2,732 research outputs found
Deep Learning How to Fit an Intravoxel Incoherent Motion Model to Diffusion-Weighted MRI
Purpose: This prospective clinical study assesses the feasibility of training
a deep neural network (DNN) for intravoxel incoherent motion (IVIM) model
fitting to diffusion-weighted magnetic resonance imaging (DW-MRI) data and
evaluates its performance. Methods: In May 2011, ten male volunteers (age
range: 29 to 53 years, mean: 37 years) underwent DW-MRI of the upper abdomen on
1.5T and 3.0T magnetic resonance scanners. Regions of interest in the left and
right liver lobe, pancreas, spleen, renal cortex, and renal medulla were
delineated independently by two readers. DNNs were trained for IVIM model
fitting using these data; results were compared to least-squares and Bayesian
approaches to IVIM fitting. Intraclass Correlation Coefficients (ICC) were used
to assess consistency of measurements between readers. Intersubject variability
was evaluated using Coefficients of Variation (CV). The fitting error was
calculated based on simulated data and the average fitting time of each method
was recorded. Results: DNNs were trained successfully for IVIM parameter
estimation. This approach was associated with high consistency between the two
readers (ICCs between 50 and 97%), low intersubject variability of estimated
parameter values (CVs between 9.2 and 28.4), and the lowest error when compared
with least-squares and Bayesian approaches. Fitting by DNNs was several orders
of magnitude quicker than the other methods but the networks may need to be
re-trained for different acquisition protocols or imaged anatomical regions.
Conclusion: DNNs are recommended for accurate and robust IVIM model fitting to
DW-MRI data. Suitable software is available at (1)
Metrological characterization of a vision-based system for relative pose measurements with fiducial marker mapping for spacecrafts
An improved approach for the measurement of the relative pose between a target and a chaser spacecraft is presented. The selected method is based on a single camera, which can be mounted on the chaser, and a plurality of fiducial markers, which can be mounted on the external surface of the target. The measurement procedure comprises of a closed-form solution of the Perspective from n Points (PnP) problem, a RANdom SAmple Consensus (RANSAC) procedure, a non-linear local optimization and a global Bundle Adjustment refinement of the marker map and relative poses. A metrological characterization of the measurement system is performed using an experimental set-up that can impose rotations combined with a linear translation and can measure them. The rotation and position measurement errors are calculated with reference instrumentations and their uncertainties are evaluated by the Monte Carlo method. The experimental laboratory tests highlight the significant improvements provided by the Bundle Adjustment refinement. Moreover, a set of possible influencing physical parameters are defined and their correlations with the rotation and position errors and uncertainties are analyzed. Using both numerical quantitative correlation coefficients and qualitative graphical representations, the most significant parameters for the final measurement errors and uncertainties are determined. The obtained results give clear indications and advice for the design of future measurement systems and for the selection of the marker positioning on a satellite surface
Plausible fluorescent Ly-alpha emitters around the z=3.1 QSO0420-388
We report the results of a survey for fluorescent Ly-alpha emission carried
out in the field surrounding the z=3.1 quasar QSO0420-388 using the FORS2
instrument on the VLT. We first review the properties expected for fluorescent
Ly-alpha emitters, compared with those of other non-fluorescent Ly-alpha
emitters. Our observational search detected 13 Ly-alpha sources sparsely
sampling a volume of ~14000 comoving Mpc^3 around the quasar. The properties of
these in terms of i) the line equivalent width, ii) the line profile and iii)
the value of the surface brightness related to the distance from the quasar,
all suggest that several of these may be plausibly fluorescent. Moreover, their
number is in good agreement with the expectation from theoretical models. One
of the best candidates for fluorescence is sufficiently far behind QSO0420-388
that it would imply that the quasar has been active for (at least) ~60 Myrs.
