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Biomedical subjects

M A Mainster

Publications and source records attributed to M A Mainster.

At least 19 recordsLinked to original sources
PubMed1992

Cellular automata: retinal cells, circulation and patterns.

Cellular automata modelling is a useful mathematical technique for simulating complex biological systems. An area to be studied is broken into a lattice of adjacent cells depicted by picture elements on a computer screen. The initial tissue pattern evolves on the computer screen, directed by a rule that considers the state of each cell and its neighbours in the lattice. Simulations of wound repair, cell proliferation, retinal circulation and pigment aggregation serve to illustrate the potential value of cellular automata modelling in ophthalmic research and practice.

Algorithms↗
PubMed1990

Hazards of laser beam reflections in eyes containing gas.

Three patients suffered photic or thermal laser injuries at unintended sites because of reflection from gas-fluid interfaces in eyes containing gas. Fresnel's equations predict the amount of light transmitted or reflected at optical interfaces. Application of Fresnel's equations demonstrate that significant laser reflection occurs at gas-fluid or fluid-gas interfaces, especially at high angles of incidence. Photocoagulation across gas-fluid or fluid-gas interfaces should be avoided to reduce the risk of unintended damage from reflected light. If photocoagulation must be performed across an interface, the surgeon should minimize the angle of incidence of the treatment laser beam, use a divergent beam, and consider the location and intensity of the reflected beam. Because of the higher phototoxicity of blue light, photocoagulation with blue-green lasers across gas-fluid or fluid-gas interfaces is undesirable. Proper polarization of the treatment laser beam may significantly reduce laser reflectance.

Aged↗
PubMed1990

The fractal properties of retinal vessels: embryological and clinical implications.

The branching patterns of retinal arterial and venous systems have characteristics of a fractal, a geometrical pattern whose parts resemble the whole. Fluorescein angiogram collages were digitised and analysed, demonstrating that retinal arterial and venous patterns have fractal dimensions of 1.63 +/- 0.05 and 1.71 +/- 0.07, respectively, consistent with the 1.68 +/- 0.05 dimension of diffusion limited aggregation. This finding prompts speculation that factors controlling retinal angiogenesis may obey Laplace's equation, with fluctuations in the distribution of embryonic cell-free spaces providing the randomness needed for fractal behaviour and for the uniqueness of each individual's retinal vascular pattern. Since fractal dimensions characterise how completely vascular patterns span the retina, they can provide insight into the relationship between vascular patterns and retinal disease. Fractal geometry offers a more accurate description of ocular anatomy and pathology than classical geometry, and provides a new language for posing questions about the complex geometrical patterns that are seen in ophthalmic practice.

Computer Graphics↗
PubMed1990

Retinal laser lenses: magnification, spot size, and field of view.

Proper use of ophthalmoscopic contact lenses for retinal photocoagulation requires knowledge of their comparative magnification, spot size, and field of view. We determined these parameters for four commonly used lenses, using data measured from optical components of the lenses and a commonly used photo-coagulator slit-lamp and spot size changer. A Krieger lens has 8% more working field of view and 29% less magnification than a Goldmann lens. A Panfundoscope lens has 84% more working field of view and 24% less magnification than a Goldmann lens. A Mainster lens has 58% more working field of view and 3% more magnification than a Goldmann lens. For Goldmann, Krieger, Panfundoscope, and Mainster lenses, respectively, retinal spot size is 8%, 53%, 41%, and 5% greater than photo-coagulator spot size settings. The field of view of each lens is increased in myopic and decreased in hyperopic patients. Anterior segment irradiance is higher than retinal irradiance for 1000 microns spot size settings with a Panfundoscope or Mainster lens, and this setting should be avoided, especially in patients with hazy ocular media.

Contact Lenses↗
PubMed1990

Argon indirect ophthalmoscopic photocoagulation: reduced potential phototoxicity with a fixed safety filter.

A new argon indirect ophthalmoscopic photocoagulator is presented which uses a red helium-neon laser aiming beam. An interference filter protects the operator from green or blue-green treatment beam reflections without impairing visualisation of the aiming beam or significant retinal anatomy. The protective filter is fixed in place, eliminating the weight, noise, and potential failure of mechanically switched filters. The red aiming beam has a negligible potential for producing photochemical retinal damage in the patient or operator.

Argon↗
PubMed1990

A slitlamp comparator system for macular photocoagulation.

A slitlamp comparator device has been developed to project the image of a 35mm angiogram frame into the field of view of a standard slitlamp during macular photocoagulation. The image may be varied in brightness, magnification, and position, and viewed either on a black background or superimposed on a slitlamp fundus image. The comparator is a useful alternative to conventional angiogram projection systems and suggests the utility of more sophisticated "heads-up" display devices.

Fluorescein Angiography↗
PubMed1987

Potential laser hazards to the clinician during photocoagulation.

We measured reflected laser beams from seven different contact lenses used during laser photocoagulation to evaluate the potential hazards to the eyes of the clinician and bystanders. We found that although collimated beam reflections from the aiming beam of an argon laser photocoagulator could produce disability glare and discomfort to the laser operator (or to an individual viewing through the auxiliary eyepiece), the levels are not hazardous, by current occupational safety limits. Reflected laser light may exceed occupational exposure limits for momentary viewing by bystanders if they are within 1 meter of the laser contact lens.

