this misrepresentation on retinal thickness measurements. Prospective case series. Five subjects with recent orbital magnetic resonance imaging (MRI) scans and normal eye examinations were consented from the clinics of the Duke Eye Center. Each subject had both eyes imaged using a retinal spectral-domain OCT system and ocular biometry measured. Two types of individualized optical models of the subject eyes-numerical and analytical-were used to determine the spatial paths of the OCT A-scans. These paths were used to reorient the A-scans in the associated retinal OCT images and generate corrected images. Using curvature as a general measure of shape, the radii of curvature of the retinal pigment epithelium in the original and corrected OCT images were compared to the ocular radii of curvature in the MRI images. Differences between the retinal thickness maps derived from the original and corrected OCT images were then determined. The retinal curvatures were substantially flatter in the original OCT than in the MRI images (mean paired difference: 52.8 ± 41.8 mm, P < .001). Correcting the OCT images decreased the paired differences between OCT and MRI (numerical: 1.6 ± 2.3 mm, P = .091; analytical: 1.9 ± 4.3 mm, P = .278). Retinal thickness measurements between the corrected and uncorrected images differed, with a root mean square difference of 5.61 μm over the entire 6-mm extent of the image; this difference was greater peripherally (6.02 μm) than centrally (2.54 μm). Optically based algorithms can be used to correct the shape of the retina as represented in OCT; this correction makes OCT more consistent with other clinical imaging techniques. Resultant retinal thickness maps were minimally affected by the change in shape. Ocular shape correction should be considered in future development of posterior segment OCT-based morphologic measurements.

Recovery of peripheral refractive errors and ocular shape in rhesus monkeys (Macaca mulatta) with experimentally induced myopia

This study aimed to investigate the changes in ocular shape and relative peripheral refraction during the recovery from myopia produced by form deprivation (FD) and hyperopic defocus. FD was imposed in six monkeys by securing a diffuser lens over one eye; hyperopic defocus was produced in another six monkeys by fitting one eye with −3D spectacle. When unrestricted vision was re-established, the treated eyes recovered from the vision-induced central and peripheral refractive errors. The recovery of peripheral refractive errors was associated with corresponding changes in the shape of the posterior globe. The results suggest that vision can actively regulate ocular shape and the development of central and peripheral refractions in infant primates.

Cross-sectional Sample of Peripheral Refraction in Four Meridians in Myopes and Emmetropes

The study of peripheral refractive error is of growing interest as degradation of the retinal image quality in the periphery is known to affect central refractive error development and ocular shape in animal models. The purpose of this study was to measure peripheral refraction across the horizontal, vertical, and two oblique meridians in a group of myopic and emmetropic adults and to investigate retinal asymmetry in the human retina. Thirty-one myopes (spherical equivalent between -2.00 and -9.62 D) and 20 emmetropes (spherical equivalent between -0.50 and +0.50 D) with astigmatism less than -0.75 D, participated in the project. Noncycloplegic peripheral refraction measurements were captured with an autorefractor (NVision K-5001; Shin-Nippon, Tokyo, Japan) at the fovea and up to 30° eccentricity in the horizontal, vertical, and two oblique meridians in 10° steps. RPR was calculated by subtracting the foveal spherical equivalent refraction from that obtained at each eccentric location. Along all measured meridians, myopic eyes showed a relative hyperopic shift in the periphery, with the superior-temporal portion of the retina exhibiting the smallest shift. Emmetropic eyes, however, exhibited a relatively consistent refractive profile across all meridians and eccentricities, confirming a spherical retinal shape for this group. In addition, off-axis astigmatism increased with eccentricity in all meridians. These results suggest that the myopic eye tends toward an ellipsoid shape, rather than the globular shape of an emmetropic eye.

