Hans Bebie

Temporal backward projection of optoacoustic pressure transients using fourier transform methods.

In medical imaging different techniques have been developed to gain information from inside a tissue. Optoacoustics is a method to generate tomography pictures of tissue using Q-switched laser pulses. Due to thermal and pressure confinement, a short light pulse generates a pressure distribution inside tissue, which mirrors absorbing structures and can be measured outside the tissue. Using a temporal back-projection method, the pressure distribution measured on the tissue surface allows us to gain a tomography picture of the absorbing structures inside tissue. This study presents a novel computational algorithm, which, at least in principle, yields an exact reconstruction of the absorbing structures in three-dimensional space inside the tissue. The reconstruction is based on 2D pressure distributions captured outside at different delay times. The algorithm is tested in a simulation and back-projection of pressure transients of a small absorber and a single point source.

STATIC PERIMETRY: ACCURACY AND FLUCTUATIONS

Threshold fluctuations are divided into short‐term and long‐term effects applying statistical methods. Sixteen normal and pathological visual fields were analysed in order to obtain the numerical values for the short‐term, long‐term and total fluctuation. Special attention is paid to fortuitous variations between the results obtained at successive examinations. Finally the concept of spatial correlations is introduced.

The cumulative defect curve: separation of local and diffuse components of visual field damage.

We present a new display mode of the results of an Octopus visual field examination, which allows for an easy and reliable assessment of the local and diffuse components of the disturbances of a visual field. Essentially, this display consists of the cumulative distribution of the local defect values. Application to typical cases is discussed.

Lens opacity meter: a new instrument to quantify lens opacity.

The lens opacity meter, a new instrument for quantifying the lens opacity, is described. The basic concept is the measurement of stray light produced by the lens. The measurement can be performed without dilating the pupil. A modulation of the light source renders the measurement insensitive to the surrounding light. The handling of the instrument is very simple. It is mounted on a slitlamp-like table. After adjusting the instrument to the eye, the measurement is initiated by depressing a console button.

Some aspects of the automation of perimetry

Manual perimetry has certain inherent limitations stemming from both the patient and the examiner. Automated perimetry, if properly developed, can offer the advantages of exact reproducibility of the examination strategy, accuracy achieved through averaging of repeated determinations, and systematic application of previous data to the current examination. The history, concepts and clinical practically of automated perimetry are discussed. Several systems capable of performing functions resembling human intellectual activity are commerically available; one, the OctopusTM, is described in detail.

Morphological studies about the efficiency of laser beams upon the structures of the angle of the anterior chamber

Working at a power density above optical breakdown threshold, irradiation effects upon the angle of the anterior chamber of the Macaca speciosa monkey by a Nd glass Q-switched laser, have been analysed with scanning electron microscopy.Two different damage effects can be identified: openings of Schlemms canal and the creation of a cyclodialysis, that is opening the uveoscleral outflow routes; the latter may be a more effective one than the former. Also a third mechanism, namely a structural alteration of the trabecular meshwork, at the molecular level by laser action, has been inferred. The physical effects leading to optically induced mechanical damage are discussed. Applicability of such effects upon the morphological findings described here, is limited and to a greater part speculative. A better definition of the physical parameters, required for optimal therapeutical damage effects in the treatment of the wide angle glaucoma may be obtained by trial and error methods, which are guided by the results of electron microscopical analyses of samples obtained from in vivo and in vitro experiments.

Computer-assisted evaluation of visual fields.

A computer program is described which provides statements in normal prose about the outcome of automated visual field examinations as well as hints concerning possible alternative interpretations and artifacts, if present. The program is intended as an aid to ophthalmologists being trained to evaluate critically visual fields. The limits of the applicability of this program are discussed.

On the effects of different laser energy sources upon the iris of the pigmented and the albino rabbit.

The irides of pigmented and albino rabbits have been irradiated by a) a CW argon laser beam (exposure duration up to 1 s), b) a 1 ms pulses Nd:YAG laser and c) a 30 ns pulses Q-switched ruby laser. The immediate and long-term pathologies were analysed by scanning and transmission electron microscopy over a period of 13 months. At both the gross and ultrastructural levels, damage configuration may differ considerably, depending on the three modes of irradiation. For each source there are both thermal and mechanical damage components and the significance of mechanical effects increases with decreasing pulse duration for a constant pulse energy. In the argon experiments, tissue destruction is predominantly a consequence of heat, resulting from conduction and convection. The subsequent regeneration of tissue after such heat-induced trauma is fast. The effects of the Nd:YAG laser, at the irradiance levels used in the present study, are again predominantly of a thermal nature and are caused by heating and local evaporation. The pigmented and the nonpigmented iris epithelium are destroyed and widespread decay of the stroma occurs over some months. Such damage never results in full repair. The most prominent feature of Q-switched ruby laser irradiations is their independence of the iris pigment content. In contrast, at the energy levels studied, the argon laser is entirely ineffective, whilst the effect of the Nd:YAG laser is much reduced in the absence of pigment.The consequences of these findings for the clinical applications of such lasers are discussed.

Basic Sciences in Ophthalmology

Basic sciences in ophthalmology : , Basic sciences in ophthalmology : , کتابخانه دیجیتال جندی شاپور اهواز

Carbon Dioxide (CO2)

Carbon dioxide is a molecule composed of two oxygen atoms covalently bonded to a single carbon atom (Fig. 11.1). It is colorless and exists in gaseous form at room temperature. In nature, carbon dioxide is used by plants during photosynthesis to make sugar, for example, and it is liberated by all organisms that depend on oxidative phosphorylation. Burning in a fire or “burning” in mitochondria also leads to carbon dioxide formation through a set of redox reactions summarized as “cellular respiration.”