1,253 research outputs found
Phenomenological model of diffuse global and regional atrophy using finite-element methods
The main goal of this work is the generation of ground-truth data for the validation of atrophy measurement techniques, commonly used in the study of neurodegenerative diseases such as dementia. Several techniques have been used to measure atrophy in cross-sectional and longitudinal studies, but it is extremely difficult to compare their performance since they have been applied to different patient populations. Furthermore, assessment of performance based on phantom measurements or simple scaled images overestimates these techniques' ability to capture the complexity of neurodegeneration of the human brain. We propose a method for atrophy simulation in structural magnetic resonance (MR) images based on finite-element methods. The method produces cohorts of brain images with known change that is physically and clinically plausible, providing data for objective evaluation of atrophy measurement techniques. Atrophy is simulated in different tissue compartments or in different neuroanatomical structures with a phenomenological model. This model of diffuse global and regional atrophy is based on volumetric measurements such as the brain or the hippocampus, from patients with known disease and guided by clinical knowledge of the relative pathological involvement of regions and tissues. The consequent biomechanical readjustment of structures is modelled using conventional physics-based techniques based on biomechanical tissue properties and simulating plausible tissue deformations with finite-element methods. A thermoelastic model of tissue deformation is employed, controlling the rate of progression of atrophy by means of a set of thermal coefficients, each one corresponding to a different type of tissue. Tissue characterization is performed by means of the meshing of a labelled brain atlas, creating a reference volumetric mesh that will be introduced to a finite-element solver to create the simulated deformations. Preliminary work on the simulation of acquisition artefa- - cts is also presented. Cross-sectional and
Control of polarization and mode mapping of small volume high Q micropillars
We show that the polarization of the emission of a single quantum dot embedded within a microcavity pillar of elliptical cross section can be completely controlled and even switched between two orthogonal linear polarizations by changing the coupling of the dot emission with the polarized photonic modes. We also measure the spatial profle of the emission of a series of pillars with
different ellipticities and show that the results can be well described by simple theoretical modeling
of the modes of an infinite length elliptical cylinder
Path-dependent initialization of a single quantum dot exciton spin in a nanophotonic waveguide
We demonstrate a scheme for in-plane initialization of a single exciton spin in an InGaAs quantum dot (QD) coupled to a GaAs nanobeam waveguide. The chiral coupling of the QD and the optical mode of the nanobeam enables spin initialization fidelity approaching unity in magnetic field B=1 T and >0.9 without the field. We further show that this in-plane excitation scheme is independent of the incident excitation laser polarization and depends solely on the excitation direction. This scheme provides a robust in-plane spin excitation basis for a photon-mediated spin network for quantum information applications
On manifolds with nonhomogeneous factors
We present simple examples of finite-dimensional connected homogeneous spaces
(they are actually topological manifolds) with nonhomogeneous and nonrigid
factors. In particular, we give an elementary solution of an old problem in
general topology concerning homogeneous spaces
Uplifted supersymmetric Higgs region
We show that the parameter space of the Minimal Supersymmetric Standard Model
includes a region where the down-type fermion masses are generated by the
loop-induced couplings to the up-type Higgs doublet. In this region the
down-type Higgs doublet does not acquire a vacuum expectation value at tree
level, and has sizable couplings in the superpotential to the tau leptons and
bottom quarks. Besides a light standard-like Higgs boson, the Higgs spectrum
includes the nearly degenerate states of a heavy spin-0 doublet which can be
produced through their couplings to the quark and decay predominantly into
\tau^+\tau^- or \tau\nu.Comment: 14 pages; Signs in Eqns. (3.1) and (4.2) corrected, appendix include
Donor Centers and Absorption Spectra in Quantum Dots
We have studied the electronic properties and optical absorption spectra of
three different cases of donor centers, D^{0}, D^{-} and D^{2-}, which are
subjected to a perpendicular magnetic field, using the exact diagonalization
method. The energies of the lowest lying states are obtained as function of the
applied magnetic field strength B and the distance zeta between the positive
ion and the confinement xy-plane. Our calculations indicate that the positive
ion induces transitions in the ground-state, which can be observed clearly in
the absorption spectra, but as zeta goes to 0 the strength of the applied
magnetic field needed for a transition to occur tends to infinity.Comment: 5 pages, 4 figures, REVTeX 4, gzipped tar fil
Complex patterns of spontaneous initiations and terminations of reentrant circulation in a loop of cardiac tissue
A two-component model is developed that consists of a discrete loop of
cardiac cells that circulates action potentials together with a cardiac pacing
mechanism. Physiological properties of cells such as restitutions of
refractoriness and of conduction velocity are given via experimentally measured
functions. The dynamics of circulating pulses and their interactions with the
pacer are regulated by two threshold relations. Patterns of spontaneous
initiations and terminations of reentry (SITR) generated by this system are
studied through numerical simulations and analytical observations. These
patterns can be regular or irregular; causes of irregularities are identified
as the threshold bistability of reentrant circulation (T-bistability) and in
some cases, also phase-resetting interactions with the pacer.Comment: 27 pages, 10 figures, 61 references; A version of this paper (same
results) is to appear in the Journal of Theoretical Biology; arXiv V2 adds
helpful commments to facilitate reading and corrects minor errors in
presentatio
Negatively Charged Excitons and Photoluminescence in Asymmetric Quantum Well
We study photoluminescence (PL) of charged excitons () in narrow
asymmetric quantum wells in high magnetic fields B. The binding of all
states strongly depends on the separation of electron and hole layers.
The most sensitive is the ``bright'' singlet, whose binding energy decreases
quickly with increasing even at relatively small B. As a result, the
value of B at which the singlet--triplet crossing occurs in the spectrum
also depends on and decreases from 35 T in a symmetric 10 nm GaAs well
to 16 T for nm. Since the critical values of at which
different states unbind are surprisingly small compared to the well
width, the observation of strongly bound states in an experimental PL
spectrum implies virtually no layer displacement in the sample. This casts
doubt on the interpretation of PL spectra of heterojunctions in terms of
recombination
Symptomatic Sinus Node Disease: Natural History After Permanent Ventricular Pacing *
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75403/1/j.1540-8159.1979.tb03650.x.pd
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