Harmonic Magnification by Time Reversal based on a Hammerstein Decomposition

International audienceIn tissue harmonic imaging, the contrast has been increased by using nonlinearities. However, enhancing the contrast requires a good harmonic level. Time reversal process is well-known to enhance the backscattered signal by optimizing the transmitted wave. However, it is not well-adjusted for second harmonics, since the nonlinearities of the time reversed harmonic signal will be double the frequency and will be removed by the transducer. To take into account the harmonics, one way consists in modelling the ultrasound system by parallel subsystems with a Hammerstein model. Therefore finding the optimal wave for harmonic generation means to create a matched filter for the subsystem describing the nonlinearity. As described by the time reversal, the first wave propagates to the medium. The backscattered second harmonic components are extracted from a Hammerstein model. The second harmonic signal is time reversed, frequency shifted by a demodulation and sent in the medium. In this study, the nonlinear components could be increased by 15 dB in comparison to the time reversal process and conventional imaging. Moreover, the SNR can be increased by 7 dB. The second harmonic time reversed waves focus better on the inclusion and wave coherence is preserved. From this point of view, the optimization process can be viewed as an extension of the matched filtering feature of the time reversal principle to second harmonics

Synchronous detector as a new paradigm for automatic microembolus detection

International audienceWe propose here a new detector paradigm based on synchronous detection. Unlike standard energy detectors, this kind of detector uses the pseudo-cyclo-stationarity properties of blood Doppler signals. The new detector, that is synchronized with cardiac rhythm, was compared to a gold standard and a standard energy detector. Detectors were evaluated according to clinical recordings from patients with carotid stenoses. The results showed that it was possible to detect inaudible microemboli. The detection rate was improved by 30% and the false alarm rate was below 2%

Optimization of Contrast-to-Tissue Ratio by Adaptation of Transmitted Ternary Signal in Ultrasound Pulse Inversion Imaging

Open access journal.Simulation program available: http://www.runmycode.org/companion/view/1956International audienceUltrasound contrast imaging has provided more accurate medical diagnoses thanks to the development of innovating modalities like the pulse inversion imaging. However, this latter modality that improves the contrast-to-tissue ratio (CTR) is not optimal, since the frequency is manually chosen jointly with the probe. However, an optimal choice of this command is possible but it requires precise information about the transducer and the medium which can be experimentally difficult to obtain, even inaccessible. It turns out that the optimization can become more complex by taking into account the kind of generators, since the generators of electrical signals in a conventional ultrasound scanner can be unipolar, bipolar or tripolar. Our aim was to seek the ternary command which maximized the CTR. By combining a genetic algorithm and a closed loop, the system automatically proposed the optimal ternary command. In simulation, the gain compared with the usual ternary signal could reach about 3.9 dB. Another interesting finding was that in contrast to what is generally accepted, the optimal command was not a fixed-frequency signal, but had harmonic components

A two-fluid hyperbolic model in a porous medium

International audienceThe paper is devoted to the computation of two-phase flows in a porous medium when applying the two-fluid approach. The basic formulation is presented first, together with the main properties of the model. A few basic analytic solutions are then provided, some of them corresponding to solutions of the one-dimensional Riemann problem. Three distinct Finite-Volume schemes are then introduced. The first two schemes, which rely on the Rusanov scheme, are shown to give wrong approximations in some cases involving sharp porous profiles. The third one, which is an extension of a scheme proposed by D. Kröner and M. D. Thanh (27) for the computation of single phase flows in varying cross section ducts , provides fair results in all situations. Properties of schemes and numerical results are presented. Analytic tests enable to compute the L 1 norm of the error

Analysis and modelling of the Optimal Command for a Ultrasound Pulse Inversion Imaging System

