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Biomedical subjects

S A Klein

Publications and source records attributed to S A Klein.

At least 73 records · Page 4Linked to original sources
PubMed1996

Limitations on position coding imposed by undersampling and univariance.

Position judgements, which are exquisitely precise in the fovea, are markedly degraded in the periphery. In a recent article [Hess & Field (1993) Vision Research, 33, 2663-2670] argue that the poor representation of positional information in peripheral vision is a consequence of uncalibrated spatial disorder of cortical connections rather than due to undersampling of the retinal image. Specifically, Hess and Field argued that if positional uncertainty is due to undersampling, then because of univariance, there should be an associated contrast uncertainty. In this report we show that the univariance model is limited in its generality since: (1) the Hess and Field data in which contrast and position discrimination are decoupled do not preclude undersampling with large univariant filters: (2) aliasing can decouple position from contrast; (3) undersampling or noise at a second stage of processing can lead to selective losses of position information without any degradation of contrast information.

Contrast Sensitivity↗
PubMed1996

The adjacent pixel nonlinearity: problems and solutions.

The adjacent pixel nonlinearity refers to the dependence of the luminance of a given pixel on the preceding pixel or pixels. We measured this nonlinearity for two CRT displays by measuring the average luminances of a variety of test patterns with different luminance jumps. A two-stage model proposed by Mulligan and Stone was used to fit the data [Mulligan, J.B. & Stone, L. S., (1989). Journal of the Optical Society of America A, 6, 1217-1227 (1989)]. The results show that the model predicts our data well. Based on our measurements and the modeling results, a double-entry look-up table was created to compensate for this nonlinearity. This compensation method works even if the current pixel depends on more than one preceding pixel. Observers commented that at small pixel sizes the compensation results in a sharp, accurate image. Advantages and problems of this compensation will be discussed.

Computer Graphics↗
PubMed1996

Introduction of new technologies to the medical undergraduate curriculum.

The rate at which new educational technologies are introduced is growing at a tremendous pace. This is in large part due to the recommendations of the AAMC Steering Committee on the Evaluation of Medical Education which strongly supported the use of Medical Informatics technologies for education. Some institutions have attempted to implement these technologies; however, a scientific study has not yet been conducted to document their potential benefit. Our attempts to conduct such a study were met with obstacles of resistance from faculty, administration, and students. To overcome these obstacles we have developed a solution which addresses all of the resistance that we encountered. This solution is based upon three premises: 1) adjunct introduction, 2) multicenter studies, and 3) scientific educational benefit analyses.

Attitude to Computers↗
PubMed1995

Decreased function of monocytes and granulocytes during HIV-1 infection correlates with CD4 cell counts.

Monocytes and neutrophils are involved in the primary immune response against opportunistic infections that occur during the progression of human immunodeficiency virus (HIV) infection towards development of acquired immune deficiency syndrome (AIDS). Phagocytic cells operate through the generation of reactive oxygen species which may be toxic for fungi, bacteria and viruses. In the present study we evaluated the function of monocytes and granulocytes in whole blood samples of 16 healthy controls, 12 HIV infected subjects who had not undergone significant infections and of 17 individuals with AIDS. Using flow cytometric methods we were able to determine phagocytosis and respiratory burst under conditions that reflect the normal environment of these cells. Compared with results in samples from controls, granulocytes and monocytes from asymptomatic HIV infected patients exhibited a significantly increased capacity to phagocytose bacteria. The production of reactive oxygen intermediates was in the normal range. In comparison to asymptomatic HIV infected individuals, patients with AIDS showed a significant reduction of phagocytosis and respiratory burst which correlated with the number of CD4+ cells. In comparison to controls, patients infected with HIV, whether they were symptomatic or not, revealed a significantly diminished number of oxygen radical producing cells compared with the number of phagocytic cells. These results indicate that monocytes and granulocytes show reduced antimicrobial activity even in early stages of HIV infection. This defect is only partly due to the HIV infection itself as neutrophils are not target cells for HIV.

Acquired Immunodeficiency Syndrome↗
PubMed1995

Microsurgery without a microscope: laboratory evaluation of a three-dimensional on-screen microsurgery system.

In microvascular surgery, procedures may be both technically and physically demanding. Precise movements sustained over long hours in addition to typically compromised surgeon and assistant positioning lead quickly to physical and mental fatigue. Many of the positioning problems encountered are related to the fact that the eyes of the surgeon must be continually fixed to the microscope eyepieces. This study explores a possible solution: a microscope system that eliminates the need to view the operative field through the microscope eyepieces. A Three-dimensional On-screen Microsurgical System (TOMS) was used and contrasted with conventional operative microvascular surgery in the laboratory setting. The surgeon's comfort, his ability to instruct microsurgical technique, pertinent technological performance, and the procedure itself were evaluated using a standardized questionnaire. Based on data collected in this study, we conclude that divorcing the surgeon's eyes from the microscope eyepieces using the TOMS may make prolonged microvascular procedures less physically demanding and may increase the comfort level of both the surgeon and his assistant, although refinements to the technology are required.

