6,027 research outputs found
Atlas-Based Prostate Segmentation Using an Hybrid Registration
Purpose: This paper presents the preliminary results of a semi-automatic
method for prostate segmentation of Magnetic Resonance Images (MRI) which aims
to be incorporated in a navigation system for prostate brachytherapy. Methods:
The method is based on the registration of an anatomical atlas computed from a
population of 18 MRI exams onto a patient image. An hybrid registration
framework which couples an intensity-based registration with a robust
point-matching algorithm is used for both atlas building and atlas
registration. Results: The method has been validated on the same dataset that
the one used to construct the atlas using the "leave-one-out method". Results
gives a mean error of 3.39 mm and a standard deviation of 1.95 mm with respect
to expert segmentations. Conclusions: We think that this segmentation tool may
be a very valuable help to the clinician for routine quantitative image
exploitation.Comment: International Journal of Computer Assisted Radiology and Surgery
(2008) 000-99
A new multipath mitigation method for GNSS receivers based on antenna array
the potential of small antenna array for multipath mitigation in GNSS systems is considered in this paper. To discriminate the different incoming signals (Line of sight and multipaths), a new implementation of the well known SAGE algorithm is proposed. This allows a significant complexity reduction and it is fully compatible with conventional GNSS receivers. Theoretical study thanks to the Cramer Rao Bound derivation and tracking simulation results (in static and dynamic scenarios) show that the proposed method is a very promising approach for the multipath mitigation problem in GNSS receivers
A new tracking approach for multipath mitigation based on antenna array
In Global Navigation Satellites Systems (GNSS), multipaths (MP) are still one of the major error sources. The additional signal replica due to reflection will introduce a bias in conventional Delay Lock Loops (DLL) which will finally cause a strong positioning error. Several techniques, based on Maximum Likelihood estimation (ML), have been developed for multipaths mitigation/estimation such as the Narrow correlator spacing [1] or the Multipath Estimating Delay-Lock-Loop (MEDLL) [2] algorithm. These techniques try to discriminate the MP from the Line Of Sight Signal (LOSS) on the time and frequency domains and thus, short delay multipaths (<0.1Chips) can not be completely mitigated. Antenna array perform a spatial sampling of the wave front what makes possible the discrimination of the sources on the space domain (azimuth and elevation). As the time-delay domain and space domain can be assumed independent, we can expect to mitigate/estimate very short delay MP by using an antenna array. However, we don't want to increase too much the size, the complexity and the cost of the receivers and thus, we focus our study on small arrays with a small number of antennas: typically a square 2x2 array. Consequently, conventional beamforming (space Fast Fourier Transform) is not directive enough to assure the mitigation of the multipaths, and then this first class of solutions was rejected. In order to improve the resolution, adaptive beamformers have also been tested. However, the LOSS and the MP signal are strongly correlated and thus, classical adaptive algorithms [3] are not able to discriminate the sources. These preliminary studies have shown that the mitigation/estimation of multipaths based on the space domain will exhibit limited performances in presence of close sources. Then, in order to propose robust algorithms, we decided to investigate a space-time-frequency estimation of the sources. Space Alternating Generalized Expectation maximisation (SAGE) algorithm [4], which is a low-complexity generalization of the Expectation Maximisation (EM) algorithm, has been considered. The basic concept of the SAGE algorithm is the hidden data space [4]. Instead of estimating the parameters of all impinging waves in parallel in one iteration step as done by the EM algorithm, the SAGE algorithm estimates the parameters of each signal sequentially. Moreover, SAGE algorithm breaks down the multi-dimensional optimization problem into several smaller problems. In [5], it can be seen that SAGE algorithm is efficient for any multipaths configurations (small relative delays, close DOAs) and space-time-frequency approach is clearly outperforming classical time-frequency approaches. Notwithstanding, SAGE algorithm is a post processing algorithm. Thus, it's necessary to memorise in the receiver the incoming signal in order to apply SAGE estimation. For example, if we want to process 10ms of signal with a 10MHz sampling rate, we need to store a matrix of m*105 with m the number of antennas. In such condition, we can understand than SAGE algorithm is hardly implemented in real time. The challenge is then to find a new type of algorithms that reach the efficiency of the SAGE algorithms, but with a reduced complexity in order to enable real time processing.