Topographic Analyses of Shape of Eyes with Pathologic Myopia by High-Resolution Three-Dimensional Magnetic Resonance Imaging

To analyze the topography of human eyes with pathologic myopia by high-resolution magnetic resonance imaging (MRI) with volume rendering of the acquired images. Observational case series. Eighty-six eyes of 44 patients with high myopia (refractive error ≥-8.00 diopters [D] or axial length >26.5 mm) were studied. Forty emmetropic eyes were examined as controls. The participants were examined with an MRI scanner (Signa HDxt 1.5T, GE Healthcare, Waukesha, WI), and T(2)-weighted cubes were obtained. Volume renderings of the images from high-resolution 3-dimensional (3D) data were done by computer workstation. The margins of globes were then identified semiautomatically by the signal intensity, and the tissues outside the globes were removed. The 3D topographic characteristic of the globes and the distribution of the 4 distinct shapes of globes according to the symmetry and the radius of curvature of the contour of the posterior segment: the barrel, cylindric, nasally distorted, and temporally distorted types. In 69.8% of the patients with bilateral high myopia, both eyes had the same ocular shape. The most protruded part of the globe existed along the central sagittal axis in 78.3% of eyes and was slightly inferior to the central axis in the remaining eyes. In 38 of 68 eyes (55.9%) with bilateral pathologic myopia, multiple protrusions were observed. The eyes with 2 protrusions were subdivided into those with nasal protrusions and those with temporal protrusions. The eyes with 3 protrusions were subdivided into nasal, temporal superior, and temporal inferior protrusions. The eyes with visual field defects that could not be explained by myopic fundus lesions significantly more frequently had a temporally distorted shape. Eyes with ≥2 protrusions had myopic chorioretinal atrophy significantly more frequently than eyes with ≤1 protrusion. Our results demonstrate that it is possible to obtain a complete topographic image of human eyes by high-resolution MRI with volume-rendering techniques. The results showed that there are different ocular shapes in eyes with pathologic myopia, and that the difference in the ocular shape is correlated with the development of vision-threatening conditions in eyes with pathologic myopia. The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Advantage of higher oxygen transmissibility material Piggyback lens for keratoconus correction

To evaluate the oxygen transmissibility of new material Piggyback lens combined with silicone hydrogel lens and higher Dk RGPCL. To investigate the improving of visual acuity, ocular shape and comfortable, also orthokeratology results and safety of this new system for keratoconus correction. Calculating the oxygen transmission (Dk/t) of the new Piggyback system involves Ohm's Law. Compare the changes of ocular surface curves and astigmatism with or without Si-SCL fitting, RGPCL vision and Piggyback lens vision. Investigate the comfortable improving and ocular complications, and changes of corneal shape after fitted with Piggyback lens by using corneal topography. Selected silicone hydrogel lens was pure vision (B&L PV, 36% water, Dk/t 110) lens, RGPCL material was Menicon Z, Dk/t 95.9 and Boston XO, Dk/t 58.8. It was calculated to 44.4 Dk/t and 34.4 Dk/t with these two combinations. Twenty two moderate and severe keratoconus patients (28 eyes) fitted with the Piggyback lens successfully. Corneal K reading and astigmatism decreased significantly after wore the Si-SCL. Compared with RGPCL vision(4.82 ± 0.11), Piggyback lens vision (4.93 ± 0.08) was significant increased (t = -10.395, P < 0.01) 90% cases improved comfort vs prior RGPCL wear alone. Only few cases appeared slight conjunctival hyperemia and corneal staining. Corneal topography showed corneal flatter, spherical and regular reshaping results after 6 months wore the Piggyback lens. By fitting Piggyback systems using new Si-SCL and higher Dk RGPCL materials may help problematic keratoconic patients improve visual acuity, comfort and safety, increase wearing time and orthokeratology results further.