Anastomosis, Surgical↗
PubMed1995

The usefulness of the Laplacian in principal component analysis and dipole source localization.

Evoked potentials are difficult to analyze because multiple sources are active simultaneously. Principal component analysis and dipole localization are two techniques that have been used to disentangle overlapping sources. Both of these techniques have problems. Principal component analysis suffers from a rotation ambiguity. Dipole localization suffers from biases when the model used to derive the sources from the scalp potentials is misspecified. Using computer simulations we demonstrate that by applying both of these techniques to the Laplacian of the voltages rather than to the raw voltages the problems associated with the two techniques are reduced. Computer programs for the analyses are presented in an Appendix.

Brain Mapping↗
PubMed1995

Vernier acuity during image rotation and translation: visual performance limits.

Our capacity to detect spatial misalignments a fraction of the distance between retinal receptors in the presence of image motion challenges our understanding of spatial vision. We find that vernier acuity, while robust to image translation, rapidly degrades during image rotation. This indicates that orientation is a critical cue utilized by the visual system in vernier acuity tasks. Moreover, vernier acuity is robust to translational motion only at high target strengths. Vernier acuity for translating 3-dot targets over midrange velocities can be predicted from vernier acuity data derived from static targets of different presentation durations. However, the degradation observed at higher velocities is greater than predicted. The high velocity degradation reveals that performance is limited by a 1 msec asynchrony sensitivity. The moving vernier stimulus appears to constitute an optimal configuration for the visual system to achieve a 1 msec asynchrony sensitivity by making use of an orientation cue.

Cues↗
PubMed1995

Location of the major corneal reference points.

INTRODUCTION: The two principal applications of corneal topography are those pertaining to corneal shape, such as contract lens posterior surface design, and those pertaining to corneal power, such as predicting power changes after refractive surgery. Corneal topography measurements for these applications have advantages when related to two major reference points, the corneal apex for shape and the corneal sighting center for power. METHOD: Instantaneous corneal power, toricity, and axis were measured by videokeratography in 20 right eyes using the standard instrument alignment and alignments at each of the 2 major reference points. RESULTS: In comparing the central corneal measurements by videokeratography using the standard alignment to alignment at the corneal sighting center, there were minimal differences of less than 0.50 D for central power, toricity, and axis. In comparing the results for regular alignment to apex alignment there were significant differences for power (greater than 0.50 D) in 5 eyes, toricity in 3 eyes, and axis shifts of greater than 10 degrees in 5 eyes. In comparing the results for the corneal sighting center to apex alignments there were significant differences for power in 4 eyes, toricity in 2 eyes, and axis in 5 eyes. Changes of greater than 1.00 D occurred in the corneal periphery. CONCLUSIONS: In comparing standard videokeratograph alignment to alignment at the corneal sighting center, there were statistically insignificant differences in videokeratography results for corneal power, toricity, and axis. Significantly larger differences were found in comparing standard alignment and alignment at the corneal apex, especially for measurements in the corneal periphery.

Adult↗
PubMed1995

Method for generating the anterior surface of an aberration-free contact lens for an arbitrary posterior surface.

Advances in accuracy of the measurement of corneal topography have made it possible to design contact lenses whose posterior surface is based on the corneal contour. Our goal is to design a contact lens of focal length, f, given an arbitrarily shaped posterior surface without assuming rotational symmetry. The shape of the anterior surface is calculated using the principle of least time, thereby going beyond paraxial assumptions and the thin lens approximation. There is no requirement that the reference ray be normal to either the anterior or posterior surface of the lens.

Contact Lenses↗
PubMed1995

A spline surface algorithm for reconstruction of corneal topography from a videokeratographic reflection pattern.

A videokeratograph quantifies one or more aspects of corneal shape by the computer analysis of a digital image. The image is the reflection formed by illuminating the cornea with a known source pattern. The type of shape information recovered and the accuracy of this information depend upon properties of the computer algorithm used in the analysis. We present a new algorithm that recovers a polynomial spline description of corneal depth.

Algorithms↗
PubMed1995

Shape and refractive powers in corneal topography.

