288 research outputs found
Illumination strategies for intensity-only imaging
We propose a new strategy for narrow band, active array imaging of localized
scat- terers when only the intensities are recorded and measured at the array.
We consider a homogeneous medium so that wave propagation is fully coherent. We
show that imaging with intensity-only measurements can be carried out using the
time reversal operator of the imaging system, which can be obtained from
intensity measurements using an appropriate illumination strategy and the
polarization identity. Once the time reversal operator has been obtained, we
show that the images can be formed using its singular value decomposition
(SVD). We use two SVD-based methods to image the scatterers. The proposed
approach is simple and efficient. It does not need prior information about the
sought image, and guarantees exact recovery in the noise-free case.
Furthermore, it is robust with respect to additive noise. Detailed numerical
simulations illustrate the performance of the proposed imaging strategy when
only the intensities are captured
Array imaging of localized objects in homogeneous and heterogeneous media
We present a comprehensive study of the resolution and stability properties
of sparse promoting optimization theories applied to narrow band array imaging
of localized scatterers. We consider homogeneous and heterogeneous media, and
multiple and single scattering situations. When the media is homogeneous with
strong multiple scattering between scatterers, we give a non-iterative
formulation to find the locations and reflectivities of the scatterers from a
nonlinear inverse problem in two steps, using either single or multiple
illuminations. We further introduce an approach that uses the top singular
vectors of the response matrix as optimal illuminations, which improves the
robustness of sparse promoting optimization with respect to additive noise.
When multiple scattering is negligible, the optimization problem becomes linear
and can be reduced to a hybrid- method when optimal illuminations are
used. When the media is random, and the interaction with the unknown
inhomogeneities can be primarily modeled by wavefront distortions, we address
the statistical stability of these methods. We analyze the fluctuations of the
images obtained with the hybrid- method, and we show that it is stable
with respect to different realizations of the random medium provided the
imaging array is large enough. We compare the performance of the
hybrid- method in random media to the widely used Kirchhoff migration
and the multiple signal classification methods
Synthetic aperture imaging with intensity-only data
We consider imaging the reflectivity of scatterers from intensity-only data
recorded by a single moving transducer that both emits and receives signals,
forming a synthetic aperture. By exploiting frequency illumination diversity,
we obtain multiple intensity measurements at each location, from which we
determine field cross-correlations using an appropriate phase controlled
illumination strategy and the inner product polarization identity. The field
cross-correlations obtained this way do not, however, provide all the missing
phase information because they are determined up to a phase that depends on the
receiver's location. The main result of this paper is an algorithm with which
we recover the field cross-correlations up to a single phase that is common to
all the data measured over the synthetic aperture, so all the data are
synchronized. Thus, we can image coherently with data over all frequencies and
measurement locations as if full phase information was recorded
Recommended from our members
Synthetic Aperture Imaging With Intensity-Only Data.
We consider imaging the reflectivity of scatterers from intensity-only data
recorded by a single moving transducer that both emits and receives signals,
forming a synthetic aperture. By exploiting frequency illumination diversity,
we obtain multiple intensity measurements at each location, from which we
determine field cross-correlations using an appropriate phase controlled
illumination strategy and the inner product polarization identity. The field
cross-correlations obtained this way do not, however, provide all the missing
phase information because they are determined up to a phase that depends on the
receiver's location. The main result of this paper is an algorithm with which
we recover the field cross-correlations up to a single phase that is common to
all the data measured over the synthetic aperture, so all the data are
synchronized. Thus, we can image coherently with data over all frequencies and
measurement locations as if full phase information was recorded
Current self-oscillations, spikes and crossover between charge monopole and dipole waves in semiconductor superlattices
Self-sustained current oscillations in weakly-coupled superlattices are
studied by means of a self-consistent microscopic model of sequential tunneling
including boundary conditions naturally. Well-to-well hopping and recycling of
charge monopole domain walls produce current spikes (high frequency modulation)
superimposed on the oscillation. For highly doped injecting contacts, the
self-oscillations are due to dynamics of monopoles. As the contact doping
decreases, a lower-frequency oscillatory mode due to recycling and motion of
charge dipoles is predicted. For low contact doping, this mode dominates and
monopole oscillations disappear. At intermediate doping, both oscillation modes
coexist as stable solutions and hysteresis between them is possible.Comment: 4 pages, 4 figure
Microscopic Model for Sequential Tunneling in Semiconductor Multiple Quantum Wells
We propose a selfconsistent microscopic model of vertical sequential
tunneling through a multi-quantum well.The model includes a detailed
description of the contacts,uses the Transfer Hamiltonian for expressions of
the current and it treats the Coulomb interaction within a mean field
approximation. We analyze the current density through a double well and a
superlattice and study the formation of electric field domains and
multistability coming from the Coulomb interaction. Phase diagrams of parameter
regions (bias, doping in the heterostructure and in the contacts,etc) where the
different solutions exist are given.Comment: 4 pages, 8 Postscript Figure
Accelerated 4D Flow MRI Using a Shared Subspace Constraint
Cardiovascular diseases are the leading cause of death in the
world, more than the next three leading causes of death combined.
Cardiovascular imaging techniques have allowed for the
study and understanding of the function and structure of the
heart as well as the detection, diagnosis, and monitoring of cardiovascular diseases in patients. One powerful technique for cardiac
imaging is 4D phase contrast magnetic resonance imaging
(PC-MRI) which allows measurement of blood flow velocity in the
heart and vessels. However, 4D PC-MRI is difficult to perform
due to low imaging speed and is therefore often carried out using
accelerated imaging techniques which reconstruct images from
reduced data. One approach for accelerating PC-MRI is explicitsubspace low-rank imaging; this project focuses on further accelerating explicit-subspace low-rank PC-MRI through the use of a
shared temporal subspace between PC-MR images with velocity
encoded in different directions. We will: a) investigate the subspace
structure of the differently encoded images to verify that
they indeed live in a shared subspace; b) evaluate the feasibility
of estimating this shared subspace from reduced auxiliary data
(which has direct implications on the frame rate of the resulting
images); and c) demonstrate the utility of exploiting this subspace
structure when performing image reconstruction from reduced
data.Ope
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