6,763 research outputs found
A deformation-based morphometry framework for disentangling Alzheimer's disease from normal aging using learned normal aging templates
Alzheimer's Disease and normal aging are both characterized by brain atrophy.
The question of whether AD-related brain atrophy represents accelerated aging
or a neurodegeneration process distinct from that in normal aging remains
unresolved. Moreover, precisely disentangling AD-related brain atrophy from
normal aging in a clinical context is complex. In this study, we propose a
deformation-based morphometry framework to estimate normal aging and
AD-specific atrophy patterns of subjects from morphological MRI scans. We first
leverage deep-learning-based methods to create age-dependent templates of
cognitively normal (CN) subjects. These templates model the normal aging
atrophy patterns in a CN population. Then, we use the learned diffeomorphic
registration to estimate the one-year normal aging pattern at the voxel level.
We register the testing image to the 60-year-old CN template in the second
step. Finally, normal aging and AD-specific scores are estimated by measuring
the alignment of this registration with the one-year normal aging pattern. The
methodology was developed and evaluated on the OASIS3 dataset with 1,014
T1-weighted MRI scans. Of these, 326 scans were from CN subjects, and 688 scans
were from individuals clinically diagnosed with AD at different stages of
clinical severity defined by clinical dementia rating (CDR) scores. The results
show that ventricles predominantly follow an accelerated normal aging pattern
in subjects with AD. In turn, hippocampi and amygdala regions were affected by
both normal aging and AD-specific factors. Interestingly, hippocampi and
amygdala regions showed more of an accelerated normal aging pattern for
subjects during the early clinical stages of the disease, while the AD-specific
score increases in later clinical stages. Our code is freely available at
https://github.com/Fjr9516/DBM_with_DL.Comment: 21 pages, 8 figure
Dynamics of the Innermost Accretion Flows Around Compact Objects: Magnetosphere-Disc Interface, Global Oscillations and Instabilities
We study global non-axisymmetric oscillation modes and instabilities in
magnetosphere- disc systems, as expected in neutron star X-ray binaries and
possibly also in accreting black hole systems. Our two-dimensional
magnetosphere-disc model consists of a Keplerian disc in contact with an
uniformly rotating magnetosphere with low plasma density. Two types of global
overstable modes exist in such systems, the interface modes and the disc
inertial-acoustic modes. We examine various physical effects and parameters
that influence the properties of these oscillation modes, particularly their
growth rates, including the magnetosphere field configuration, the velocity and
density contrasts across the magnetosphere-disc interface, the rotation profile
(with Newtonian or General Relativistic potential), the sound speed and
magnetic field of the disc. The interface modes are driven unstable by
Rayleigh-Taylor and Kelvin-Helmholtz in- stabilities, but can be stabilized by
the toroidal field (through magnetic tension) and disc differential rotation
(through finite vorticity). General relativity increases their growth rates by
modifying the disc vorticity outside the magnetosphere boundary. The interface
modes may also be affected by wave absorption associated with corotation
resonance in the disc. In the presence of a magnetosphere, the
inertial-acoustic modes are effectively trapped at the innermost region of the
relativistic disc just outside the interface. They are driven unstable by wave
absorption at the corotation resonance, but can be stabilized by modest disc
magnetic fields. The overstable oscillation modes studied in this paper have
characteristic properties that make them possible candidates for the
quasi-periodic oscillations observed in X-ray binaries.Comment: 18 pages, 9 figures, MNRAS accepte
Excitation of Trapped Waves in Simulations of Tilted Black Hole Accretion Disks with Magnetorotational Turbulence
We analyze the time dependence of fluid variables in general relativistic,
magnetohydrodynamic simulations of accretion flows onto a black hole with
dimensionless spin parameter a/M=0.9. We consider both the case where the
angular momentum of the accretion material is aligned with the black hole spin
axis (an untilted flow) and where it is misaligned by 15 degrees (a tilted
flow). In comparison to the untilted simulation, the tilted simulation exhibits
a clear excess of inertial variability, that is, variability at frequencies
below the local radial epicyclic frequency. We further study the radial
structure of this inertial-like power by focusing on a radially extended band
at 118 (M/10Msol)^-1 Hz found in each of the three analyzed fluid variables.
The three dimensional density structure at this frequency suggests that the
power is a composite oscillation whose dominant components are an over dense
clump corotating with the background flow, a low order inertial wave, and a low
order inertial-acoustic wave. Our results provide preliminary confirmation of
earlier suggestions that disk tilt can be an important excitation mechanism for
inertial waves.Comment: 8 Pages, 6 Figures, accepted for publication in Ap
Phase Transition in the Number Partitioning Problem
Number partitioning is an NP-complete problem of combinatorial optimization.
