296 research outputs found
Reproducibility of quantitative indices of lung function and microstructure from 129Xe chemical shift saturation recovery (CSSR) MR spectroscopy
Purpose
To evaluate the reproducibility of indices of lung microstructure and function derived from 129Xe chemical shift saturation recovery (CSSR) spectroscopy in healthy volunteers and patients with chronic obstructive pulmonary disease (COPD), and to study the sensitivity of CSSR-derived parameters to pulse sequence design and lung inflation level.
Methods
Preliminary data were collected from five volunteers on three occasions, using two implementations of the CSSR sequence. Separately, three volunteers each underwent CSSR at three different lung inflation levels. After analysis of these preliminary data, five COPD patients were scanned on three separate days, and nine age-matched volunteers were scanned three times on one day, to assess reproducibility.
Results
CSSR-derived alveolar septal thickness (ST) and surface-area-to-volume (S/V) ratio values decreased with lung inflation level (P < 0.001; P = 0.057, respectively). Intra-subject standard deviations of ST were lower than the previously measured differences between volunteers and subjects with interstitial lung disease. The mean coefficient of variation (CV) values of ST were 3.9 ± 1.9% and 6.0 ± 4.5% in volunteers and COPD patients, respectively, similar to CV values for whole-lung carbon monoxide diffusing capacity. The mean CV of S/V in volunteers and patients was 14.1 ± 8.0% and 18.0 ± 19.3%, respectively.
Conclusion
129Xe CSSR presents a reproducible method for estimation of alveolar septal thickness
Multiple breath washout of hyperpolarized 129Xe and 3He in human lungs with 3D bSSFP imaging
Purpose: (i) To compare quantitative fractional ventilation measurements from multiple breath washout imaging (MBW-I) using hyperpolarized (HP) 3He with both spoiled gradient echo (SPGR) and balanced steady-state free-precession (bSSFP) 3D pulse sequences and (ii) to evaluate the feasibility of MBW-I with HP 129Xe.
Methods: Seven healthy volunteers were scanned using 3He MBW-I with 3D SPGR and bSSFP sequences. Five also underwent MBW-I with 129Xe. A dual-tuned coil was used to acquire MBW-I data from both nuclei in the same subject position, enabling direct comparison of regional information.
Results: High-quality MBW images were obtained with bSSFP sequences using a reduced dose (100ml) of inhaled hyperpolarized 3He. 3D MBW-I with 129Xe was also successfully demonstrated with a bSSFP sequence. Regional quantitative ventilation measures derived from 3He and 129Xe MBW-I correlated well in all subjects (p<0.001) with mean Pearson’s correlation coefficients of r=0.61 and r=0.52 for 3He SPGR-bSSFP and 129Xe-3He (bSSFP) comparisons. The average inter-volunteer mean difference (and standard deviation) in fractional ventilation in SPGR-bSSFP and 129Xe-3He comparisons was 15% (28%) and 9% (38%), respectively.
Conclusions: Improved sensitivity in MBW-I can be achieved with polarization-efficient bSSFP sequences. Same scan-session 3D MBW-I with 3He and 129Xe has been demonstrated using a dual-tuned coil
Lorenz-like systems and classical dynamical equations with memory forcing: a new point of view for singling out the origin of chaos
A novel view for the emergence of chaos in Lorenz-like systems is presented.
For such purpose, the Lorenz problem is reformulated in a classical mechanical
form and it turns out to be equivalent to the problem of a damped and forced
one dimensional motion of a particle in a two-well potential, with a forcing
term depending on the ``memory'' of the particle past motion. The dynamics of
the original Lorenz system in the new particle phase space can then be
rewritten in terms of an one-dimensional first-exit-time problem. The emergence
of chaos turns out to be due to the discontinuous solutions of the
transcendental equation ruling the time for the particle to cross the
intermediate potential wall. The whole problem is tackled analytically deriving
a piecewise linearized Lorenz-like system which preserves all the essential
properties of the original model.Comment: 48 pages, 25 figure
Natural Inflation From Fermion Loops
``Natural'' inflationary theories are a class of models in which inflation is
driven by a pseudo-Nambu-Goldstone boson. In this paper we consider two models,
one old and one new, in which the potential for inflation is generated by loop
effects from a fermion sector which explicitly breaks a global symmetry.