International audienceOver the past twenty years, in ultrasound imaging, contrast and resolution were improved by using the nonlinearties of the medium. One of the most common techniques which used this properties is the pulse inversion imaging. The optimization of this imaging system that we proposed has consisted in finding the optimal command. However, the properties which enable to make an optimal command was not known and that is why we seek the best optimal command by exciting the system by random sequences. In this study, we proposed two steps in our analysis: an analysis and a modelling stage. The proposed model took into account the nonlinearity of the optimal command and enabled to describe the optimal command by using some parameters. If the synthetic model was used in the pulse inversion imaging system, the contrast can reach the same performances

Contrast Optimization by Metaheuristic for Inclusion Detection in Nonlinear Ultrasound Imaging

International Congress on Ultrasonics, Metz, May 2015International audienceIn ultrasound imaging, improvements have been made possible by taking into account the harmonic frequencies. However, the transmitted signal often consists of providing empirically pre-set transmit frequencies, even if the medium to be explored should be taken into account during the optimization process. To resolve this waveform optimization, transmission of stochastic sequences were proposed combined with a genetic algorithm. A medium with an inclusion was compared in term of contrast to a reference medium without defect. Two media were distinguished thanks an Euclidean distance. In simulation, the optimal distance could be multiplied by 4 in comparison with an usual excitation

Multi-scale sample entropy and recurrence plots distinguish healthy from suffering foetus

International audienceCurrently, the assessment of the state of fetal well-being using ultrasound is a challenge in the obstetrical world. To assess the fetal well-being, parameters are derived from the fetal heart rate and fetal movements. We estimated the fetal heart rate using a multi-sensor, multi-gate pulsed Doppler system and we propose to study the complexity of heart rate by calculating the multi-scale entropy and the parameters deduced from the recurrence plots. The article presents a preliminary study that evaluates the relevance of complexity parameters in assessing the state of fetal well-being. Our results show that complexity parameters can distinguish healthy from suffering foetus

Similarity Quantification Analysis to Detect Suffering Fetus

International audienceTo characterize a dynamical system, a number of descriptors or invariants can be used. Here we proposed to review and generalize the analysis of recurrence by introducing the con-cept of similarity. In this study, we showed that it was possible to distinguish the fetal heart rate of healthy from suffering fe-tus using the similarities of a time series, while the analysis of recurrence does not allowed it

Optimal Control by Transmit Frequency in Tissue Harmonic Imaging

ISBN: 978-2-919340-01-9; EAN: 9782919340019International audienceUltrasound imaging systems usually work in open loop. The system control is thus a sine wave whose frequency is often fixed around two-thirds of the center frequency of the transducer in tissue harmonic imaging. However, this choice requires a knowledge of the transducer and does not take into account the medium properties. Our aim is to seek the command which maximizes the tissue harmonic contrast. We proposed an iterative optimization algorithm that automatically saught for the optimal frequency of the command. Both experimentally and in simulation, its value did not correspond to the usual value. The contrast can be improved by 5 dB. By providing a closed loop system, the system automatically proposes the optimal control without any a priori knowledge of the system or of the medium explored

Random excitation by optimized pulse inversion in contrast harmonic imaging

ISBN: 978-2-919340-01-9 EAN: 9782919340019International audienceOver the past twenty years, in ultrasound contrast imaging, new physiological information are obtained by the detection of non-linearities generated by the microbubbles. One of the most used techniques is the pulse inversion imaging. The usual command of this system is a fixed-frequency sinus wave. An optimal choice of this command requires the knowledge of the transducer and of the medium to obtain the best contrast-to-tissue ratio. However, these information are experimentally inaccessible. Our goal is to seek the command which maximizes the contrast-to-tissue ratio. Among several noises, we identified the one which maximized the contrast-to-tissue ratio. A new suboptimal control was made from the parameters of a nonlinear autoregressive filter and from suboptimal noise. The contrast-to-tissue ratio was then iteratively optimized by the method of Nelder-Mead which adjusted the filter parameters. The gain compared to the case in which we used at the optimal frequency can reach about 1 dB and 5 dB in comparison to the center frequency of the transducer. By adding a closed loop, the system automatically proposes the optimal command without any a priori knowledge of the system or of the medium explored and without any hypothesis about the shape of the command