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

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

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

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

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

Estimation du rythme cardiaque fœtal par l'estimateur YIN

International audienceTechniques dedicated to the fetal heart rate detection identify the patterns that repeat themselves over the time. The heart rate estimation is algorithmically similar to the estimation of the fundamental frequency (pitch) of voice signals. The new YIN technique introduced for the estimation of the fundamental frequency is applied to fetal heart rate estimation from the directional Doppler signals. We compare the performances in terms of probability of detection and accuracy of the estimation of the technique YIN with those of the cross-correlation, implemented in the Oxford SONICAID™ monitors. A better detection probability and accuracy of estimation of fetal heart rate was obtained in case of YIN.Les techniques dédiées à la détection du rythme cardiaque fœtal (RCF) identifient des motifs du signal qui se répètent dans le temps. Algorithmiquement, l’estimation du rythme cardiaque est similaire à l’estimation de la fréquence fondamentale des signaux de parole. Nous proposons ici de comparer la technique existante basée sur la corrélation-croisée à la nouvelle technique YIN pour l’estimation du RCF à partir des signaux Doppler directionnels. Nous avons comparé les performances en termes de probabilité de détection et de précision d’estimation de la technique YIN avec celles de la corrélation-croisée, implémentée dans les moniteurs Oxford SONICAID™. Une meilleure probabilité de détection et une meilleure précision d’estimation du RCF a été obtenue dans le cas de la technique YIN

Amélioration de l'estimation du rythme cardiaque foetal en utilisant les signaux Doppler directionnels et YIN

National audienceTechniques dedicated to the fetal heart rate detection identify the patterns that repeat themselves over the time. The heart rate estimation is algorithmically similar to the estimation of the fundamental frequency (pitch) of voice signals. The new YIN technique introduced for the estimation of the fundamental frequency is applied to fetal heart rate estimation from the directional Doppler signals. We compare the performance of YIN with those of cross-correlation, which is implemented in the Oxford SONICAID monitors. A better detection probability of fetal heart rate was obtained in case of YIN.Les techniques dédiées à la détection du rythme cardiaque fœtal (RCF) identifient des motifs du signal qui se répètent dans le temps. Algorithmiquement, l'estimation du rythme cardiaque est similaire à l'estimation de la fréquence fondamentale des signaux de parole. Nous proposons ici de comparer la technique existante basée sur la corrélation-croisée à la nouvelle technique YIN pour l'estimation du RCF à partir des signaux Doppler directionnels. Nous avons comparé les performances en termes de probabilité de détection et de précision d'estimation de la technique YIN avec celles de la corrélation-croisée, implémentée dans les moniteurs Oxford SONICAID™. Une meilleure probabilité de détection et une meilleure précision d'estimation du RCF a été obtenue dans le cas de la technique YIN

Optimal Prefiltered Stochastic Transmitted Waves for Fat Inclusion Detection in Milk with Harmonic Ultrasound

International audienceIn non destructive testing, improvements have been made possible by taking into account the harmonic frequencies, as in agri-food domain. The transmitted signal are often selected empirically as a fixed-frequency Gaussian pulse, by taking into account the transducer bandwidth only. However, waveform should take into account all the features of the ultrasound system and of the medium. To design the waveform, a genetic algorithm looks for the best stochastic wave. However, one of limitations in this optimization process is the high amount of transmitted waves. To reduce this number, instead of transmitting wide band stochastic waves, narrow band stochastic waves limited by the transducer bandwidth are preferred. The optimization was thus applied on the detection of fat cluster in milk by maximizing the signal-to-noise ratio (SNR), while decreasing the amount of transmitted waves. Twelve combinations from different limited bandwidths of transmitted waves were tested. Whereas the low cut-off frequencies did not change the performances, the high cut-off frequencies affected the convergence speed. In this study, it is shown that the best optimization was twelve times faster with the high cut-off frequency of 5.6 MHz and led to a gain of 62% compared to the SNR obtained with a best fixed-frequency sine wave

Optimization of Contrast to Tissue Ratio by Frequency Adaptation in Pulse Inversion Imaging

Simulation program available : http://www.runmycode.org/companion/view/1956International audienceContrast imaging has significantly improved clinical diagnosis by increasing the contrast-to-tissue ratio after microbubble injection. Pulse inversion imaging is the most commonly used contrast imaging technique, as it greatly increases the contrast-to-tissue ratio by extracting microbubble nonlinearities. The main purpose of our study was to propose an automatic technique providing the best contrast-to-tissue ratio throughout the experiment. For reasons of simplicity, we proposed to maximize the contrast-to-tissue ratio with an appropriate choice of the transmit frequency. The contrast-to-tissue ratio was maximized by a closed loop system including the pulse inversion technique. An algorithm based on the gradient provided iterative determination of the optimal transmit frequency. The optimization method converged quickly after six iterations. This optimal control method is easy to implement and it optimizes the contrast-to-tissue ratio by selecting the transmit frequency adaptively