In both models, we retrieve the ``standard'' natural inflation potential,
, as a limiting case of the exact one-loop potential, but we
carry out a general analysis of the models including the limiting case.
Constraints from the COBE DMR observation and from theoretical consistency are
used to limit the parameters of the models, and successful inflation occurs
without the necessity of fine-tuning the parameters.Comment: (Revised) 15 pages, LaTeX (revTeX), 8 figures in uuencoded PostScript
format. Version accepted for publication in Phys. Rev. D 15. Corrected
definition of power spectrum and added three reference
Duality Invariance of Cosmological Perturbation Spectra
I show that cosmological perturbation spectra produced from quantum
fluctuations in massless or self-interacting scalar fields during an
inflationary era remain invariant under a two parameter family of
transformations of the homogeneous background fields. This relates slow-roll
inflation models to solutions which may be far from the usual slow-roll limit.
For example, a scale-invariant spectrum of perturbations in a minimally
coupled, massless field can be produced by an exponential expansion with
, or by a collapsing universe with .Comment: 5 pages, Latex with Revtex. Hamiltonian formulation added and
discussion expanded. Version to appear in Phys Rev
Comparison of 3He and129Xe MRI for evaluation of lung microstructure and ventilation at 1.5T
BACKGROUND: To support translational lung MRI research with hyperpolarized129Xe gas, comprehensive evaluation of derived quantitative lung function measures against established measures from3He MRI is required. Few comparative studies have been performed to date, only at 3T, and multisession repeatability of129Xe functional metrics have not been reported. PURPOSE/HYPOTHESIS: To compare hyperpolarized129Xe and3He MRI-derived quantitative metrics of lung ventilation and microstructure, and their repeatability, at 1.5T. STUDY TYPE: Retrospective. POPULATION: Fourteen healthy nonsmokers (HN), five exsmokers (ES), five patients with chronic obstructive pulmonary disease (COPD), and 16 patients with nonsmall-cell lung cancer (NSCLC). FIELD STRENGTH/SEQUENCE: 1.5T. NSCLC, COPD patients and selected HN subjects underwent 3D balanced steady-state free-precession lung ventilation MRI using both3He and129Xe. Selected HN, all ES, and COPD patients underwent 2D multislice spoiled gradient-echo diffusion-weighted lung MRI using both hyperpolarized gas nuclei. ASSESSMENT: Ventilated volume percentages (VV%) and mean apparent diffusion coefficients (ADC) were derived from imaging. COPD patients performed the whole MR protocol in four separate scan sessions to assess repeatability. Same-day pulmonary function tests were performed. STATISTICAL TESTS: Intermetric correlations: Spearman's coefficient. Intergroup/internuclei differences: analysis of variance / Wilcoxon's signed rank. Repeatability: coefficient of variation (CV), intraclass correlation (ICC) coefficient. RESULTS: A significant positive correlation between3He and129Xe VV% was observed (r = 0.860, P < 0.001). VV% was larger for3He than129Xe (P = 0.001); average bias, 8.79%. A strong correlation between mean3He and129Xe ADC was obtained (r = 0.922, P < 0.001). MR parameters exhibited good correlations with pulmonary function tests. In COPD patients, mean CV of3He and129Xe VV% was 4.08% and 13.01%, respectively, with ICC coefficients of 0.541 (P = 0.061) and 0.458 (P = 0.095). Mean3He and129Xe ADC values were highly repeatable (mean CV: 2.98%, 2.77%, respectively; ICC: 0.995, P < 0.001; 0.936, P < 0.001). DATA CONCLUSION:129Xe lung MRI provides near-equivalent information to3He for quantitative lung ventilation and microstructural MRI at 1.5T. LEVEL OF EVIDENCE: 3 Technical Efficacy Stage
Inflation, cold dark matter, and the central density problem
A problem with high central densities in dark halos has arisen in the context
of LCDM cosmologies with scale-invariant initial power spectra. Although n=1 is
often justified by appealing to the inflation scenario, inflationary models
with mild deviations from scale-invariance are not uncommon and models with
significant running of the spectral index are plausible. Even mild deviations
from scale-invariance can be important because halo collapse times and
densities depend on the relative amount of small-scale power. We choose several
popular models of inflation and work out the ramifications for galaxy central