Further studies on such objects will give information about proto-galactic
clouds and on the radiative history (and beaming) of the high-redshift quasars.Comment: 10 pages, 4 figures.Update to match the version published on ApJ 657,
135, 2007 March
Spitzer Microlensing Program as a Probe for Globular Cluster Planets. Analysis of OGLE-2015-BLG-0448
The microlensing event OGLE-2015-BLG-0448 was observed by Spitzer and lay within the tidal radius of the globular cluster NGC 6558. The event had moderate magnification and was intensively observed, hence it had the potential to probe the distribution of planets in globular clusters. We measure the proper motion of NGC 6558 µ_(cl)(N,E)=(+0.36 ± 0.10,+1.42 ± 0.10, + yr^(-1)) as well as the source and show that the lens is not a cluster member. Even though this particular event does not probe the distribution of planets in globular clusters, other potential cluster lens events can be verified using our methodology. Additionally, we find that microlens parallax measured using Optical Gravitational Lens Experiment (OGLE) photometry is consistent with the value found based on the light curve displacement between the Earth and Spitzer
The QUAX proposal: a search of galactic axion with magnetic materials
Aim of the QUAX (QUaerere AXion) proposal is to exploit the interaction of
cosmological axions with the spin of electrons in a magnetized sample. Their
effect is equivalent to the application of an oscillating rf field with
frequency and amplitude which are fixed by axion mass and coupling constant,
respectively. The rf receiver module of the QUAX detector consists of
magnetized samples with the Larmor resonance frequency tuned to the axion mass
by a polarizing static magnetic field. The interaction of electrons with the
axion-equivalent rf field produces oscillations in the total magnetization of
the samples. To amplify such a tiny field, a pump field at the same frequency
is applied in a direction orthogonal to the polarizing field. The induced
oscillatory magnetization along the polarizing field is measured by a SQUID
amplifier operated at its quantum noise level.Comment: 5 pages, Contribution for the proceedings of the TAUP2015,
International Conference on Topics in Astroparticle and Underground Physics,
7-11 September 2015, Torino, Ital
Hawking-like radiation does not require a trapped region
We discuss the issue of quasi-particle production by ``analogue black holes''
with particular attention to the possibility of reproducing Hawking radiation
in a laboratory. By constructing simple geometric acoustic models, we obtain a
somewhat unexpected result: We show that in order to obtain a stationary and
Planckian emission of quasi-particles, it is not necessary to create a trapped
region in the acoustic spacetime (corresponding to a supersonic regime in the
fluid flow). It is sufficient to set up a dynamically changing flow
asymptotically approaching a sonic regime with sufficient rapidity in
laboratory time.Comment: revtex4, 4 pages, 1 figur
Thyroid-specific transcription factors control Hex promoter activity
The homeobox-containing gene Hex is expressed in several cell types, including thyroid follicular cells, in which it regulates the transcription of tissue-specific genes. In this study the regulation of Hex promoter activity was investigated. Using co-transfection experiments, we demonstrated that the transcriptional activity of the Hex gene promoter in rat thyroid FRTL-5 cells is ∼10-fold greater than that observed in HeLa and NIH 3T3 cell lines (which do not normally express the Hex gene). To identify the molecular mechanisms underlying these differences, we evaluated the effect of the thyroid-specific transcription factor TTF-1 on the Hex promoter activity. TTF-1 produced 3-4-fold increases in the Hex promoter activity. Gel-retardation assays and mutagenesis experiments revealed the presence of functionally relevant TTF-1 binding sites in the Hex promoter region. These in vitro data may also have functional relevance in vivo, since a positive correlation between TTF-1 and Hex mRNAs was demonstrated in human thyroid tissues by means of RT-PCR analysis. The TTF-1 effect, however, is not sufficient to explain the difference in Hex promoter activity between FRTL-5 and cells that do not express the Hex gene. For this reason, we tested whether Hex protein is able to activate the Hex promoter. Indeed, co-transfection experiments indicate that Hex protein is able to increase the activity of its own promoter in HeLa cells ∼4-fold. TTF-1 and Hex effects are additive: when transfected together in HeLa cells, the Hex promoter activity is increased 6-7-fold. Thus, the contemporary presence of both TTF-1 and Hex could be sufficient to explain the higher transcriptional activity of the Hex promoter in thyroid cells with respect to cell lines that do not express the Hex gene. These findings demonstrate the existence of direct cross-regulation between thyroid-specific transcription factors
Quantifying dynamical high-order interdependencies from the O-information: an application to neural spiking dynamics
We address the problem of efficiently and informatively quantifying how
multiplets of variables carry information about the future of the dynamical
system they belong to. In particular we want to identify groups of variables
carrying redundant or synergistic information, and track how the size and the
composition of these multiplets changes as the collective behavior of the
system evolves. In order to afford a parsimonious expansion of shared
information, and at the same time control for lagged interactions and common
effect, we develop a dynamical, conditioned version of the O-information, a
framework recently proposed to quantify high-order interdependencies via
multivariate extension of the mutual information. We thus obtain an expansion
of the transfer entropy in which synergistic and redundant effects are
separated. We apply this framework to a dataset of spiking neurons from a
monkey performing a perceptual discrimination task. The method identifies
synergistic multiplets that include neurons previously categorized as
containing little relevant information individually
Quantum Breathing of an Impurity in a One-dimensional Bath of Interacting Bosons
By means of time-dependent density-matrix renormalization-group (TDMRG) we
are able to follow the real-time dynamics of a single impurity embedded in a
one-dimensional bath of interacting bosons. We focus on the impurity breathing
mode, which is found to be well-described by a single oscillation frequency and
a damping rate. If the impurity is very weakly coupled to the bath, a
Luttinger-liquid description is valid and the impurity suffers an
Abraham-Lorentz radiation-reaction friction. For a large portion of the
explored parameter space, the TDMRG results fall well beyond the
Luttinger-liquid paradigm.Comment: 10 pages, 7 figures, main text and supplementary material merged in a
single PRB style documen
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