13,647 research outputs found
Motion correction of SPEC projection before reconstruction
In Single Photon Emission Computed Tomography (SPECT), the data acquisition occurs over a relatively long
time, typically in the range of 5-30 minutes. During this period, the patient must lie still to guarantee the image
quality. Nevertheless, patient movement has frequently been reported in clinical applications. This movement
causes misalignment of the projection frames, which degrades the reconstructed image and may introduce
artifacts. However, the ability to detect and correct for the motion using a computational method is valuable for
quality assurance of SPECT imaging.
In this work a correlation function based on Linogram and Sinogram of the projection is evaluated in order to
estimate the occurred motion and correct it for the best alignment.
By our implemented method the motion artifacts of our cases reduced considerably and our results showed that
the misalignment (motion) between the projections could be found with a small error depending on the
resolution of the images (pixel size), and the time and duration of the applied motion during the course of
projection acquisition
Generation of Vorticity and Velocity Dispersion by Orbit Crossing
We study the generation of vorticity and velocity dispersion by orbit
crossing using cosmological numerical simulations, and calculate the
backreaction of these effects on the evolution of large-scale density and
velocity divergence power spectra. We use Delaunay tessellations to define the
velocity field, showing that the power spectra of velocity divergence and
vorticity measured in this way are unbiased and have better noise properties
than for standard interpolation methods that deal with mass weighted
velocities. We show that high resolution simulations are required to recover
the correct large-scale vorticity power spectrum, while poor resolution can
spuriously amplify its amplitude by more than one order of magnitude. We
measure the scalar and vector modes of the stress tensor induced by orbit
crossing using an adaptive technique, showing that its vector modes lead, when
input into the vorticity evolution equation, to the same vorticity power
spectrum obtained from the Delaunay method. We incorporate orbit crossing
corrections to the evolution of large scale density and velocity fields in
perturbation theory by using the measured stress tensor modes. We find that at
large scales (k~0.1 h/Mpc) vector modes have very little effect in the density
power spectrum, while scalar modes (velocity dispersion) can induce percent
level corrections at z=0, particularly in the velocity divergence power
spectrum. In addition, we show that the velocity power spectrum is smaller than
predicted by linear theory until well into the nonlinear regime, with little
contribution from virial velocities.Comment: 27 pages, 14 figures. v2: reorganization of the material, new
appendix. Accepted by PR
Imaging Jupiter's radiation belts down to 127 MHz with LOFAR
Context. Observing Jupiter's synchrotron emission from the Earth remains
today the sole method to scrutinize the distribution and dynamical behavior of
the ultra energetic electrons magnetically trapped around the planet (because
in-situ particle data are limited in the inner magnetosphere). Aims. We perform
the first resolved and low-frequency imaging of the synchrotron emission with
LOFAR at 127 MHz. The radiation comes from low energy electrons (~1-30 MeV)
which map a broad region of Jupiter's inner magnetosphere. Methods (see article
for complete abstract) Results. The first resolved images of Jupiter's
radiation belts at 127-172 MHz are obtained along with total integrated flux
densities. They are compared with previous observations at higher frequencies
and show a larger extent of the synchrotron emission source (>=4 ). The
asymmetry and the dynamic of east-west emission peaks are measured and the
presence of a hot spot at lambda_III=230 {\deg} 25 {\deg}. Spectral flux
density measurements are on the low side of previous (unresolved) ones,
suggesting a low-frequency turnover and/or time variations of the emission
spectrum. Conclusions. LOFAR is a powerful and flexible planetary imager. The
observations at 127 MHz depict an extended emission up to ~4-5 planetary radii.
The similarities with high frequency results reinforce the conclusion that: i)
the magnetic field morphology primarily shapes the brightness distribution of
the emission and ii) the radiating electrons are likely radially and
latitudinally distributed inside about 2 . Nonetheless, the larger extent
of the brightness combined with the overall lower flux density, yields new
information on Jupiter's electron distribution, that may shed light on the
origin and mode of transport of these particles.Comment: 10 pages, 12 figures, accepted for publication in A&A (27/11/2015) -
abstract edited because of limited character
Testing SPECT Motion Correction Algorithms
Frequently, testing of Single Photon Emission Computed Tomography (SPECT) motion correction algorithms is done either by using simplistic deformations that do not accurately simulate true patient motion or by applying the algorithms directly to data acquired from a real patient, where the true internal motion is unknown. In this work, we describe a way to combine these two approaches by using imaging data acquired from real volunteers to simulate the data that the motion correction algorithms would normally observe. The goal is to provide an assessment framework which can both: simulate realistic SPECT acquisitions that incorporate realistic body deformations and provide a ground truth volume to compare against. Every part of the motion correction algorithm needs to be exercised: from parameter estimation of the motion model, to the final reconstruction results. In order to build the ground truth anthropomorphic numerical phantoms, we acquire high resolution MRI scans and motion observation data of a volunteer in multiple different configurations. We then extract the organ boundaries using thresholding, active contours, and morphology. Phantoms of radioactivity uptake and density inside the body can be generated from these boundaries to be used to simulate SPECT acquisitions. We present results on extraction of the ribs, lungs, heart, spine, and the rest of the soft tissue in the thorax using our segmentation approach. In general, extracting the lungs, heart, and ribs in images that do not contain the spine works well, but the spine could be better extracted using other methods that we discuss. We also go in depth into the software development component of this work, describing the C++ coding framework we used and the High Level Interactive GUI Language (HLING). HLING solved a lot of problems but introduced a fair bit of its own. We include a set of requirements to provide a foundation for the next attempt at developing a declarative and minimally restrictive methodology for writing interactive image processing applications in C++ based on lessons learned during the development of HLING
The Inverse Redshift-Space Operator: Reconstructing Cosmological Density and Velocity Fields
We present the linear inverse redshift space operator which maps the galaxy
density field derived from redshift surveys from redshift space to real space.