densities. For each model, we calculate its COBE-normalized power spectrum and
deduce the implied halo densities using a semi-analytic method calibrated
against N-body simulations. We compare our predictions to a sample of dark
matter-dominated galaxies using a non-parametric measure of the density. While
standard n=1, LCDM halos are overdense by a factor of 6, several of our example
inflation+CDM models predict halo densities well within the range preferred by
observations. We also show how the presence of massive (0.5 eV) neutrinos may
help to alleviate the central density problem even with n=1. We conclude that
galaxy central densities may not be as problematic for the CDM paradigm as is
sometimes assumed: rather than telling us something about the nature of the
dark matter, galaxy rotation curves may be telling us something about inflation
and/or neutrinos. An important test of this idea will be an eventual consensus
on the value of sigma_8, the rms overdensity on the scale 8 h^-1 Mpc. Our
successful models have values of sigma_8 approximately 0.75, which is within
the range of recent determinations. Finally, models with n>1 (or sigma_8 > 1)
are highly disfavored.Comment: 13 pages, 6 figures. Minor changes made to reflect referee's
Comments, error in Eq. (18) corrected, references updated and corrected,
conclusions unchanged. Version accepted for publication in Phys. Rev. D,
scheduled for 15 August 200
Inflation at Low Scales: General Analysis and a Detailed Model
Models of inflationary cosmology based on spontaneous symmetry breaking
typically suffer from the shortcoming that the symmetry breaking scale is
driven to nearly the Planck scale by observational constraints. In this paper
we investigate inflationary potentials in a general context, and show that this
difficulty is characteristic only of potentials dominated near their
maxima by terms of order . We find that potentials dominated by terms
of order with \hbox{} can satisfy observational constraints at
an arbitrary symmetry breaking scale. Of particular interest, the spectral
index of density fluctuations is shown to depend only on the order of the
lowest non-vanishing derivative of near the maximum. This result is
illustrated in the context of a specific model, with a broken
symmetry, in which the potential is generated by gauge boson loops.Comment: Submitted to Phys. Rev. D. 32 Pages, REVTeX. No figure
What Can WMAP Tell Us About The Very Early Universe? New Physics as an Explanation of Suppressed Large Scale Power and Running Spectral Index
The Wilkinson Microwave Anisotropy Probe microwave background data may be
giving us clues about new physics at the transition from a ``stringy'' epoch of
the universe to the standard Friedmann Robertson Walker description. Deviations
on large angular scales of the data, as compared to theoretical expectations,
as well as running of the spectral index of density perturbations, can be
explained by new physics whose scale is set by the height of an inflationary
potential. As examples of possible signatures for this new physics, we study
the cosmic microwave background spectrum for two string inspired models: 1)
modifications to the Friedmann equations and 2) velocity dependent potentials.
The suppression of low ``l'' modes in the microwave background data arises due
to the new physics. In addition, the spectral index is red (n<1) on small
scales and blue (n>1) on large scales, in agreement with data.Comment: 18 pages, 2 figures, submitted for publication in Physical Review D,
references added in this versio
Unconventional Cosmology
I review two cosmological paradigms which are alternative to the current
inflationary scenario. The first alternative is the "matter bounce", a
non-singular bouncing cosmology with a matter-dominated phase of contraction.
The second is an "emergent" scenario, which can be implemented in the context
of "string gas cosmology". I will compare these scenarios with the inflationary
one and demonstrate that all three lead to an approximately scale-invariant
spectrum of cosmological perturbations.Comment: 45 pages, 10 figures; invited lectures at the 6th Aegean Summer
School "Quantum Gravity and Quantum Cosmology", Chora, Naxos, Greece, Sept.
12 - 17 2012, to be publ. in the proceedings; these lecture notes form an
updated version of arXiv:1003.1745 and arXiv:1103.227
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