Peripheral Refraction in Different Ethnicities

Peripheral refraction is commonly used to infer retinal shape. Because of the different prevalence of myopia in the white compared with East Asian populations, peripheral refraction along the horizontal meridian was compared in white and East Asian young adults with emmetropic, low myopic, and moderately myopic refractive errors. Thirty-five white and 37 East Asian subjects were recruited with central refraction between +1.00 DS and -5.50 DS and ≤ -1.50 DC. Central and peripheral noncycloplegic autorefraction was measured along the horizontal meridian at 5° intervals up to ± 35°, and corneal topography maps were also quantified. There were no significant differences between whites and East Asians in peripheral refraction profiles in the emmetrope and low myope groups, which had peripheral myopic and emmetropic refraction, respectively. However, in the moderate myope group, there was a statistically significant difference between whites and East Asians in peripheral refraction, which was generally hyperopic. East Asian moderate myopes had more relative peripheral hyperopia than did whites of similar central refractive error. Corneal shape and power were comparable between white and East Asian subjects in the three refractive groups. East Asian moderate myopes have a greater degree of relative peripheral hyperopia and hence a more prolate ocular shape than do white subjects of similar central refractive error. Differences in ocular shape may play a role in the greater propensity for East Asians to develop and progress in myopia compared with whites.

Effects of Optical Defocus on Refractive Development in Monkeys: Evidence for Local, Regionally Selective Mechanisms

To characterize the influence of optical defocus on ocular shape and the pattern of peripheral refraction in infant rhesus monkeys. Starting at 3 weeks of age, eight infant monkeys were reared wearing -3 diopter (D) spectacle lenses over one eye that produced relative hyperopic defocus in the nasal field (NF) but allowed unrestricted vision in the temporal field (NF group). Six infants were reared with monocular -3 D lenses that produced relative hyperopic defocus across the entire field of view. Control data were obtained from 11 normal monkeys. Refractive development was assessed by streak retinoscopy performed along the pupillary axis and at eccentricities of 15 degrees, 30 degrees, and 45 degrees along the vertical and horizontal meridians. Central axial dimensions and eye shape were assessed with magnetic resonance imaging. In response to full-field hyperopic defocus, the eye developed relative central axial myopia, became less oblate, and exhibited relative peripheral hyperopia in both the nasal and the temporal hemifields. Conversely, nasal-field hyperopic defocus produced relative myopia that was largely restricted to the nasal hemifield; these alterations in the patterns of peripheral refraction in the NF monkeys were associated with local, region-specific alterations in vitreous chamber depth in the treated hemiretina. Optically imposed defocus can alter the shape and pattern of peripheral refraction in infant primates. Like those of form deprivation, the effects of optical defocus in primates are dominated by mechanisms that integrate visual signals in a spatially restricted manner and exert their influence in a regionally selective manner.

An In Vitro Intact Globe Expansion Method for Evaluation of Cross-linking Treatments

To measure the tissue mechanical response to elevated intraocular pressure (IOP) using intact globe expansion of rabbit eyes. This method examined rabbit kit (2-3 weeks old) eyes as a model for weakened tissue and evaluated riboflavin/UVA and glyceraldehyde cross-linking treatments. The ocular shape of enucleated eyes was photographed during a 24-hour period while a controlled IOP was imposed (either low IOP = 22 mm Hg or high IOP = 85 mm Hg). Untreated controls consisted of kit eyes tested at both low- and high IOP and adult eyes tested at high IOP. Treated kit eyes (dextran controls, riboflavin/UVA treatment of the cornea, and glyceraldehyde treatment of the entire globe) were tested at high IOP. Low IOP elicited negligible creep of the sclera and very gradual creep of the cornea. In contrast, high IOP induced up to an 8% strain in the sclera and a 15% strain in the cornea of rabbit kit eyes. The expansion of adult eyes was less than one third that of kit eyes at the same, high IOP. Riboflavin/UVA treatment of corneas reduced expansion compared with that in both dextran-treated and untreated control corneas. Glyceraldehyde treatment prevented expansion of the cornea and sclera. The intact globe expansion method (GEM) imposes a loading geometry comparable to in vivo conditions and can quantify changes in mechanical stability as a function of testing conditions (e.g., IOP, tissue maturation, and therapeutic cross-linking) with small sample sizes and small variability. Rabbit kit eyes provide a model of weak tissue suitable for screening treatments that strengthen the cornea and sclera.

