Improvement of the power response in contrast imaging with transmit frequency optimization

© 2009 IEEE. Reprinted, with permission, from Sébastien Ménigot, Anthony Novell, Ayache Bouakaz and Jean-Marc Girault, Improvement of the power response in contrast imaging with transmit frequency optimization, 2009 IEEE International Ultrasonics Symposium (IUS), 20---23 Sept. 2009. This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of the Université François Rabelais de Tours' products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to [email protected] audienceConventionnal ultrasound contrast imaging systems use a fixed transmit frequency. However it is known that the insonified medium (microbubbles) is time-varying and therefore an adapted time-varying excitation is expected. We suggest an adaptive imaging technique which selects the optimal transmit frequency that maximizes the contrast tissue ratio (CTR). Two algorithms have been proposed to find an US excitation for which the frequency has been optimal with microbubbles. Simulations were carried out for encapsulated microbubbles of 2µm-radius by considering the modified Rayleigh-Plesset equation for a 2.25MHz transmitted frequency and for various pressure levels (20 kPa up to 420kPa). In vitro experiments have been carried out using a 2.25 MHz transducer and using a programmable waveform generator. Responses of a 1/2000 blood mimicking fluid-diluted solution of Sonovue(TM) were measured by a 3.5 MHz transducer. We show through simulations that our adaptive imaging technique allows to reduce the transmit maximal pressure. As for in vitro experiments the CTR can reach 10 dB. By proposing a close loop system whose frequency adapts itself with the perfused media, throughout the examination, the optimization system adapt itself to the remaining bubbles population thus allowing an increase of the 30\% examination duration

Optimization of the Contrast Tissue Ratio in Ultrasound Contrast Imaging by an Adaptive Transmit Frequency

Conventionnal ultrasound contrast imaging systems use a fixed transmit frequency. However it is known that the insonified medium (microbubbles) is time-varying and therefore an adapted time-varying excitation is expected. We suggest an adaptive imaging technique which selects the optimal transmit frequency that maximizes the contrast tissue ratio (CTR). The method is proposed with the contrast imaging and the harmonic contrast imaging. Simulations were carried out for encapsulated microbubbles of 2µm-radius by considering the modified Rayleigh-Plesset equation for various pressure levels (80 kPa up to 420kPa). In vitro experiments have been carried out using using a 2.25 MHz transmitter transducer and using a programmable waveform. We show, through experiments, that our adaptive imaging technique increases the CTR of 2 dB compared to the standard method looking at the central frequency. By proposing a close loop system whose frequency adapts itself with the perfused media, throughout the examination, the optimization system adapt itself to the bubble population to seek the best trade-off between the bubble response and the transducer bandwidth

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

Optimization of the Contrast Tissue Ratio in Ultrasound Contrast Imaging by an Adaptive Transmit Frequency

Conventionnal ultrasound contrast imaging systems use a fixed transmit frequency. However it is known that the insonified medium (microbubbles) is time-varying and therefore an adapted time-varying excitation is expected. We suggest an adaptive imaging technique which selects the optimal transmit frequency that maximizes the contrast tissue ratio (CTR). The method is proposed with the contrast imaging and the harmonic contrast imaging. Simulations were carried out for encapsulated microbubbles of 2µm-radius by considering the modified Rayleigh-Plesset equation for various pressure levels (80 kPa up to 420kPa). In vitro experiments have been carried out using using a 2.25 MHz transmitter transducer and using a programmable waveform. We show, through experiments, that our adaptive imaging technique increases the CTR of 2 dB compared to the standard method looking at the central frequency. By proposing a close loop system whose frequency adapts itself with the perfused media, throughout the examination, the optimization system adapt itself to the bubble population to seek the best trade-off between the bubble response and the transducer bandwidth

Wideband acoustic activation and detection of droplet vaporization events using a capacitive micromachined ultrasonic transducer

An ongoing challenge exists in understanding and optimizing the acoustic droplet vaporization (ADV) process to enhance contrast agent effectiveness for biomedical applications. Acoustic signatures from vaporization events can be identified and differentiated from microbubble or tissue signals based on their frequency content. The present study exploited the wide bandwidth of a 128-element capacitive micromachined ultrasonic transducer (CMUT) array for activation (8 MHz) and real-time imaging (1 MHz) of ADV events from droplets circulating in a tube. Compared to a commercial piezoelectric probe, the CMUT array provides a substantial increase of the contrast-to-noise ratio

