22,802 research outputs found
Fluorescein angiography compared to three-dimensional measurements by the retinal thickness analyzer in classic choroidal neovascularization
Purpose: To compare and correlate imaging of classic subfoveal choroidal neovascularization (CNV) with noninvasive 3-dimensional imaging by the retinal thickness analyzer (RTA) to conventional fluorescein angiography (FA). Methods: A total of 29 eyes of 29 consecutive patients with predominantly classic CNV eligible for photodynamic therapy underwent FA and RTA imaging. The FA dimensions of the CNV were measured independently by two graders. With the RTA, masked to FA the size of the CNV itself as imaged in 3-dimensional reconstruction, the size of significantly thickened retina overlying the CNV and the maximum retinal thickness were measured. Results: The mean diameter of the CNV determined from 3-dimensional RTA reconstructions showed an excellent correlation with measurements from FA (r = 0.91, p < 0.001). The area of retinal thickening was by a mean of 0.7 mm in diameter larger and correlated moderately well with the size of the CNV on FA (r = 0.65, p < 0.001). In contrast, there was no correlation between the absolute retinal thickness and the CNV size on FA. Conclusions: Noninvasive quantitative mapping of predominantly classic CNV by RTA is feasible and also allows 3-dimensional measurement of the lesion itself. The results correlate well with FA assessment but visualize different properties of the disease. Copyright (c) 2007 S. Karger AG, Basel
Power assist EVA glove development
Structural modeling of the EVA glove indicates that flexibility in the metacarpophalangeal (MCP) joint can be improved by selectively lowering the elasticity of the glove fabric. Two strategies are used to accomplish this. One method uses coil springs on the back of the glove to carry the tension in the glove skin due to pressurization. These springs carry the loads normally borne by the glove fabric, but are more easily deformed. An active system was also designed for the same purpose and uses gas filled bladders attached to the back of the EVA glove that change the dimensions of the back of the glove and allow the glove to bend at the MCP joint, thus providing greater flexibility at this joint. A threshold control scheme was devised to control the action of the joint actuators. Input to the controller was provided by thin resistive pressure sensors placed between the hand and the pressurized glove. The pressure sensors consist of a layer of polyester film that has a thin layer of ink screened on the surface. The resistivity of the ink is pressure dependent, so an extremely thin pressure sensor can be fabricated by covering the ink patch with another layer of polyester film and measuring the changing resistance of the ink with a bridge circuit. In order to sense the force between the hand and the glove at the MCP joint, a sensor was placed on the palmar face of the middle finger. The resultant signal was used by the controller to decide whether to fill or exhaust the bladder actuators on the back of the glove. The information from the sensor can also be used to evaluate the effectiveness of a given control scheme or glove design since the magnitude of the measured pressures gives some idea of the torque required to bend a glove finger at the MCP joint. Tests of this actuator, sensor, and control system were conducted in an 57.2 kPa glove box by performing a series of 90 degree finger bends with a glove without an MCP joint assembly, a glove with the coil spring assembly, and with the four fingered actuated glove. The tests of these three glove designs confirm the validity of the model
A preliminary structural analysis of space-based inflatable tubular frame structures
The use of inflatable structures has often been proposed for aerospace and planetary applications. The advantages of such structures include low launch weight and easy assembly. The use of inflatables for applications requiring very large frame structures intended for aerospace use are proposed. In order to consider using an inflated truss, the structural behavior of the inflated frame must be examined. The statics of inflated tubes as beams was discussed in the literature, but the dynamics of these elements has not received much attention. In an effort to evaluate the vibration characteristics of the inflated beam a series of free vibration tests of an inflated fabric cantilevers were performed. Results of the tests are presented and models for system behavior posed
Very Long Baseline Array Imaging of Type-2 Seyferts with Double-Peaked Narrow Emission Lines: Searches for Sub-kpc Dual AGNs and Jet-Powered Outflows
This paper presents Very Long Baseline Array (VLBA) observations of 13
double-peaked [O III] emission-line type-2 Active Galactic Nuclei (AGNs) at
redshifts 0.06 < z < 0.41 (with a median redshift of z~0.15) identified in the
Sloan Digital Sky Survey. Such double-peaked emission-line objects may result
from jets or outflows from the central engine or from a dual AGN. The VLBA
provides an angular resolution of <~10 pc at the distance of many of these
galaxies, sufficient to resolve the radio emission from extremely close dual
AGNs and to contribute to understanding the origin of double-peaked [O III]
emission lines. Of the 13 galaxies observed at 3.6 cm (8.4 GHz), we detect six
at a 1\sigma\ sensitivity level of ~0.15 mJy/beam, two of which show clear jet
structures on scales ranging from a few milliarcseconds to tens of
milliarcseconds (corresponding to a few pc to tens of pc at a median redshift
of 0.15). We suggest that radio-loud double-peaked emission-line type-2 AGNs
may be indicative of jet produced structures, but a larger sample of
double-peaked [O III] AGNs with high angular resolution radio observations will
be required to confirm this suggestion.Comment: 14 pages, 7 figures; ApJ in pres
Nonharmonic oscillations of nanosized cantilevers due to quantum-size effects
Using a one-dimensional jellium model and standard beam theory we calculate
the spring constant of a vibrating nanowire cantilever. By using the asymptotic
energy eigenvalues of the standing electron waves over the nanometer-sized
cross-section area, the change in the grand canonical potential is calculated
and hence the force and the spring constant. As the wire is bent more electron
states fits in its cross section. This has an impact on the spring"constant"
which oscillates slightly with the bending of the wire. In this way we obtain
an amplitude-dependent resonance frequency of the oscillations that should be
detectable.Comment: 6 pages, 5 figure
Well-posedness, energy and charge conservation for nonlinear wave equations in discrete space-time
We consider the problem of discretization for the U(1)-invariant nonlinear
wave equations in any dimension. We show that the classical finite-difference
scheme used by Strauss and Vazquez \cite{MR0503140} conserves the
positive-definite discrete analog of the energy if the grid ratio is , where and are the mesh sizes of the time and space
variables and is the spatial dimension. We also show that if the grid ratio
is , then there is the discrete analog of the charge which is
conserved.
