14,163 research outputs found
GPU-based Fast Cone Beam CT Reconstruction from Undersampled and Noisy Projection Data via Total Variation
Purpose: Cone-beam CT (CBCT) plays an important role in image guided
radiation therapy (IGRT). However, the large radiation dose from serial CBCT
scans in most IGRT procedures raises a clinical concern, especially for
pediatric patients who are essentially excluded from receiving IGRT for this
reason. The goal of this work is to develop a fast GPU-based algorithm to
reconstruct CBCT from undersampled and noisy projection data so as to lower the
imaging dose. Methods: The CBCT is reconstructed by minimizing an energy
functional consisting of a data fidelity term and a total variation
regularization term. We developed a GPU-friendly version of the
forward-backward splitting algorithm to solve this model. A multi-grid
technique is also employed. Results: It is found that 20~40 x-ray projections
are sufficient to reconstruct images with satisfactory quality for IGRT. The
reconstruction time ranges from 77 to 130 sec on a NVIDIA Tesla C1060 GPU card,
depending on the number of projections used, which is estimated about 100 times
faster than similar iterative reconstruction approaches. Moreover, phantom
studies indicate that our algorithm enables the CBCT to be reconstructed under
a scanning protocol with as low as 0.1 mAs/projection. Comparing with currently
widely used full-fan head and neck scanning protocol of ~360 projections with
0.4 mAs/projection, it is estimated that an overall 36~72 times dose reduction
has been achieved in our fast CBCT reconstruction algorithm. Conclusions: This
work indicates that the developed GPU-based CBCT reconstruction algorithm is
capable of lowering imaging dose considerably. The high computation efficiency
in this algorithm makes the iterative CBCT reconstruction approach applicable
in real clinical environments.Comment: Accepted as a letter in Med. Phys., brief clarifying comments and
updated references. 6 pages and 2 figure
The influence of ultrasonic surface rolling on the fatigue and wear properties of 23-8N engine valve steel
An ultrasonic surface rolling (USR) technique was employed for the first time as a method to enhance the fatigue and wear resistance of 33Cr23Ni8Mn3N (23–8N) austenitic engine valve steel. The microstructure of the modified layer on the material surface was characterised by scanning electron microscopy (SEM) coupled with electron back scatter diffraction (EBSD) and transmission electron microscope (TEM) methods. Nanoscale lamellar grains were discovered on the top surface of the treated material, and an increase of compressive residual stress and microhardness of the surface material observed. A comparative fretting wear test and a rotating bending fatigue test were performed out to verify the surface enhancement effect. Fractured and worn faces of specimens were evaluated through utilizing SEM and energy–dispersive spectroscopy (EDS). Compared to the untreated material, the coefficient of friction of USR treated material was significantly reduced, and the wear resistance was improved. The fatigue strength of a specimen treated at 25 °C was increased from 528 MPa to 730 MPa (38.3 %). At 650 °C, the fatigue strength increased from 345 MPa to 400 MPa (15.9 %). The fatigue resistance extension and wear resistance improvement of treated specimen can be attributed to a combination of beneficial compressive residual stress, work hardening, and the modified microstructure with fine-grains in the surface layer, and thus demonstrates the validity of this novel technique
Disparities in health information-seeking behaviors and fatalistic views of cancer by sexual orientation identity: A nationally representative study of adults in the United States
Orbital character of O 2p unoccupied states near the Fermi level in CrO2
The orbital character, orientation, and magnetic polarization of the O 2
unoccupied states near the Fermi level () in CrO was determined using
polarization-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic
circular dichroism (XMCD) from high-quality, single-crystal films. A sharp peak
observed just above is excited only by the electric field vector () normal to the tetragonal -axis, characteristic of a narrow band
( 0.7 eV bandwidth) constituted from O 2 orbitals perpendicular to
(O 2) hybridized with Cr 3 states. By comparison
with band-structure and configuration-interaction (CI) cluster calculations our
results support a model of CrO as a half-metallic ferromagnet with large
exchange-splitting energy ( 3.0 eV) and
substantial correlation effects.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. B Rapid
Com
Efficiency in nanostructured thermionic and thermoelectric devices
Advances in solid-state device design now allow the spectrum of transmitted
electrons in thermionic and thermoelectric devices to be engineered in ways
that were not previously possible. Here we show that the shape of the electron
energy spectrum in these devices has a significant impact on their performance.