Effects of form deprivation on peripheral refractions and ocular shape in infant rhesus monkeys (Macaca mulatta).

To determine whether visual experience can alter ocular shape and peripheral refractive error pattern, the authors investigated the effects of form deprivation on refractive development in infant rhesus monkeys. Monocular form deprivation was imposed in 10 rhesus monkeys by securing diffuser lenses in front of their treated eyes between 22 +/- 2 and 163 +/- 17 days of age. Each eye's refractive status was measured longitudinally by retinoscopy along the pupillary axis and at 15 degrees intervals along the horizontal meridian to eccentricities of 45 degrees . Control data for peripheral refraction were obtained from the nontreated fellow eyes and six untreated monkeys. Near the end of the diffuser-rearing period, the shape of the posterior globe was assessed by magnetic resonance imaging. Central axial dimensions were also determined by A-scan ultrasonography. Form deprivation produced interocular differences in central refractive errors that varied between +2.69 and -10.31 D (treated eye-fellow eye). All seven diffuser-reared monkeys that developed at least 2.00 D of relative central axial myopia also showed relative hyperopia in the periphery that increased in magnitude with eccentricity. Alterations in peripheral refraction were highly correlated with eccentricity-dependent changes in vitreous chamber depth and the shape of the posterior globe. Like humans with myopia, monkeys with form-deprivation myopia exhibit relative peripheral hyperopia and eyes that are less oblate and more prolate. Thus, in addition to producing central refractive errors, abnormal visual experience can alter the shape of the posterior globe and the pattern of peripheral refractive errors in infant primates.

Peripheral retinal shape measurements, using the IOLMaster, in myopia and hyperopia

Purpose: When measuring peripheral retinal shape using the IOLMaster, measurements have been taken with the instrument in a fixed position and the subjects rotating their eye to measure peripherally. However, eye rotation could affect the peripheral ocular shape measures. The aim of this experiment is to determine the effect of eye rotation upon the measurement of peripheral retinal shape in myopic and hyperopic subjects using the IOLMaster.Methods: 20 subjects participated with informed consent in the experiment [10 myopes (MYO) and 10 hyperopes (HYP)]. All subjects were visually normal with visual acuity of 0.0 logMAR or better. The mean age of the subjects was 21.4 ± 2.2 years, and all were students in the University. Cycloplegia and mydriasis were induced using 1.0% cyclopentolate and the measurements were taken 30 min after instillation of the eyedrops. Peripheral retinal dimensions were measured with the IOLMaster (Carl Zeiss Ltd., Welwyn Garden City, UK) which measures axial length using partial coherence interferometry. 5 measures of axial length were taken at 10 degree intervals from 0–40 degree nasally and temporally using two different experimental paradigms. Experiment 1: Subjects rotated their eye to view an LED target array, attached to the front of the instrument. Experiment 2: Subjects maintained fixation upon a stationary target in the primary position while the IOL‐Master was rotated about the centre of rotation of the eye. The data was fitted with second order polynomials with separate curves being fitted to the temporal and nasal profiles to take into account any retinal asymmetry.Results: A significant difference (p &lt; 0.01) was found in the mean second order coefficients for the temporal and nasal retinal profiles in the MYO subjects, with the temporal retina having the lower coefficient. There was no significant difference between temporal and nasal second order coefficients in the HYP subjects. Eye rotation produced a significant shift in the second order coefficients for both temporal (p &lt; 0.01) and nasal (p &lt; 0.01) retinal profiles in the MYO group, with the coefficients becoming smaller with ocular rotation. No significant effect of eye rotation was found in the HYP group.Conclusions: Asymmetry between the temporal and nasal retinal profiles is more pronounced in the myopic eye. Ocular rotation has a substantial and measurable effect on the shape of the myopic eye, but no measurable effect is found in the hyperopic eye. These results are indicative of substantial structural differences between the eyes of subjects with myopia and hyperopia. When measuring peripheral retinal shape these factors should be taken into account.

