A method for profiling biometric changes during disaccommodation

PURPOSE: To demonstrate the application of low-coherence reflectometry to the study of biometric changes during disaccommodation responses in human eyes after cessation of a near task and to evaluate the effect of contact lenses on low-coherence reflectometry biometric measurements. METHODS: Ocular biometric parameters of crystalline lens thickness (LT) and anterior chamber depth (ACD) were measured with the LenStar device during and immediately after a 5 D accommodative task in 10 participants. In a separate trial, accommodation responses were recorded with a Shin-Nippon WAM-5500 optometer in a subset of two participants. Biometric data were interleaved to form a profile of post-task anterior segment changes. In a further experiment, the effect of soft contact lenses on LenStar measurements was evaluated in 15 participants. RESULTS: In 10 adult participants, increased LT and reduced ACD was seen during the 5 D task. Post-task, during fixation of a 0 D target, a profile of the change in LT and ACD against time was observed. In the two participants with accommodation data (one a sufferer of nearwork-induced transient myopia and other a non-sufferer), the post-task changes in refraction compared favorably with the interleaved LenStar biometry data. The insertion of soft contact lenses did not have a significant effect on LenStar measures of ACD or LT (mean change: -0.007 mm, p = 0.265 and + 0.001 mm, p = 0.875, respectively). CONCLUSIONS: With the addition of a relatively simple stimulus modification, the LenStar instrument can be used to produce a profile of post-task changes in LT and ACD. The spatial and temporal resolution of the system is sufficient for the investigation of nearwork-induced transient myopia from a biometric viewpoint. LenStar measurements of ACD and LT remain valid after the fitting of soft contact lenses.

The interaction of fast neutrons with shielding and fusion blanket materials

In the present work the neutron emission spectra from a graphite cube, and from natural uranium, lithium fluoride, graphite, lead and steel slabs bombarded with 14.1 MeV neutrons were measured to test nuclear data and calculational methods for D - T fusion reactor neutronics. The neutron spectra measured were performed by an organic scintillator using a pulse shape discrimination technique based on a charge comparison method to reject the gamma rays counts. A computer programme was used to analyse the experimental data by the differentiation unfolding method. The 14.1 MeV neutron source was obtained from T(d,n)4He reaction by the bombardment of T - Ti target with a deuteron beam of energy 130 KeV. The total neutron yield was monitored by the associated particle method using a silicon surface barrier detector. The numerical calculations were performed using the one-dimensional discrete-ordinate neutron transport code ANISN with the ZZ-FEWG 1/ 31-1F cross section library. A computer programme based on Gaussian smoothing function was used to smooth the calculated data and to match the experimental data. There was general agreement between measured and calculated spectra for the range of materials studied. The ANISN calculations carried out with P3 - S8 calculations together with representation of the slab assemblies by a hollow sphere with no reflection at the internal boundary were adequate to model the experimental data and hence it appears that the cross section set is satisfactory and for the materials tested needs no modification in the range 14.1 MeV to 2 MeV. Also it would be possible to carry out a study on fusion reactor blankets, using cylindrical geometry and including a series of concentric cylindrical shells to represent the torus wall, possible neutron converter and breeder regions, and reflector and shielding regions.

Elastic and inelastic scattering of fast neutrons in fusion reactor materials

In this work, the angular distributions for elastic and. inelastic scattering of fast neutrons in fusion .reactor materials have been studied. Lithium and lead material are likely to be common components of fusion reactor wall configuration design. The measurements were performed using an associated particle time-of- flight technique. The 14 and 14.44 Mev neutrons were produced by the T(d,n} 4He reaction with deuterons being accelerated in a 150kev SAMES type J accelerator at ASTON and in.the 3. Mev DYNAMITRON at the Joint Radiation Centre, Birmingham respectively. The associated alpha-particles and fast. neutrons were detected.by means of a plastic scintillator mounted on a fast focused photomultiplier tube. The samples used were extended flat plates of thicknesses up to 0.9 mean-free-path for Lithium and 1.562 mean-free-path for Lead. The differential elastic scattering cross-sections were measured for 14 Mev neutrons for various thicknesses of Lithium and Lead in the angular range from zero to; 90º. In addition, the angular distributions of elastically scattered 14,.44 Mev .neutrons from Lithium samples were studied in the same angular range. Inelastic scattering to the 4.63 Mev state in 7Li and the 2.6 Mev state, and 4.1 Mev state in 208Pb have:been :measured.The results are compared to ENDF/B-IV data files and to previous measurements. For the Lead samples the differential neutron scattering:cross-sections for discrete 3 Mev ranges and the angular distributions were measured. The increase in effective cross-section due to multiple scattering effects,as the sample thickness increased:was found to be predicted by the empirical .relation ....... A good fit to the exoerimental data was obtained using the universal constant............ The differential elastic scattering cross-section data for thin samples of Lithium and Lead were analyzed in terms of optical model calculations using the. computer code. RAROMP. Parameter search procedures produced good fits to the·cross-sections. For the case of thick samples of Lithium and Lead, the measured angular distributions of :the scattered neutrons were compared to the predictions of the continuous slowing down model.