Expressions are presented for observers in both the CMBR and Local Group rest
frames. We show how these results can be generalised to flux--limited galaxy
redshift surveys. These results allow the straightforward reconstruction of
real space density and velocity fields without resort to iterative or
numerically intensive inverse methods. As a corollary to the inversion of the
density in the Local Group rest frame we present an expression for estimating
the real space velocity dipole from redshift space, allowing one to estimate
the Local Group dipole without full reconstruction of the redshift survey. We
test these results on some simple models and find the reconstruction is very
accurate. A new spherical harmonic representation of the redshift distortion
and its inverse is developed, which simplifies the reconstruction and allows
analytic calculation of the properties of the reconstructed redshift survey. We
use this representation to analyse the uncertainties in the reconstruction of
the density and velocity fields from redshift space, due to only a finite
volume being available. Both sampling and shot-noise variance terms are derived
and we discuss the limits of reconstruction analysis. We compare the
reconstructed velocity field with the true velocity field and show that
reconstruction in the Local Group rest frame is preferable, since this
eliminates the major source of uncertainty from the dipole mode. These results
can be used to transform redshift surveys to real space and may be used as part
of a full likelihood analysis to extract cosmological parameters.Comment: 13 pages (Latex), 6 postscript figures included, accepted for
publication in MNRA
Software Corrections of Vocal Disorders
We discuss how vocal disorders can be post-corrected via a simple nonlinear
noise reduction scheme. This work is motivated by the need of a better
understanding of voice dysfunctions. This would entail a twofold advantage for
affected patients: Physicians can perform better surgical interventions and on
the other hand researchers can try to build up devices that can help to improve
voice quality, i.e. in a phone conversation, avoiding any surgigal treatment.
As a first step, a proper signal classification is performed, through the idea
of geometric signal separation in a feature space. Then through the analysis of
the different regions populated by the samples coming from healthy people and
from patients affected by T1A glottis cancer, one is able to understand which
kind of interventions are necessary in order to correct the illness, i.e. to
move the corresponding feature vector from the sick region to the healthy one.
We discuss such a filter and show its performance.Comment: Computer Methods and Programs in Biomedicine, accepted for
publicatio
Conjugate Hard X-ray Footpoints in the 2003 October 29 X10 Flare: Unshearing Motions, Correlations, and Asymmetries
We present a detailed imaging and spectroscopic study of the conjugate hard
X-ray (HXR) footpoints (FPs) observed with RHESSI in the 2003 October 29 X10
flare. The double FPs first move toward and then away from each other, mainly
parallel and perpendicular to the magnetic neutral line, respectively. The
transition of these two phases of FP unshearing motions coincides with the
direction reversal of the motion of the loop-top (LT) source, and with the
minima of the estimated loop length and LT height. The FPs show temporal
correlations between HXR flux, spectral index, and magnetic field strength. The
HXR flux exponentially correlates with the magnetic field strength, which also
anti-correlates with the spectral index before the second HXR peak's maximum,
suggesting that particle acceleration sensitively depends on the magnetic field
strength and/or reconnection rate. Asymmetries are observed between the FPs: on
average, the eastern FP is 2.2 times brighter in HXR flux and 1.8 times weaker
in magnetic field strength, and moves 2.8 times faster away from the neutral
line than the western FP; the estimated coronal column density to the eastern
FP from the LT source is 1.7 times smaller. The two FPs have marginally
different spectral indexes. The eastern-to-western FP HXR flux ratio and
magnetic field strength ratio are anti-correlated only before the second HXR
peak's maximum. Neither magnetic mirroring nor column density alone can explain
the totality of these observations, but their combination, together with other
transport effects, might provide a full explanation. We have also developed
novel techniques to remove particle contamination from HXR counts and to
estimate effects of pulse pileup in imaging spectroscopy, which can be applied
to other RHESSI flares in similar circumstances.Comment: 22 pages, 14 figures, 4 tables; ApJ 2009, in pres
Attosecond electron-spin dynamics in Xe 4d photoionization
The photoionization of xenon atoms in the 70-100 eV range reveals several
fascinating physical phenomena such as a giant resonance induced by the dynamic
rearrangement of the electron cloud after photon absorption, an anomalous
branching ratio between intermediate Xe states separated by the spin-orbit
interaction and multiple Auger decay processes. These phenomena have been
studied in the past, using in particular synchrotron radiation, but without
access to real-time dynamics. Here, we study the dynamics of Xe 4d
photoionization on its natural time scale combining attosecond interferometry
and coincidence spectroscopy. A time-frequency analysis of the involved
transitions allows us to identify two interfering ionization mechanisms: the
broad giant dipole resonance with a fast decay time less than 50 as and a
narrow resonance at threshold induced by spin-flip transitions, with much
longer decay times of several hundred as. Our results provide new insight into
the complex electron-spin dynamics of photo-induced phenomena
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