Three-Dimensional Modeling of the Human Eye Based on Magnetic Resonance Imaging

A methodology for noninvasively characterizing the three-dimensional (3-D) shape of the complete human eye is not currently available for research into ocular diseases that have a structural substrate, such as myopia. A novel application of a magnetic resonance imaging (MRI) acquisition and analysis technique is presented that, for the first time, allows the 3-D shape of the eye to be investigated fully. The technique involves the acquisition of a T2-weighted MRI, which is optimized to reveal the fluid-filled chambers of the eye. Automatic segmentation and meshing algorithms generate a 3-D surface model, which can be shaded with morphologic parameters such as distance from the posterior corneal pole and deviation from sphericity. Full details of the method are illustrated with data from 14 eyes of seven individuals. The spatial accuracy of the calculated models is demonstrated by comparing the MRI-derived axial lengths with values measured in the same eyes using interferometry. The color-coded eye models showed substantial variation in the absolute size of the 14 eyes. Variations in the sphericity of the eyes were also evident, with some appearing approximately spherical whereas others were clearly oblate and one was slightly prolate. Nasal-temporal asymmetries were noted in some subjects. The MRI acquisition and analysis technique allows a novel way of examining 3-D ocular shape. The ability to stratify and analyze eye shape, ocular volume, and sphericity will further extend the understanding of which specific biometric parameters predispose emmetropic children subsequently to develop myopia.

The human eye is an example of robust optical design

In most eyes, in the fovea and at best focus, the resolution capabilities of the eye's optics and the retinal mosaic are remarkably well adapted. Although there is a large individual variability, the average magnitude of the high order aberrations is similar in groups of eyes with different refractive errors. This is surprising because these eyes are comparatively different in shape: Myopic eyes are longer whereas hyperopic eyes are shorter. In most young eyes, the amount of aberrations for the isolated cornea is larger than for the complete eye, indicating that the internal ocular optics (mainly the crystalline lens) play a significant role in compensating for the corneal aberrations, thereby producing an improved retinal image. In this paper, we show that this compensation is larger in the less optically centered eyes that mostly correspond to hyperopic eyes. This suggests a type of mechanism in the eye's design that is the most likely responsible for this compensation. Spherical aberration of the cornea is partially compensated by that of the lens in most eyes. Lateral coma is also compensated mainly in hyperopic eyes. We found that the distribution of aberrations between the cornea and lens appears to allow the optical properties of the eye to be relatively insensitive to variations arising from eye growth or exact centration and alignment of the eye's optics relative to the fovea. These results may suggest the presence of an auto-compensation mechanism that renders the eye's optics robust despite large variation in the ocular shape and geometry.

Posterior retinal contour in adult human anisomyopia.

It is well documented that myopia is associated with an increase in axial length or, more specifically, in vitreous chamber depth. Whether the transverse dimensions of the eye also increase in myopia is relevant to further understanding of its development. The posterior retinal surface was localized in two-dimensional space in both eyes of young adult white and Taiwanese-Chinese iso- and anisomyopes (N = 56), from measured keratometry, A-scan ultrasonography, and central and peripheral refraction (+/-35 degrees) data, with the aid of a computer modeling program designed for this purpose. Anisomyopes had 2 D or more interocular difference in their refractive errors, with mean values in their more myopic eyes of -5.57 D and in their less myopic eyes of -3.25 D, similar to the means of the two isomyopic groups. The derived retinal contours for the more and less myopic eyes were compared by way of investigating ocular shape changes that accompany myopia, in the posterior region of the vitreous chamber. The presence and size of optic disc crescents were also investigated as an index of retinal stretching in myopia. Relative to the less myopic eyes of anisometropic subjects, the more myopic eyes were more elongated and also distorted into a more prolate shape in both the white and Chinese groups. However, the Chinese eyes showed a greater and more uniform relative expansion of the posterior retinal surface in their more myopic eyes, and this was associated with larger optic disc crescents. The changes in the eyes of whites displayed a nasal-temporal axial asymmetry, reflecting greater enlargement of the nasal retinal sector. Myopia is associated with increased axial length and a prolate shape. This prolate shape is consistent with the proposed idea that axial and transverse dimensions of the eye are regulated differently. The observations that ocular shape changes are larger but more symmetrical in Chinese eyes than in eyes of whites warrant further investigation.