We distinguish between traditional thermionic devices where electron momentum
is filtered in the direction of transport only and a second type, in which the
electron filtering occurs according to total electron momentum. Such 'total
momentum filtered' kr thermionic devices could potentially be implemented in,
for example, quantum dot superlattices. It is shown that whilst total momentum
filtered thermionic devices may achieve efficiency equal to the Carnot value,
traditional thermionic devices are limited to efficiency below this. Our second
main result is that the electronic efficiency of a device is not only improved
by reducing the width of the transmission filter as has previously been shown,
but also strongly depends on whether the transmission probability rises sharply
from zero to full transmission. The benefit of increasing efficiency through a
sharply rising transmission probability is that it can be achieved without
sacrificing device power, in contrast to the use of a narrow transmission
filter which can greatly reduce power. We show that devices which have a
sharply-rising transmission probability significantly outperform those which do
not and it is shown such transmission probabilities may be achieved with
practical single and multibarrier devices. Finally, we comment on the
implications of the effect the shape of the electron energy spectrum on the
efficiency of thermoelectric devices.Comment: 11 pages, 15 figure
CMBfit: Rapid WMAP likelihood calculations with normal parameters
We present a method for ultra-fast confrontation of the WMAP cosmic microwave
background observations with theoretical models, implemented as a publicly
available software package called CMBfit, useful for anyone wishing to measure
cosmological parameters by combining WMAP with other observations. The method
takes advantage of the underlying physics by transforming into a set of
parameters where the WMAP likelihood surface is accurately fit by the
exponential of a quartic or sextic polynomial. Building on previous physics
based approximations by Hu et.al., Kosowsky et.al. and Chu et.al., it combines
their speed with precision cosmology grade accuracy. A Fortran code for
computing the WMAP likelihood for a given set of parameters is provided,
pre-calibrated against CMBfast, accurate to Delta lnL ~ 0.05 over the entire
2sigma region of the parameter space for 6 parameter ``vanilla'' Lambda CDM
models. We also provide 7-parameter fits including spatial curvature,
gravitational waves and a running spectral index.Comment: 14 pages, 8 figures, References added, accepted for publication in
Phys.Rev.D., a Fortran code can be downloaded from
http://space.mit.edu/home/tegmark/cmbfit
Benchmark Parameters for CMB Polarization Experiments
The recently detected polarization of the cosmic microwave background (CMB)
holds the potential for revealing the physics of inflation and gravitationally
mapping the large-scale structure of the universe, if so called B-mode signals
below 10^{-7}, or tenths of a uK, can be reliably detected. We provide a
language for describing systematic effects which distort the observed CMB
temperature and polarization fields and so contaminate the B-modes. We identify
7 types of effects, described by 11 distortion fields, and show their
association with known instrumental systematics such as common mode and
differential gain fluctuations, line cross-coupling, pointing errors, and
differential polarized beam effects. Because of aliasing from the small-scale
structure in the CMB, even uncorrelated fluctuations in these effects can
affect the large-scale B modes relevant to gravitational waves. Many of these
problems are greatly reduced by having an instrumental beam that resolves the
primary anisotropies (FWHM << 10'). To reach the ultimate goal of an
inflationary energy scale of 3 \times 10^{15} GeV, polarization distortion
fluctuations must be controlled at the 10^{-2}-10^{-3} level and temperature
leakage to the 10^{-4}-10^{-3} level depending on effect. For example pointing
errors must be controlled to 1.5'' rms for arcminute scale beams or a percent
of the Gaussian beam width for larger beams; low spatial frequency differential
gain fluctuations or line cross-coupling must be eliminated at the level of
10^{-4} rms.Comment: 11 pages, 5 figures, submitted to PR
Diffusion of Pt dimers on Pt(111)
We report the results of a density-functional study of the diffusion of Pt
dimers on the (111) surface of Pt. The calculated activation energy of 0.37 eV
is in {\em exact} agreement with the recent experiment of Kyuno {\em et al.}
\protect{[}Surf. Sci. {\bf 397}, 191 (1998)\protect{]}. Our calculations
establish that the dimers are mobile at temperatures of interest for adatom
diffusion, and thus contribute to mass transport. They also indicate that the
diffusion path for dimers consists of a sequence of one-atom and (concerted)
two-atom jumps.Comment: Pour pages postscript formatted, including one figure; submitted to
Physical Review B; other papers of interest can be found at url
http://www.centrcn.umontreal.ca/~lewi
Heisenberg-picture approach to the evolution of the scalar fields in an expanding universe
We present the Heisenberg-picture approach to the quantum evolution of the
scalar fields in an expanding FRW universe which incorporates relatively simply
the initial quantum conditions such as the vacuum state, the thermal
equilibrium state, and the coherent state. We calculate the Wightman function,
two-point function, and correlation function of a massive scalar field. We find
the quantum evolution of fluctuations of a self-interacting field
perturbatively and discuss the renormalization of field equations.Comment: 15 pages, RevTeX, no figure
- …