1,649 research outputs found
Towards efficient modelling of optical micromanipulation of complex structures
Computational methods for electromagnetic and light scattering can be used
for the calculation of optical forces and torques. Since typical particles that
are optically trapped or manipulated are on the order of the wavelength in
size, approximate methods such as geometric optics or Rayleigh scattering are
inapplicable, and solution or either the Maxwell equations or the vector
Helmholtz equation must be resorted to. Traditionally, such solutions were only
feasible for the simplest geometries; modern computational power enable the
rapid solution of more general--but still simple--geometries such as
axisymmetric, homogeneous, and isotropic scatterers. However, optically-driven
micromachines necessarily require more complex geometries, and their
computational modelling thus remains in the realm of challenging computational
problems. We review our progress towards efficient computational modelling of
optical tweezers and micromanipulation, including the trapping and manipulation
of complex structures such as optical micromachines. In particular, we consider
the exploitation of symmetry in the modelling of such devices.Comment: 5 pages, 4 figure
Modelling optical micro-machines
A strongly focused laser beam can be used to trap, manipulate and exert
torque on a microparticle. The torque is the result of transfer of angular
momentum by scattering of the laser beam. The laser could be used to drive a
rotor, impeller, cog wheel or some other microdevice of a few microns in size,
perhaps fabricated from a birefringent material. We review our methods of
computationally simulating the torque and force imparted by a laser beam. We
introduce a method of hybridizing the T-matrix with the Finite Difference
Frequency Domain (FDFD) method to allow the modelling of materials that are
anisotropic and inhomogeneous, and structures that have complex shapes. The
high degree of symmetry of a microrotor, such as discrete or continuous
rotational symmetry, can be exploited to reduce computational time and memory
requirements by orders of magnitude. This is achieved by performing
calculations for only a given segment or plane that is repeated across the
whole structure. This can be demonstrated by modelling the optical trapping and
rotation of a cube.Comment: 4 pages, 3 figure
Efficacy of antiplatelet therapy in secondary prevention following lacunar stroke:Pooled analysis of randomized trials
Background and Purpose: Lacunar stroke accounts for ≈25% of ischemic stroke, but optimal antiplatelet regimen to prevent stroke recurrence remains unclear. We aimed to evaluate the efficacy of antiplatelet agents in secondary stroke prevention after a lacunar stroke. Methods: We searched MEDLINE, Embase, and the Cochrane library for randomized controlled trials that reported risk of recurrent stroke or death with antiplatelet therapy in patients with lacunar stroke. We used random effects meta-analysis and evaluated heterogeneity with I2. Results: We included 17 trials with 42 234 participants (mean age 64.4 years, 65% male) and follow up ranging from 4 weeks to 3.5 years. Compared with placebo, any single antiplatelet agent was associated with a significant reduction in recurrence of any stroke (risk ratio [RR] 0.77, 0.62–0.97, 2 studies) and ischemic stroke (RR 0.48, 0.30–0.78, 2 studies), but not for the composite outcome of any stroke, myocardial infarction, or death (RR 0.89, 0.75–1.05, 2 studies). When other antiplatelet agents (ticlodipine, cilostazol, and dipyridamole) were compared with aspirin, there was no consistent reduction in stroke recurrence (RR 0.91, 0.75–1.10, 3 studies). Dual antiplatelet therapy did not confer clear benefit over monotherapy (any stroke RR 0.83, 0.68–1.00, 3 studies; ischemic stroke RR 0.80, 0.62–1.02, 3 studies; composite outcome RR 0.90, 0.80–1.02, 3 studies). Conclusions: Our results suggest that any of the single antiplatelet agents compared with placebo in the included trials is adequate for secondary stroke prevention after lacunar stroke. Dual antiplatelet therapy should not be used for long-term stroke prevention in this stroke subtype
Transfer of K-types on local theta lifts of characters and unitary lowest weight modules
In this paper we study representations of the indefinite orthogonal group
O(n,m) which are local theta lifts of one dimensional characters or unitary
lowest weight modules of the double covers of the symplectic groups. We apply
the transfer of K-types on these representations of O(n,m), and we study their
effects on the dual pair correspondences. These results provide examples that
the theta lifting is compatible with the transfer of K-types. Finally we will
use these results to study subquotients of some cohomologically induced
modules
A background-priority discrete boundary triangulation method
Discrete boundary triangulation methods generate triangular meshes through the centers of the boundary voxels of a volumetric object. At some voxel configurations it may be arbitrary whether a part of the volume should be included in the object or could be classified as background. Consequently, important details such as concave and convex edges and corners are not consistently preserved in the describing geometry. We present a "background priority" version of an existing "object priority" algorithm [6]. We show that the ad hoc configurations of the well-known Discretized Marching Cubes algorithm [13] can be derived from our method and that a combined triangulation with "object priority" and "background priority" better would preserve object details
Segmentação de dados 3D
Este artigo apresenta um algoritmo para efectuar uma segmentação não supervisionada em 3D, mostrando a sua aplicação a diferentes volumes, tais como: volumes artificiais, acústica submarina e de ressonância magnética (NMR- Nuclear Magnetic Resonance). O trabalho tem por base o efectuado em 2D e já apresentado numa anterior Tecnovisão [I]
Fast segmentation of 3D data using an octree
The algorithm developed uses an octree pyramid
in which noise is reduced at the expense of the spatial
resolution. At a certain level an unsupervised clustering
without spatial connectivity constraints is applied.
After the classification, isolated voxels and insignificant
regions are removed by assigning them to their neighbours.
The spatial resolution is then increased by the
downprojection of the regions, level by level. At each
level the uncertainty of the boundary voxels is minimised
by a dynamic selection and classification of these, using
an adaptive 3D filtering. The algorithm is tested using
different data sets, including NMR data
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