Biomedical subjects

D E Freund

Publications and source records attributed to D E Freund.

4 recordsLinked to original sources

PubMed1995

Ultrastructure in anterior and posterior stroma of perfused human and rabbit corneas. Relation to transparency.

PURPOSE: The authors sought to discover whether there are differences in the degree of spatial order in the fibrillar ultrastructure between anterior and posterior stroma. METHODS: Human corneas were obtained from eye bank eyes. Although they had been classified as normal, some swelling remained after 3 hours of deturgescence. Freshly excised, unswollen rabbit corneas also were used. Image analysis methods were applied to transmission electron micrographs of the anterior, middle, and posterior stroma of these corneas to determine the positions and radii of fibrils, the fraction of total area occupied by fibrils, and the fibril number density. Results were used to calculate the interference factor that appears in the direct summation of the fields for light scattering theory and to estimate the total scattering cross-section per fibril. The interference factor is a measure of the spatial order in the positions and sizes of the fibrils. RESULTS: Electron micrographs showed anterior-posterior variations in size and number density of fibrils. The interference factor at wavelengths of visible light was lower in posterior stroma than in anterior stroma for humans and rabbits. In some instances in humans, the anterior interference factor was characteristic of mildly swollen cornea. When averaged for the electron micrographs analyzed, the anterior stroma was predicted to scatter approximately twice as much light per unit depth as the posterior stroma in humans (at any given wavelength) and approximately three times as much in rabbits. CONCLUSIONS: Calculations of the interference factor showed that there were differences in the anterior-posterior spatial ordering of fibrils. In human corneas, the differences could have been caused by intrinsic in vivo differences between anterior and posterior stroma; however, possible anterior-posterior variations in swelling between the two regions in vitro also could have affected the results.

Aged↗

PubMed1993

Image processing of electron micrographs for light scattering calculations.

Theoretical light scattering calculations, when applied to models based on structures seen in electron micrographs of corneal stroma, require knowledge of the constituent fibril positions and radii. Obtaining this information manually is a difficult and time-consuming task. In order to facilitate this problem, we present a simple and flexible computer algorithm that allows the process to be automated using a Macintosh computer. The accuracy of the results is checked by comparing light scattering calculations using fibril positions and radii found manually with those found by the computer. The results show that the computer algorithm is a viable and accurate means of obtaining the needed data.

Algorithms↗

PubMed1990

A variational principle for the scattered wave.

A Schwinger-type variational principle is presented for the scattered field in the case of scalar wave scattering with an arbitrary field incident on an object of arbitrary shape with homogeneous Dirichlet boundary conditions. The result is variationally invariant at field points ranging from the surface of the scatterer to the farfield and is an important extension of the usual Schwinger variational principle for the scattering amplitude, which is a farfield quantity. Also, a generic procedure, physically motivated by the general principles of boundary conditions and shadowing, is presented for constructing simple trial functions to approximate the fields. The variational principle and the trial function design are tested for the special case of a spherical scatterer and accurate answers are found over the entire frequency range.

Acoustics↗

PubMed1986

Effects of fibril orientations on light scattering in the cornea.

It is generally assumed that the collagen fibrils in the stroma are the primary scatters of light in the nearly transparent cornea of the eye. We derive a scaling relationship between scattering angle and light wavelength that should hold if this hypothesis is valid. The derivation accounts for the cornea's layered nature and the azimuthal orientations of the fibrils in the different layers. The fibrils are treated as finite-length cylinders, and the scaling relation is obtained in both the far- and the intermediate-field zones. The predicted relationship is verified experimentally for normal-thickness rabbit corneas.

Animals↗