121,978 research outputs found
Time-resolved quantitative multiphase interferometric imaging of a highly focused ultrasound pulse
Interferometric imaging is a well established method to image phase objects
by mixing the image wavefront with a reference one on a CCD camera. It has also
been applied to fast transient phenomena, mostly through the analysis of single
interferograms. It is shown that for repetitive phenomena multiphase
acquisition brings significant advantages. A 1 MHz focused sound field emitted
by a hemispherical piezotransducer in water is imaged as an example.
Quantitative image analysis provides high resolution sound field profiles.
Pressure at focus determined by this method agrees with measurements from a
fiber-optic probe hydrophone. This confirms that multiphase interferometric
imaging can indeed provide quantitative measurements
Gas perturbations in cool cores of galaxy clusters: effective equation of state, velocity power spectra and turbulent heating
We present the statistical analysis of X-ray surface brightness and gas
density fluctuations in cool cores of ten, nearby and bright galaxy clusters
that have deep Chandra observations and show observational indications of
radio-mechanical AGN feedback. Within the central parts of cool cores the total
variance of fluctuations is dominated by isobaric and/or isothermal
fluctuations on spatial scales ~ 10-60 kpc, which are likely associated with
slow gas motions and bubbles of relativistic plasma. Adiabatic fluctuations
associated with weak shocks constitute less than 10 per cent of the total
variance in all clusters. The typical amplitude of density fluctuations is
small, ~ 10 per cent or less on scales of ~ 10-15 kpc. Subdominant contribution
of adiabatic fluctuations and small amplitude of density fluctuations support a
model of gentle AGN feedback as opposed to periodically explosive scenarios
which are implemented in some numerical simulations. Measured one-component
velocities of gas motions are typically below 100-150 km/s on scales < 50 kpc,
and can be up to ~ 300 km/s on ~ 100 kpc scales. The non-thermal energy is < 12
per cent of the thermal energy. Regardless of the source that drives these
motions the dissipation of the energy in such motions provides heat that is
sufficient to balance radiative cooling on average, albeit the uncertainties
are large. Presented results here support previous conclusions based on the
analysis of the Virgo and Perseus Clusters, and agree with the Hitomi
measurements. With next generation observatories like Athena and Lynx, these
techniques will be yet more powerful.Comment: 16 pages, 11 figures, submitted to MNRA
On spatial selectivity and prediction across conditions with fMRI
Researchers in functional neuroimaging mostly use activation coordinates to
formulate their hypotheses. Instead, we propose to use the full statistical
images to define regions of interest (ROIs). This paper presents two machine
learning approaches, transfer learning and selection transfer, that are
compared upon their ability to identify the common patterns between brain
activation maps related to two functional tasks. We provide some preliminary
quantification of these similarities, and show that selection transfer makes it
possible to set a spatial scale yielding ROIs that are more specific to the
context of interest than with transfer learning. In particular, selection
transfer outlines well known regions such as the Visual Word Form Area when
discriminating between different visual tasks.Comment: PRNI 2012 : 2nd International Workshop on Pattern Recognition in
NeuroImaging, London : United Kingdom (2012
An NMR study on internal browning in pears
Internal browning in pears (Pyrus communis L. cv. Blanquilla) has been studied by NMR and MRI in order to develop a non-destructive procedure for on-line disorder identification. For NMR relaxometry, disordered tissue shows higher transverse relaxation rates compared to sound tissue, especially at higher magnetic field strength and for long pulse spacing. Membrane alteration and therefore tissue disintegration, as well as water evaporation, appear to be the main causes of this response. Correlation between relaxation times and diffusion showed that the proton pools in disordered tissue are grouped into a smaller number of populations compared to sound tissue, also highlighting cell decompartmentation in disordered tissue. At a macroscopic level, fast low angle shot MR images, effective transverse relaxation-weighted (TR 11 ms and TE 3.7 ms) and proton density-weighted (TR 7.6 ms and TE 2.5 ms), were acquired for pears at a rate of 54 mm/s. Images have been discriminated for internal breakdown according to histogram characteristics. Up to 94 and 96% of pears, respectively, were correctly classified in the former and the latter type of images. In this study a minimum value of 12% of tissue affected by breakdown was always clearly identifie
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