High level data fusion

We address the question of how to obtain effective fusion of identification information such that it is robust to the quality of this information. As well as technical issues data fusion is encumbered with a collection of (potentially confusing) practical considerations. These considerations are described during the early chapters in which a framework for data fusion is developed. Following this process of diversification it becomes clear that the original question is not well posed and requires more precise specification. We use the framework to focus on some of the technical issues relevant to the question being addressed. We show that fusion of hard decisions through use of an adaptive version of the maximum a posteriori decision rule yields acceptable performance. Better performance is possible using probability level fusion as long as the probabilities are accurate. Of particular interest is the prevalence of overconfidence and the effect it has on fused performance. The production of accurate probabilities from poor quality data forms the latter part of the thesis. Two approaches are taken. Firstly the probabilities may be moderated at source (either analytically or numerically). Secondly, the probabilities may be transformed at the fusion centre. In each case an improvement in fused performance is demonstrated. We therefore conclude that in order to obtain robust fusion care should be taken to model the probabilities accurately; either at the source or centrally.

Gamma rays associated with neutron scattering in fusion reactor materials

The gamma-rays produced by the inelastic scattering of 14 MeV neutrons. in fusion reactor materials have been studied using a gamma-ray spectrometer employing a sodium iodide scintillation detector. The source neutrons are produced by the T(d,n)4He reaction using the SAMES accelerator at the University of Aston in Birmingham. In order to eliminate the large gamma-ray background and neutron signal due to the sensitivity of the sodium iodide detector to neutrons, the gamma-ray detector is heavily shielded and is used together with a particle time of flight discrimination system based on the associated particle time of flight method. The instant of production of a source neutron is determined by detecting the associated alpha-particle enabling discrimination between the neutrons and gamma-rays by their different time of flight times. The electronic system used for measuring the time of flight of the neutrons and gamrna-rays over the fixed flight path is described. The materials studied in this work were Lithium and Lead because of their importance as fuel breeding and shielding materials in conceptual fusion reactor designs. Several sample thicknesses were studied to determine the multiple scattering effects. The observed gamma-ray spectra from each sample at several scattering angles in the angular range Oº - 90° enabled absolute differential gamma-ray production cross-sections and angular distributions of the resolved gamma-rays from Lithium to be measured and compared with published data. For the Lead sample, the absolute differential gamma-ray production cross-sections for discrete 1 MeV ranges and the angular distributions were measured. The measured angular distributions of the present work and those on Iron from previous work are compared to the predictions of the Monte Carlo programme M.O.R.S.E. Good agreement was obtained between the experimental results and the theoretical predictions. In addition an empirical relation has been constructed which describes the multiple scattering effects by a single parameter and is capable of predicting the gamma-ray production cross-sections for the materials to an accuracy of ± 25%.

Ocular biometric correlates of early-and late-onset myopia

Myopia is a refractive condition and develops because either the optical power of the eye is abnormally great or the eye is abnormally long, the optical consequences being that the focal length of the eye is too short for the physical length of the eye. The increase in axial length has been shown to match closely the dioptric error of the eye, in that a lmm increase in axial length usually generates 2 to 3D of myopia. The most common form of myopia is early-onset myopia (EO M) which occurs between 6 to 14 years of age. The second most common form of myopia is late-onset myopia (LOM) which emerges in late teens or early twenties, at a time when the eye should have ceased growing. The prevalence of LOM is increasing and research has indicated a link with excessive and sustained nearwork. The aim of this thesis was to examine the ocular biometric correlates associated with LOM and EOM development and progression. Biometric data was recorded on SO subjects, aged 16 to 26 years. The group was divided into 26 emmetropic subjects and 24 myopic subjects. Keratometry, corneal topography, ultrasonography, lens shape, central and peripheral refractive error, ocular blood flow and assessment of accommodation were measured on three occasions during an ISmonth to 2-year longitudinal study. Retinal contours were derived using a specially derived computer program. The thesis shows that myopia progression is related to an increase in vitreous chamber depth, a finding which supports previous work. The myopes exhibited hyperopic relative peripheral refractive error (PRE) and the emmetropes exhibited myopic relative PRE. Myopes demonstrated a prolate retinal shape and the retina became more prolate with myopia progression. The results show that a longitudinal, rather than equatorial, increase in the posterior segment is the principal structural correlate of myopia. Retinal shape, relative PRE and the ratio of axial length to corneal curvature have been indicated, in this thesis, as predictive factors for myopia onset and development. Data from this thesis demonstrates that myopia progression in the LOM group is the result of an increase in anterior segment power, owing to an increase in lens thickness, in conjunction with posterior segment elongation. Myopia progression in the EOM group is the product of a long posterior segment, which over-compensates for a weak anterior segment power. The weak anterior segment power in the EOM group is related to a combination of crystalline lens thinning and surface flattening. The results presented in this thesis confirm that posterior segment elongation is the main structural correlate in both EOM and LOM progression. The techniques and computer programs employed in the thesis are reproducible and robust providing a valuable framework for further myopia research and assessment of predictive factors.