Transmit frequency optimization for ultrasound constrast agent response

International Federation for Medical and Biological Engineering copyrightInternational audienceIntroduction: Since the introduction of ultrasound (US) contrast imaging, the imaging systems use a fixed emitting frequency. However it is known that the insonified medium is time-varying and therefore an adapted time-varying excitation is expected. We suggest an adaptive imaging technique which selects the optimal transmit frequency that maximizes the acoustic contrast. Two algorithms have been proposed to find an US excitation for which the frequency was optimal with microbubbles. Methods and Materials: Simulations were carried out for encapsulated microbubbles of 2 microns by considering the modified Rayleigh-Plesset equation for 2 MHz transmit fre-quency and for various pressure levels (20 kPa up to 420kPa). In vitro experiments were carried out using a transducer oper-ating at 2 MHz and using a programmable waveform genera-tor. Contrast agent was then injected into a small container filled with water. Results and discussions: We show through simulations and in vitro experiments that our adaptive imaging technique gives: 1) in case of simulations, a gain of acoustic contrast which can reach 9 dB compared to the traditional technique without optimization and 2) for in vitro experiments, a gain which can reach 18 dB. There is a non negligible discrepancy between simulations and experiments. These differences are certainly due to the fact that our simulations do not take into account the diffraction and nonlinear propagation effects. Further optimizations are underway

Blood-brain barrier disruption with focused ultrasound enhances delivery of dopamine transporter tracer (PE2I) into the brain

International audiencePE2I is one of the most selective ligands for dopamine transporter. However it is associated with blood-brain barrier (BBB) permeability limitations. The aim of this study was to investigate the use of ultrasound and microbubbles to increase its delivery through the BBB and by determining the optimal experimental conditions that achieve a transient and safe BBB disruption. First, we stablished the ultrasound conditions that achieved a transient BBB disruption in rats using a non-permeant marker, Evans blue. Hence SonoVue® (450μL/kg) and Evans blue (100mg/kg) were intravenously administered. BBB leakage was obtained using ultrasound insonation through the rat skull at 1.6MPa, PRF 1Hz, duty cycle 1%, burst 10ms during 120sec. BBB disruption was observed in all treated animals (N=4) by histological analysis. The same experimental conditions were applied to enhance brain uptake of PE2I. Biological samples were analyzed using a scintillation counter apparatus. The results showed 50% and 20% increase of 125I-PE2I uptake in the striatum and cerebral cortex, respectively, in the treated rats (N=5) versus control (N=4). Similar enhancements were observed using SonoVue® at half concentration. This innovative method provides a great potential for intracerebral delivery of molecular ligands that could be used for the therapy of brain diseases

Adaptive matched filters for contrast imaging

2010 IEEE. Reprinted, with permission, from Sébastien Ménigot, Iulian Voicu, Anthony Novell, Melouka Elkateb Hachemi Amar and Jean-Marc Girault,Adaptive matched filters for contrast imaging, 2010 IEEE International Ultrasonics Symposium (IUS), 11---14 Oct. 2010. This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of the Université François Rabelais de Tours' products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to [email protected] audienceThe ultrasound (US) contrast imaging is a promising technique. Currently the scientific community of this field seeks US excitations which should make possible the optimization of the acoustic contrast. Two matched filters (MF) techniques are used to improve the image contrast. The first technique is an adaptive MF technique and the second is a RLS technique derived from identification theory. The system proposed is a close loop system which optimizes the power backscattered by microbubbles. After having transmit a first signal, the backscattered power is optimized by transmitting the matched filtered signal at each iteration. This process is iterated until convergence. Simulations are carried out for encapsulated microbubbles of 2 microns by considering the modified Rayleigh-Plesset equation for a 2.25 MHz transmitted frequency and for various pressure levels (20 kPa up to 420 kPa). In vitro, experiments are carried out by using two transducers a transducers which were placed perpendicularly. The signal was transmitted through a 2.25 MHz transducer. Responses of a 1/2000 diluted solution of SonoVue were measured by a 3.5 MHz transducer. Each experiment has been realized with three pressure levels (127, 244 and 370 kPa). We show through simulations and through in vitro experiments that our adaptive imaging technique gives in case of simulations a gain which can reach 12 dB compared to the traditional technique and for in vitro experiments, the MF gives a gain which can reach 4.5 dB whereas the MF derived from identification theory can reach 6 dB