Furthermore, the implementation should be compatible with conventional GNSS tracking loops (DLL and PLL). To cope with these two constraints, we propose to apply the SAGE algorithm on the post-correlated signal. Indeed, the correlation step can be seen as a compression step and thus, the size of the studied signal is strongly reduced. In such a way, SAGE algorithm is able to provide estimates of the relative delay and Doppler of the received signals with respect to the local replicas. Thus, a post correlation implementation of SAGE can be seen as a discriminator for both the DLL and the PLL
Real-Time Nearfield Acoustic Holography: Implementation of the Direct and Inverse Impulse Responses in the Time-Wavenumber Domain
The aim of the study is to demonstrate that some methods are more relevant
for implementing the Real-Time Nearfield Acoustic Holography than others. First
by focusing on the forward propagation problem, different approaches are
compared to build the impulse response to be used. One of them in particular is
computed by an inverse Fourier transform applied to the theoretical transfer
function for propagation in the frequency-wavenumber domain. Others are
obtained by directly sampling an analytical impulse response in the
time-wavenumber domain or by additional low-pass filtering. To estimate the
performance of each impulse response, a simulation test involving several
monopoles excited by non stationary signals is presented and some features are
proposed to assess the accuracy of the temporal signals resulting from
reconstruction processing on a forward plane. Then several inverse impulse
responses used to solve the inverse problem, which consists in back propagating
the acoustic signals acquired by the microphone array, are built directly from
a transfer function or by using Wiener inverse filtering from the direct
impulse responses obtained for the direct problem. Another simulation test is
performed to compare the signals reconstructed on the source plane. The same
indicators as for the propagation study are used to highlight the differences
between the methods tested for solving the Holography inverse problem.Comment: 15 th International Congress on Sound and Vibration, Daejeon :
Cor\'ee, R\'epublique de (2008
Radiation effects on CMOS image sensors with sub-2”m pinned photodiodes
A group of four commercial sensors with pixel pitches below 2ÎŒm has been irradiated with 60Co source at several total ionizing dose levels related to space applications. A phenomenological approach is proposed through behavior analysis of multiple sensors embedding different technological choices (pitch, isolation or buried oxide). A complete characterization including dark current, activation energy and temporal noise analysis allows to discuss about a degradation scheme
Locality of triad interaction and Kolmogorov constant in inertial wave turbulence
Using the theory of wave turbulence for rapidly rotating incompressible
fluids derived by Galtier (2003), we find the locality conditions that the
solutions of the kinetic equation must satisfy. We show that the exact
anisotropic Kolmogorov-Zakharov (KZ) spectrum satisfies these conditions, which
justifies the existence of this constant (positive) energy flux solution.
Although a direct cascade is predicted in the transverse () and parallel
() directions to the rotation axis, we show numerically that in the
latter case some triadic interactions can have a negative contribution to the
energy flux, while in the former case all interactions contribute to a positive
flux. Neglecting the parallel energy flux, we estimate the Kolmogorov constant
at . These results provide theoretical support for recent
numerical and experimental studies.Comment: 10 pages, 4 figure
Topic segmentation of TV-streams by watershed transform and vectorization
International audienceA fine-grained segmentation of Radio or TV broadcasts is an essential step for most multimedia processing tasks. Applying segmentation algorithms to the speech transcripts seems straightforward. Yet, most of these algorithms are not suited when dealing with short segments or noisy data. In this paper, we present a new segmentation technique inspired from the image analysis field and relying on a new way to compute similarities between candidate segments called Vectorization. Vectorization makes it possible to match text segments that do not share common words; this property is shown to be particularly useful when dealing with transcripts in which transcription errors and short segments makes the segmentation difficult. This new topic segmen-tation technique is evaluated on two corpora of transcripts from French TV broadcasts on which it largely outperforms other existing approaches from the state-of-the-art
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