A statistical mechanics analysis reveals the existence of a phase transition
that separates the easy from the hard to solve instances and that reflects the
pseudo-polynomiality of number partitioning. The phase diagram and the value of
the typical ground state energy are calculated.Comment: minor changes (references, typos and discussion of results
Efficient green-emitting Tb3+-doped di-ureasil coating phosphors for near-UV excited light-emitting diodes
Light-emitting diodes (LEDs) are replacing conventional lighting sources, like incandescent and fluorescent lamps, due to their higher efficiency, lower energy consumption and environmental friendliness characteristics. Additional applications envisaging “engineered light” able to control the human circadian rhythm are now in place with emphases on green-emitting LEDs. In this work, transparent and flexible coatings based on organic–inorganic di-ureasil hybrids doped in-situ with a terbium (Tb3+) complex involving salicylic acid as ligands were synthesized. The materials are transparent, essentially amorphous and thermal stable up to 180 °C. Under near-UV excitation, bright green emission with high quantum yield (0.565 ± 0.057) and enhanced photostability are observed. Green-emitting prototypes were fabricated using a commercial near-UV-emitting LED (NUV-LED) combined with the Tb3+-doped di-ureasil coating showing narrow-band green emission with yellowish-green color coordinates (Commission Internationale de l'Éclairage, CIE 1931) of (0.329, 0.606) and high luminous efficacy (21.5 lm/W). This efficacy is the largest one reported for analogous prototypes formed by an NUV-LED coated with a green-emitting phosphor prepared under mild synthetic conditions (<100 °C), demonstrating that in-situ formation of carboxylate lanthanide-based complexes is an energy saving process with potential for solid-state lighting and backlight for flexible displays.publishe
Random Costs in Combinatorial Optimization
The random cost problem is the problem of finding the minimum in an
exponentially long list of random numbers. By definition, this problem cannot
be solved faster than by exhaustive search. It is shown that a classical
NP-hard optimization problem, number partitioning, is essentially equivalent to
the random cost problem. This explains the bad performance of heuristic
approaches to the number partitioning problem and allows us to calculate the
probability distributions of the optimum and sub-optimum costs.Comment: 4 pages, Revtex, 2 figures (eps), submitted to PR
A size-dependent functionally graded sinusoidal plate model based on a modified couple stress theory
A size-dependent model for bending and free vibration of functionally graded plate is developed based on the modified couple stress theory and sinusoidal shear deformation theory. In the former theory, the small scale effect is taken into consideration, while the effect of shear deformation is accounted for in the latter theory. The equations of motion and boundary conditions are derived from Hamilton’s principle. Analytical solutions for the bending and vibration problems of simply supported plates are obtained. Numerical examples are presented to illustrate the influences of small scale on the responses of functionally graded microplates. The results indicate that the inclusion of small scale effects results in an increase in plate stiffness, and consequently, leads to a reduction of deflection and an increase in frequency. Such small scale effects are significant when the plate thickness is small, but become negligible with increasing plate thickness
Corotational Instability, Magnetic Resonances and Global Inertial-Acoustic Oscillations in Magnetized Black-Hole Accretion Discs
Low-order, non-axisymmetric p-modes (also referred as inertial-acoustic
modes) trapped in the inner-most region of hydrodynamic accretion discs around
black holes, are plausible candidates for high-frequency quasi-periodic
oscillations (QPOs) observed in a number of accreting black-hole systems. These
modes are subject to global instabilities due to wave absorption at the
corotation resonance (where the wave pattern frequency equals the
disc rotation rate ), when the fluid vortensity,
(where and are the radial
epicyclic frequency and disc surface density, respectively), has a positive
gradient. We investigate the effects of disc magnetic fields on the wave
absorption at corotation and the related wave super-reflection of the
corotation barrier, and on the overstability of disc p-modes. For discs with a
pure toroidal field, the corotation resonance is split into two magnetic
resonances, where the wave frequency in the corotating frame of the fluid,
\tomega=\omega-m\Omega, matches the slow magnetosonic wave frequency.
Significant wave energy/angular momentum absorption occurs at both magnetic
resonances, but with opposite signs. The combined effect of the two magnetic
resonances is to reduce the super-reflection and the growth rate of the
overstable p-modes. We show that even a subthermal toroidal field may suppress
the overstability of hydrodynamic (B=0) p-modes. For accretion discs with mixed
(toroidal and vertical) magnetic fields, two additional Alfven resonances
appear, where \tomega matches the local Alfven wave frequency. They further
reduce the growth rate of p-modes. Our results suggest that in order for the
non-axisymmetric p-modes to be a viable candidate for the observed
high-frequency QPOs, the disc magnetic field must be appreciably subthermal, or
other mode excitation mechanisms are at work.Comment: 21 pages, 11 figures, MNRAS accepte
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