We prove the existence and uniqueness of solutions to the discrete Cauchy
problem. We use the energy conservation to obtain the a priori bounds for
finite energy solutions, thus showing that the Strauss -- Vazquez
finite-difference scheme for the nonlinear Klein-Gordon equation with positive
nonlinear term in the Hamiltonian is conditionally stable.Comment: 10 page
Matter and dynamics in closed cosmologies
To systematically analyze the dynamical implications of the matter content in
cosmology, we generalize earlier dynamical systems approaches so that perfect
fluids with a general barotropic equation of state can be treated. We focus on
locally rotationally symmetric Bianchi type IX and Kantowski-Sachs orthogonal
perfect fluid models, since such models exhibit a particularly rich dynamical
structure and also illustrate typical features of more general cases. For these
models, we recast Einstein's field equations into a regular system on a compact
state space, which is the basis for our analysis. We prove that models expand
from a singularity and recollapse to a singularity when the perfect fluid
satisfies the strong energy condition. When the matter source admits Einstein's
static model, we present a comprehensive dynamical description, which includes
asymptotic behavior, of models in the neighborhood of the Einstein model; these
results make earlier claims about ``homoclinic phenomena and chaos'' highly
questionable. We also discuss aspects of the global asymptotic dynamics, in
particular, we give criteria for the collapse to a singularity, and we describe
when models expand forever to a state of infinite dilution; possible initial
and final states are analyzed. Numerical investigations complement the
analytical results.Comment: 23 pages, 24 figures (compressed), LaTe
Soliton ratchets in homogeneous nonlinear Klein-Gordon systems
We study in detail the ratchet-like dynamics of topological solitons in
homogeneous nonlinear Klein-Gordon systems driven by a bi-harmonic force. By
using a collective coordinate approach with two degrees of freedom, namely the
center of the soliton, , and its width, , we show, first, that
energy is inhomogeneously pumped into the system, generating as result a
directed motion; and, second, that the breaking of the time shift symmetry
gives rise to a resonance mechanism that takes place whenever the width
oscillates with at least one frequency of the external ac force. In addition,
we show that for the appearance of soliton ratchets, it is also necesary to
break the time-reversal symmetry. We analyze in detail the effects of
dissipation in the system, calculating the average velocity of the soliton as a
function of the ac force and the damping. We find current reversal phenomena
depending on the parameter choice and discuss the important role played by the
phases of the ac force. Our analytical calculations are confirmed by numerical
simulations of the full partial differential equations of the sine-Gordon and
systems, which are seen to exhibit the same qualitative behavior. Our
results are in agreement with recent experimental work on dissipation induced
symmetry breaking.Comment: Minor corrections, several references added, accepted for publication
in Chao
A feasibility study of hand kinematics for EVA analysis using magnetic resonance imaging
A new method of analyzing the kinematics of joint motion is developed. Magnetic Resonance Imaging (MRI) offers several distinct advantages. Past methods of studying anatomic joint motion have usually centered on four approaches. These methods are x-ray projection, goniometric linkage analysis, sonic digitization, and landmark measurement of photogrammetry. Of these four, only x-ray is applicable for in vivo studies. The remaining three methods utilize other types of projections of inter-joint measurements, which can cause various types of error. MRI offers accuracy in measurement due to its tomographic nature (as opposed to projection) without the problems associated with x-ray dosage. Once the data acquisition of MR images was complete, the images were processed using a 3D volume rendering workstation. The metacarpalphalangeal (MCP) joint of the left index finger was selected and reconstructed into a three-dimensional graphic display. From the reconstructed volumetric images, measurements of the angles of movement of the applicable bones were obtained and processed by analyzing the screw motion of the MCP joint. Landmark positions were chosen at distinctive locations of the joint at fixed image threshold intensity levels to ensure repeatability. The primarily two dimensional planar motion of this joint was then studied using a method of constructing coordinate systems using three (or more) points. A transformation matrix based on a world coordinate system described the location and orientation of a local target coordinate system. Future research involving volume rendering of MRI data focusing on the internal kinematics of the hand's individual ligaments, cartilage, tendons, etc. will follow. Its findings will show the applicability of MRI to joint kinematics for gaining further knowledge of the hand-glove (power assisted) design for extravehicular activity (EVA)
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