Ocular Shape and Myopia

Introduction: To learn if eye shape might be a useful parameter in refractive research. Materials and Methods: Laboratory research on eye growth mechanisms is summarised. The available clinical literature relating refraction to eye shape and peripheral refraction is critically assessed in the context of the laboratory research on refractive development. Results: Almost all refraction research assesses optical and length parameters exclusively along the visual axis. Contemporary laboratory research demonstrates a remarkable phylogenic conservation of the neural mechanisms regulating refractive development. On-axis image quality regulates central refractive development in animals and probably, to some extent, in humans. Off-axis image quality at the retina depends on anterior segment geometry and optics, and on the 3-dimensional conformation of the retina. In chicks, eye shape is a predictable parameter linked to the underlying neural mechanisms modulating eye development. Based on the sparse clinical literature in human adults and children, the eye shapes induced in chicks are also seen in human subjects in patterns suggesting that eye shape may be a useful parameter in clinical studies. Conclusion: The diverse findings suggest that incorporating the 3-dimensional conformation of the eye into future clinical studies may help resolve many of the ambiguities in contemporary refractive research.

The effect of accommodation on ocular shape.

Ocular shape is altered in myopia, and accommodation during nearwork is a proposed risk factor for myopia. Using relative peripheral refractive error (RPRE), ocular shape was assessed before, during, and after a period of sustained nearwork to determine whether accommodation affects ocular shape. Measurements of RPRE at 30 degrees in the nasal visual field were obtained using the spherical equivalent calculated from Canon R-1 autorefraction. The RPRE of 41 young adults was measured on two separate occasions separated by at least 1 week to assess RPRE repeatability. Later, the RPRE of 22 young adults was measured at a 0 D accommodative stimulus and then at a 3 D stimulus level at 0, 1, and 2 h during which subjects performed sustained nearwork at 33 cm. After 2 h of nearwork, subjects had RPRE measured at prescribed time intervals over a 1-h period in which they looked in the distance (0 D stimulus). The measurement of RPRE had adequate repeatability (mean difference +/- SD, -0.05 +/- 0.35 D) with +/- 0.68 D as the 95% limits of agreement. The onset of accommodation produced an immediate hyperopic shift of RPRE relative to baseline (+0.37 +/- 0.44 D; p = 0.0007), indicating that ocular shape had become more prolate. This shape remained unchanged after 1 h of sustained accommodation (RPRE difference from baseline, +0.25 +/- 0.55 D; p = 0.04) and then returned to baseline dimensions after 2h of accommodation (RPRE difference from baseline, +0.11 +/- 0.39 D; p = 0.21). At the 0 D stimulus level one minute after the period of nearwork, RPRE became more myopic relative to baseline (RPRE difference from baseline, -0.28 +/- 0.50 D; p = 0.016). Ocular shape returned to baseline dimensions after 45 min of accommodative relaxation. Accommodation induced the ocular shape to become more prolate. The opposite occurred after accommodation was relaxed, namely a change toward a more oblate ocular shape. The transient nature of these changes suggests that tension on the choroid and choroidal hysteresis may play a role in influencing ocular shape.