PURPOSE: To compare the relative advantages and disadvantages of four different representations of corneal power--identified as instantaneous, axial, position, and refractive--based on curvature, slope, coordinate position, and focal properties, respectively. METHOD: The four types of corneal power were evaluated by examining their interrelationship for 12 hypothetical corneal shapes chosen to represent the general characteristics of regular, irregular, and surgically altered corneas. RESULTS: There is only a limited association between refractive power and the other three powers, which are based on shape properties. For corneal shapes represented by ellipsoids of low eccentricity (e), refractive power increased as a function of the distance from the reference axis, whereas axial and instantaneous powers were either constant (for e = 0) or decreased, with instantaneous power having the largest decrease. Refractive power decreased for elliptical corneal shapes with eccentricities greater than the reciprocal of the index of refraction, although always decreasing less than the axial or instantaneous power. For various corneal shapes formed by segments of circles (or a polynomial) with large differences in instantaneous power, the refractive power was more closely associated with axial than instantaneous or position powers. CONCLUSIONS: The Purkinje image measurements from videokeratograpy are most closely related to the slope-based axial power because it is slope that determines the direction of reflected rays forming the corneal image. Because of this direct connection, axial power is less sensitive to noise than is refractive, instantaneous, or position power. Present videokeratographs that report axial power provide an approximation of refractive power, but if an exact refractive power is needed it can be calculated easily. Instantaneous power provides the most sensitive measure of local curvature changes, such as those occurring in keratoconus or refractive surgery. There are unique practical applications for each of the four powers.

Cornea↗
PubMed1995

Axial and instantaneous power conversion in corneal topography.

PURPOSE: To determine if a direct relationship exists between the two common measures of corneal topography--axial and instantaneous powers--that are based on corneal slope or curvature, respectively. METHOD: The theoretical relationship between axial and instantaneous powers was derived for unrestricted shapes using equations of basic calculus. RESULTS: It was found that axial power at any point y on the cornea is exactly equal to the average of the instantaneous powers from the axis to point y. CONCLUSIONS: A simple relationship exists between axial and instantaneous powers that is valid for the intersection of any surface by a meridional plane. This provides a practical means for converting between axial and instantaneous powers for clinical applications of corneal topography.

Cornea↗
PubMed1994

The use of generalized cell-survival data in a physiologically based objective function for hyperthermia treatment planning: a sensitivity study with a simple tissue model implanted with an array of ferromagnetic thermoseeds.

PURPOSE: A physiologically based objective function for identifying a combination of ferromagnetic seed temperatures and locations that maximizes the fraction of tumor cells killed in pretreatment planning of local hyperthermia. METHODS AND MATERIALS: An objective-function is developed and coupled to finite element software that solves the bioheat transfer equation. The sensitivity of the objective function is studied in the optimization of a ferromagnetic hyperthermia treatment. The objective function has several salient features including (a) a physiological basis that considers increasing the fraction of cells killed with increasing temperatures above a minimum therapeutic temperature (Tmin,thera), (b) a term to penalize for heating of normal tissues above Tmin,thera, and (c) a scalar weighting factor (gamma) that has treatment implications. Reasonable estimates for gamma are provided and their influence on the objective function is demonstrated. The cell-kill algorithm formulated in the objective function is based empirically upon the behavior of published hyperthermic cell-survival data. The objective function is shown to be independent of normal tissue size and shape when subjected to a known outer-surface, thermal boundary condition. Therefore, fractions of cells killed in tumors of different shapes and sizes can be compared to determine the relative performance of thermoseed arrays to heat different tumors. RESULTS: In simulations with an idealized tissue model perfused by blood at various rates, maxima of the objective function are unique and identify seed spacings and Curie-point temperatures that maximize the fraction of tumor cells killed. In ferromagnetic hyperthermia treatment planning, seed spacing can be based on maximizing the minimum tumor temperature and minimizing the maximum normal tissue temperature. It is shown that this treatment plan is less effective than a plan based on seed spacings that maximize the objective function. CONCLUSIONS: It is shown that under the assumptions of the model and based on a desired therapeutic goal, the objective function identifies a combination of thermoseed temperatures and locations that maximizes the fraction of tumor cells killed.

Cell Survival↗
PubMed1994

The topography of visual evoked response properties across the visual field.

Visual evoked potentials (VEPs) to luminance and pattern reversal stimulation were derived for a large number of small areas throughout the central visual field. In one study, the field was tested with a stimulus array consisting of 64 equal-area patches. Local response components were extracted by independent m-sequence modulation of the patches. Field topographies were compared between and within subjects using different electrode placements. The subject-dependent local variability observed in response characteristics is attributed to contributions from two or more cortical representations of the visual field and to inter-subject variations in gross cortical anatomy. The second study used luminance modulation of 56 patches across a 15 degrees field, scaled to activate approximately equal cortical areas in area V1. This produced many robust signals at all eccentricities. Bipolar and double differential ("1-dimensional Laplacian") signals were compared. The double differencing reduced contributions from distant or distributed sources, enhancing nearby current source activity, and greatly improved S/N for many stimulus locations. The high-resolution visual field maps demonstrated that clinical field testing using the VEP is not feasible because of effects of cortical convolutions on responses. However, the vast improvement in data quality and quantity make it a useful tool for VEP source localization and identification.

Brain↗