Equipment Safety↗
PubMed1987

Light and macular degeneration: a biophysical and clinical perspective.

The evidence linking photic retinopathy to ageing macular degeneration (AMD) is compelling but circumstantial. The biophysical foundations of ageing theory are presented, in addition to an analysis of retinal senescence and the potential contributory role of photochemical retinal damage. Although there is pressure to implement clinical therapy for AMD based on laboratory studies of photic retinopathy, there is no evidence at this time that any such therapy is effective. Nonetheless, until the relationship between photic retinopathy and AMD is better understood, it is appropriate for individuals to use ultraviolet and deep blue protective sunglasses in bright environments, particularly if they have reduced ocular pigmentation or if they are aphakes or pseudophakes without an ultraviolet-protective intraocular lens.

Aging↗
PubMed1986

The spectra, classification, and rationale of ultraviolet-protective intraocular lenses.

I measured the spectral transmittance of 16 implantable intraocular lenses from 12 manufacturers and examined the rationale for using ultraviolet-absorptive intraocular lenses to protect pseudophakic individuals from photic retinopathy. Each ultraviolet-protective lens was classified by the wavelength at which its spectral transmittance fell to 10% in the blue or ultraviolet region of the spectrum. Current ultraviolet-protective intraocular lenses differ in the effectiveness of their protection against photic retinopathy, and product descriptions may be misleading.

Advertising↗
PubMed1986

Wavelength selection in macular photocoagulation. Tissue optics, thermal effects, and laser systems.

The therapeutic effects of macular photocoagulation result from focal heating of the retina and choroid. The magnitude, spatial extent, and duration of temperature increases produced by laser exposures are influenced by light scattering in intraocular and intraretinal transit; light absorption by melanin, hemoglobin, and xanthophyll in target tissues; and beam parameters, such as wavelength, spot size, and exposure duration. Tissue optics and thermodynamics provide a useful guide for selecting new laser systems of potential value in macular photocoagulation, but laser-tissue interactions and subsequent chorioretinal responses are poorly understood, and therapeutic efficacy can be established only by controlled clinical trials.

Absorption↗
PubMed1986

Reading with a macular scotoma. I. Retinal location of scotoma and fixation area.

To investigate how patients with macular scotomas use residual functional retinal areas to inspect visual detail, a scanning laser ophthalmoscope (SLO) was used to map the retinal locations of scotomas and areas used to fixate. Three patients with dense macular scotomas of at least 20 months duration and with no explicit low vision training were tested. SLO stimuli were produced by computer modulation of the scanned laser beam, and could be placed on known retinal loci by direct observation of the retina on a television monitor. Videotaped SLO images were analyzed to produce retinal maps that are corrected for shifts of stimulus position due to fixational eye movement, thus showing the true retinal locations of scotomas and fixation loci. Major findings were as follows: 1) each patient used a single, idiosyncratic retinal area, immediately adjacent to the scotoma to fixate, and did not attempt to use the nonfunctional foveola, 2) fixation stability with the eccentric fixation locus was as good as, or better than, that of ocularly normal subjects trying to fixate at comparable eccentricities, 3) fixation stability was not systematically related to clinical visual acuity, and 4) there is good agreement as to the shape and overall size of SLO and standard clinical tangent screen scotoma maps for these three patients.

Adult↗
PubMed1985

Photofield mapping in the evaluation and management of subretinal neovascularization.

Photofield mapping is a useful method for classifying subretinal neovascularization based on its area and location. Careful central field mapping is performed to delineate scotomata during evaluation of patients with subretinal new vessels. A photofield map is then constructed by projecting 35 mm fundus photographs and angiograms onto an appropriately-oriented central visual field map. The composite map (photofield) documents the location and relationship of scotomata, subretinal neovascularization, foveal avascular zone, and retinal blood vessels. The areas of subretinal neovascularization and scotomata are measured in square millimeters with a planimeter. The distance between their closest edge and the center of the foveal avascular zone is measured in millimeters. This method permits quantitative comparison of results with different photocoagulation protocols and instrumentation. Such comparative analysis may help improve the technique and instrumentation used in laser photocoagulation of subretinal neovascularization.

Adult↗
PubMed1984

Visual acuity related to retinal distance from the fovea in macular disease.

The visual potential of patients with macular diseases involving the fovea has not been adequately documented. We evaluated 55 eyes of patients having discrete macular lesions and related best visual acuity to retinal distance from the fovea. Best visual acuity in the parafoveal area extending from the center of the fovea to a distance of 0.25 disk diameters (DD) away was 20/25 to 20/50; from 0.25 to 0.5 DD, 20/50 to 20/100; from 0.5 to 0.75 DD, 20/100 to 20/200; from 0.75 to 1.0 DD, 20/200 to 20/400; and greater than 1.0 DD, counting fingers. The clinical importance of the findings as they relate to the rationale of laser treatment of macular lesions and to the evaluation of the low-vision is discussed.

Adult↗