The Interaction between Spiral Galaxies IC 2163 and NGC 2207. I. Observations

High-resolution H I observations taken at the VLA of the interacting pair of galaxies NGC 2207 and IC 2163 are presented, compared with optical and radio continuum images, and analyzed with detailed computer simulations (in Paper II) in order to determine the orbits and study the effects of in-plane and out-of-plane tidal forcing during a recent, close, nonmerging encounter between galaxies of comparable mass. <P />IC 2163 has an ocular shape (an eye-shaped central oval with a sharp apex at each end) and a double- parallel arm structure on the side opposite NGC 2207. Our observations of IC 2163 find (1) an intrinsically oval shape to the disk, (2) large streaming motions along the oval, (3) H I tidal arms located symmetrically on opposite sides of the nucleus (even though one side is obscured by NGC 2207 at optical wavelengths), and (4) a line-of-sight velocity difference of 70-100 km s<SUP>-1</SUP> between the two components of the double-parallel arm. Optical surface photometry of IC 2163 indicates that most of the stars in the interarm region have been cleared away and put into the central oval and tidal arms. The kinematic and structural anomalies of IC 2163 are consistent with the predictions of N-body galaxy encounter simulations if NGC 2207 moved approximately in the plane of IC 2163 in a prograde sense. <P />The companion, NGC 2207, shows different types of disturbances. The main body of H I gas in NGC 2207 forms a broad, clumpy ring that contains relatively thin stellar arms and corresponds to a plateau in the radial distribution of optical light. The most massive H I clouds in the ring do not always coincide with the stellar arms. The ring is broken in the south, and a filamentary pool of H I extends 40 kpc farther south. The velocity field in the main disk of NGC 2207 is highly distorted with isovelocity contours that are shaped like an open trailing spiral, and the H I line profiles in this region are very broad with Gaussian dispersions of 40-50 km s<SUP>-1</SUP>. The kinematic disturbances in NGC 2207 suggest that the main tidal force on NGC 2207 was perpendicular to its disk. The companion side of the H I ring is unusually bright in λ20 cm radio continuum emission, perhaps indicating a shock front. <P />There is no significant excess of star formation in these galaxies, but there are several 10<SUP>8</SUP> M<SUB>sun</SUB> gas clouds in each disk, some of which may eventually become detached dwarf galaxies.

Magnetic Resonance Imaging of the Human Eye in Vivo

Because of the broad range of T1 (longitudinal relaxation time), T2 (transverse relaxation time), and proton density of the component tissues in the eye, proton magnetic resonance imaging (MRI) can provide ocular images of high contrast. The use of surface coils further enhances the signal-to-noise (S/N) ratio and shortens the image acquisition time to 1 to 6 min. High-resolution MR images of the eye therefore can be acquired readily and routinely. At present, with conventional spin-echo imaging, the in-plane resolution of MR images can be as high as (but not limited to) 0.3 x 0.3 mm with a 3-mm slice thickness. MRI can be used to detect and differentiate ocular lesions and to determine the ocular shape and measure ocular dimensions such as tissue thickness, curvature, and volume.

The Effect of Scleral Buckling on Ocular Rigidity

In a study of enucleated human eyes, the authors investigated the effect of scleral buckling on the ocular pressure-volume relationship. Intraocular pressure was recorded continuously during intravitreal infusion of saline solution before and after the application of encircling silicone elements. Scleral buckling produced a marked reduction in ocular rigidity, with reversibility of the effect on removal of the buckling elements. Similar results were obtained during incremental intravitreal air injection. The authors propose that the greater extensibility of silicone compared with sclera and the induced alterations in ocular shape are the primary factors responsible for the observed change in ocular rigidity. The clinical implications of these findings for intravitreal gas injection are discussed.

Ocular size and shape: Regulation during development Edited by ? and ?, 211 pp., 80 figs, Springer, Berlin, 1981, DM66.00

Ocular size and shape: Regulation during development Edited by ? and ?, 211 pp., 80 figs, Springer, Berlin, 1981, DM66.00

Regulation of ocular size and shape during development

During the last few years, much information has accumulated on the embryonic processes that shape organs and the study of ocular morphogenesis has been particularly productive. New contributions to knowledge on organogenesis of the eye was the subject of the Fifth Symposium on Ocular and Visual development, held in Philadelphia, PA, U.S.A. on 9 and 10 June 1980.