Optimisation de l'énergie rétrodiffusée en imagerie de contraste par la fréquence de transmission

National audienceConventionnellement, les systèmes d'imagerie de contraste ultrasonore utilisent une fréquence d'émission fixe. Cependant, le milieu perfusé (microbulles) change au cours du temps, et donc nous suggérons une excitation adaptative au cours du temps. Nous proposons une technique d'imagerie adaptative qui sélectionne la fréquence optimale d'émission pour maximiser le contraste. Deux algorithmes ont été proposés pour trouver une excitation ultrasonore pour laquelle la fréquence est optimale pour les microbulles. Les simulations ont été réalisées avec une microbulle encapsulée de 2 microns à partir de l'équation de Rayleigh-Plesset modifiée pour différents niveaux de pression (de 20 kPa à 420 kPa). In vitro, les expériences ont été menées en utilisant 2 transducteurs placés perpendiculairement. Le signal était transmit par un transducteur à 2,25 MHz, tandis que la réponse d'une solution de Sonovue\texttrademark~ diluée à 1/2000 était mesurée par un transducteur à 3,5 MHz. Chaque expérience a été réalisée avec 2 niveaux de pression (244 et 370 kPa). Nous montrons à travers les simulations et les expériences in vitro que notre technique adaptative donne 1) pour les simulations, un gain qui peut atteindre 9 dB comparée aux techniques traditionnelles de l'imagerie de contraste 2) pour les expériences in vitro un gain qui peut atteindre 14 dB. En proposant un système en boucle fermée qui adapte de lui-même la fréquence au milieu perfusé tout au long de l'examen, le système d'optimisation s'adapte de lui-même à la population de microbulles restantes, et donc permet une augmentation de 30% de la durée d'examen. La divergence entre les résultats expérimentaux et ceux de simulations est non-négligeable. Ces différences sont certainement dues à la simulation qui ne prend pas en compte les imperfections des transducteurs, l'atténuation, la diffraction et les effets de la propagation. Cependant, ces résultats sont encourageants et nous suggèrent de continuer dans cette voie

Optimization of multi-pulse sequences for nonlinear contrast agent imaging using a cMUT array

Capacitive micromachined ultrasonic transducer (cMUT) technology provides advantages such as wide frequency bandwidth, which can be exploited for contrast agent imaging. Nevertheless, the efficiency of traditional multi-pulse imaging schemes, such as pulse inversion (PI), remains limited because of the intrinsic nonlinear character of cMUTs. Recently, a new contrast imaging sequence, called bias voltage modulation sequence (BVM), had been specifically developed for cMUTs to suppress their unwanted nonlinear behavior. In this study, we propose to optimize contrast agent detection by combining the BVM sequence with PI and/or chirp reversal (CR). An aqueous dispersion of lipid encapsulated microbubbles was exposed to several combinations of multi-pulse imaging sequences. Approaches were evaluated in vitro using 9 inter-connected elements of a cMUT linear array (excitation frequency of 4 MHz; peak negative pressure of 100 kPa). For sequences using chirp excitations, a specific compression filter was designed to compress and extract several nonlinear components from the received microbubble responses. A satisfactory cancellation of the nonlinear signal from the source is achieved when BVM is combined with PI and CR. In comparison with PI and CR imaging modes alone, using sequences incorporating BVM increases the contrast-to-tissue ratio by 10.0 dB and 4.6 dB, respectively. Furthermore, the combination of BVM with CR and PI results in a significant increase of the contrast-to-noise ratio (+29 dB). This enhancement is attributed to the use of chirps as excitation signals and the improved preservation of several nonlinear components contained within the contrast agent response