7,322 research outputs found
Compressive 3D ultrasound imaging using a single sensor
Three-dimensional ultrasound is a powerful imaging technique, but it requires thousands of sensors and complex hardware. Very recently, the discovery of compressive sensing has shown that the signal structure can be exploited to reduce the burden posed by traditional sensing requirements. In this spirit, we have designed a simple ultrasound imaging device that can perform three-dimensional imaging using just a single ultrasound sensor. Our device makes a compressed measurement of the spatial ultrasound field using a plastic aperture mask placed in front of the ultrasound sensor. The aperture mask ensures that every pixel in the image is uniquely identifiable in the compressed measurement. We demonstrate that this device can successfully image two structured objects placed in water. The need for just one sensor instead of thousands paves the way for cheaper, faster, simpler, and smaller sensing devices and possible new clinical applications
The Sun's position in the sky
We express the position of the Sun in the sky as a function of time and the
observer's geographic coordinates. Our method is based on applying rotation
matrices to vectors describing points on the celestial sphere. We also derive
direct expressions, as functions of date of the year and geographic latitude,
for the duration of daylight, the maximum and minimum altitudes of the Sun, and
the cardinal directions to sunrise and sunset. We discuss how to account for
the eccentricity of the earth's orbit, the precessions of the equinoxes and the
perihelion, the size of the solar disk, and atmospheric refraction. We
illustrate these results by computing the dates of "Manhattanhenge" (when
sunset aligns with the east-west streets on the main traffic grid for
Manhattan, in New York City), by plotting the altitude of the Sun over
representative cities as a function of time, and by showing plots ("analemmas")
for the position of the Sun in the sky at a given hour of the day.Comment: 19 pages, 16 figures. v3: Replaced to match published version and to
re-package Mathematica notebook as an ancillary fil
Geometric-phase-induced false electric dipole moment signals for particles in traps
Theories are developed to evaluate Larmor frequency shifts, derived from geometric phases, in experiments to measure electric dipole moments (EDMs) of trapped, atoms, molecules and neutrons. A part of these shifts is proportional to the applied electric field and can be interpreted falsely as an electric dipole moment. A comparison is made between our theoretical predictions for these shifts and some results from our recent experiments, which shows agreement to within the experimental errors of 15 %. The comparison also demonstrates that some trapped particle EDM experiments have reached the sensitivity where stringent precautions are needed to minimise and control such false EDMs. Computer simulations of these processes are also described. They give good agreement with the analytical results and they extend the study by investigating the influence of varying surface reflection laws in the hard walled traps considered. They also explore the possibility to suppress such false EDMs by introducing collisions with buffer gas particles. Some analytic results for frequency shifts proportional to the square of the E-field are also given and there are results for the averaging of the B-field in the absence of an E-field
Efficient Bayesian-based Multi-View Deconvolution
Light sheet fluorescence microscopy is able to image large specimen with high
resolution by imaging the sam- ples from multiple angles. Multi-view
deconvolution can significantly improve the resolution and contrast of the
images, but its application has been limited due to the large size of the
datasets. Here we present a Bayesian- based derivation of multi-view
deconvolution that drastically improves the convergence time and provide a fast
implementation utilizing graphics hardware.Comment: 48 pages, 20 figures, 1 table, under review at Nature Method
The magnetic field structure of the central region in M31
The Andromeda Galaxy (M31) is the nearest grand-design spiral galaxy. Thus
far most studies in the radio regime concentrated on the 10 kpc ring. The
central region of M31 has significantly different properties than the outer
parts: The star formation rate is low, and inclination and position angle are
largely different from the outer disk. The existing model of the magnetic field
in the radial range 6<=r<=14 kpc is extended to the innermost part r<=0.5 kpc
to ultimately achieve a picture of the entire magnetic field in M31. We
combined observations taken with the VLA at 3.6 cm and 6.2 cm with data from
the Effelsberg 100-m telescope to fill the missing spacings of the synthesis
data. The resulting polarization maps were averaged in sectors to analyse the
azimuthal behaviour of the polarized intensity (PI), rotation measure (RM), and
apparent pitch angle (\phi_obs). We developed a simplified 3-D model for the
magnetic field in the central region to explain the azimuthal behaviour of the
three observables. Our 3-D model of a quadrupolar or dipolar dynamo field can
explain the observed patterns in PI, RM, and \phi_obs, while a 2-D
configuration is not sufficient to explain the azimuthal behaviour. In addition
and independent of our model, the RM pattern shows that the spiral magnetic
field in the inner 0.5 kpc points outward, which is opposite to that in the
outer disk, and has a pitch angle of about 33 degrees, which is much larger
than that of 8-19 degrees in the outer disk. The physical conditions in the
central region differ significantly from those in the 10 kpc ring. In addition,
the orientation of this region with respect to the outer disk is completely
different. The opposite magnetic field directions suggest that the central
region is decoupled from the outer disk, and we propose that an independent
dynamo is active in the central region.Comment: Astronomy & Aatrophysics, in pres
- …