Autonomic, biometric and oculomotor correlates of myopia in young adults

The principal work reported in this thesis is the examination of autonomic profile of ciliary muscle innervation as a risk factor in myopia development. Deficiency in sympathetic inhibitory control of accommodation has been proposed as a contributory factor in the development of late onset myopia (LOM). Complementary measurements of ocular biometry, oculomotor function and dynamic accommodation response were carried out on the same subject cohort, thus allowing cross-correlation of these factors with. autonomic profile. Subjects were undergraduate and postgraduate students of Aston University. A 2.5 year longitudinal study of refractive error progression in 40 subjects revealed the onset of LOM in 10, initially emmetropic, young adult subjects (age range 18-24 years) undertaking substantial amounts of near work. A controlled, double blind experimental protocol was conducted concurrently to measure post-task open-loop accommodative regression following distance (0 D) or near (3 D above baseline tonic accommodation) closed-loop tasks of short (10 second) or long (3 minute) duration. Closed-loop tasks consisted of observation of a high contrast Maltese cross target; open-loop conditions were imposed by observation of a 0.2 c/deg Difference of Gaussian target. Accommodation responses were recorded continuously at 42 Hz using a modified Shin-Nippon SRW-5000 open-view infra-red optometer. Blockade of the sympathetic branch of accommodative control was achieved by topical instillation of the non-selective b-adrenoceptor antagonist timolol maleate. Betaxolol hydrochloride (non-selective b1-adrenoceptor antagonist) and normal saline were employed as control agents. Retarded open-loop accommodative regression under b2 blockade following the 3 minute near task indicated the presence of sympathetic facility. Sympathetic inhibitory facility in accommodation control was found in similar proportions between LOM and stable emmetropic subjects. A cross-sectional study (N=60) of autonomic profile showed that sympathetic innervation of ciliary muscle is present in similar proportions between emmetropes, early-, and late-onset myopes. Sympathetic facility was identified in 27% of emmetropes, 21% of EOMs and 29% of LOMs.

Ocular biometric investigation of anisometropia

The thesis aims to define further the biometric correlates in anisometropic eyes in order to provide a structural foundation for propositions concerning the development of ametropia.Biometric data are presented for 40 anisometropes and 40 isometropic controls drawn from Caucasian and Chinese populations.The principal finding was that the main structural correlate of myopia is an increase in axial rather than equatorial dimensions of the posterior globe. This finding has not been previously reported for in vivo work on humans. The computational method described in the thesis is a more accessible method for determination of eye shape than current imaging techniques such as magnetic resonance imaging or laser Doppler interferometry (LDI). Retinal contours derived from LDI and computation were shown to be closely matched. Corneal topography revealed no differences in corneal characteristics in anisometropic eyes, which supports the finding that anisometropia arises from differences in vitreous chamber depth.The corollary to axial expansion in myopia, that is retinal stretch in central regions of the posterior pole, was investigated by measurement of disc-to-fovea distances (DFD) using a scanning laser ophthalmoscope. DFD was found to increase with increased myopia, which demonstrates the primary contribution made by posterior central regions of the globe to axial expansion.The ocular pulse volume and choroidal blood flow, measured with the Ocular Blood Flow Tonograph, were found to be reduced in myopia; the reductions were found to be significantly correlated with vitreous chamber depth. The thesis includes preliminary data on whether the relationship arises from the influx of a blood bolus into eyes of different posterior volumes or represents actual differences in choroidal blood flow.The results presented in this thesis show the utility of computed retinal contour and demonstrate that the structural correlate of myopia is axial rather than equatorial expansion of the vitreous chamber. The technique is suitable for large population studies and its relative simplicity makes it feasible for longitudinal studies on the development of ametropia in, for example, children.

Biometric and physiological factors in human ocular perfusion

By addressing the vascular features that characterise myopia, this thesis aims to provide an understanding of the early structural changes associated with human myopia and the progression to co-morbidity with age. This thesis addresses three main areas of study: 1. Ocular perfusion features and autoregulatory mechanisms in human myopia; 2. Choroidal thickness at the macular area of myopic eyes; 3. Effect of chronic smoking on the ocular haemodynamics and autoregulation. This thesis demonstrated a reduced resting ocular pulse amplitude and retrobulbar blood flow in human myopia, associated with an apparent oversensitivity to the vasodilatory effects of hypercapnia, which may be due to anatomical differences in the volume of the vessel beds. In young smokers, normal resting state vascular characteristics were present; however there also appeared to be increased reactivity to hypercapnia, possibly due to relative chronic hypoxia. The systemic circulation in myopes and smokers over-reacted similarly to hypercapnia suggesting that physiologic differences are not confined to the eye. Age also showed a negative effect on autoregulatory capacity in otherwise normal eyes. Collectively, these findings suggest that myopes and smokers require greater autoregulatory capacity to maintain appropriate oxygenation of retinal tissue, and since the capacity for such regulation reduces with age, these groups are at greater risk of insufficient autoregulation and relative hypoxia with age.