Non-invasive Evaluation of Aortic Stiffness Dependence with Aortic Blood Pressure and Internal Radius by Shear Wave Elastography and Ultrafast Imaging

Elastic properties of arteries have long been recognized as playing a major role in the cardiovascular system. However, non-invasive in vivo assessment of local arterial stiffness remains challenging and imprecise as current techniques rely on indirect estimates such as wall deformation or pulse wave velocity. Recently, Shear Wave Elastography (SWE) has been proposed to non-invasively assess the intrinsic arterial stiffness. In this study, we applied SWE in the abdominal aortas of rats while increasing blood pressure (BP) to investigate the dependence of shear wave speed with invasive arterial pressure and non-invasive arterial diameter measurements. A 15MHz linear array connected to an ultrafast ultrasonic scanner, set non-invasively, on the abdominal aorta of anesthetized rats (N=5) was used. The SWE acquisition followed by an ultrafast (UF) acquisition was repeated at different moment of the cardiac cycle to assess shear wave speed and arterial diameter variations respectively. Invasive arterial BP catheter placed in the carotid, allowed the accurate measurement of pressure responses to increasing does of phenylephrine infused via a venous catheter. The SWE acquisition coupled to the UF acquisition was repeated for different range of pressure. For normal range of BP, the shear wave speed was found to follow the aortic BP variation during a cardiac cycle. A minimum of (5.06$\pm$0.82) m/s during diastole and a maximum of (5.97$\pm$0.90) m/s during systole was measured. After injection of phenylephrine, a strong increase of shear wave speed (13.85$\pm$5.51) m/s was observed for a peak systolic arterial pressure of (190$\pm$10) mmHg. A non-linear relationship between shear wave speed and arterial BP was found. A complete non-invasive method was proposed to characterize the artery with shear wave speed combined with arterial diameter variations. Finally, the results were validated against two parameters the incremental elastic modulus and the pressure elastic modulus derived from BP and arterial diameter variations

Imaging the effect of acoustically induced cavitation bubbles on the generation of shear-waves by ultrasonic radiation force

In soft solids, the acoustic radiation force on bubbles generates a shear wave. This bubble-based shear wave can be imaged using high frame rate ultrasound imaging. We report here an experiment where cavitation is induced in a tissue mimicking material by an ultrasonic tone-burst excitation, which also pushes the bubbles. The generated shear wave was imaged and the energy backscattered by the bubbles measured. The tone burst excitation was iterated at the same location and the decrease of both the amplitude of the particle velocity induced by the shear wave and the backscattered energy was shown. Data treatment to extract the bubbles' contribution to this two quantities, and a simple theoretical model allowed us to point out their linear dependence.http://deepblue.lib.umich.edu/bitstream/2027.42/84304/1/CAV2009-final129.pd

Acoustically induced and controlled micro-cavitation bubbles as active source for transcranial adaptive focusing

The skull bone is a strong aberrating medium for ultrasound in the low MHz range. Brain treatment with High Intensity Focused Ultrasound (HIFU) can however be achieved through the skull by multichannel arrays using an adaptive focusing technique. Time-reversal is a robust adaptive technique for correction of aberrations. It achieves moreover a matched filter and then allows the optimal energy concentration for thermal therapy. Nevertheless, this method requires a reference signal sent by a source embedded in brain tissues. Acoustically generated cavitation bubbles are active acoustic sources which can be remotely generated. Therefore, they are suited for this non-invasive time reversal aberration correction. We report here in vitro experiments where micro-cavitation was induced transcranially in agar gel at targeted positions using a coarse aberration correction either obtained from CT-scan based simulations or conventional steering. The bubbles' ultrasonic signature received by the array were then successfully used to optimally focus at the designated locations.http://deepblue.lib.umich.edu/bitstream/2027.42/84308/1/CAV2009-final134.pd

Intervertebral disc characterisation by elastography: a preliminary study.

BiomecAM chai

A versatile and robust microfluidic device for capillary-sized simple or multiple emulsions production

International audienceUltrasound-vaporizable microdroplets can be exploited for targeted drug delivery. However, it requires customized microfluidic techniques able to produce monodisperse, capillary-sized and biocompatible multiple emulsions. Recent development of microfluidic devices led to the optimization of microdroplet production with high yields, low polydispersity and well-defined diameters. So far, only few were shown to be efficient for simple droplets or multiple emulsions production below 5 microns in diameter, which is required to prevent microembolism after intravenous injection. Here, we present a versatile microchip for both simple and multiple emulsion production. This parallelized system based on microchannel emulsification was designed to produce perfluorocarbon in water or water within perfluorocarbon in water emulsions with capillary sizes (<5 μm) and polydispersity index down to 5 % for in vivo applications such as spatiotemporally-triggered drug delivery using Ultrasound. We show that droplet production at this scale is mainly controlled by interfacial tension forces, how capillary and viscosity ratios influence droplet characteristics and how different production regimes may take place. The better understanding of droplet formation and its relation to applied pressures is supported by observations with a high-speed camera. Compared to previous microchips, this device opens perspectives to produce injectable and biocompatible droplets with a reasonable yield in order to realize preclinical studies in mice

Shear Wave Imaging of Passive Diastolic Myocardial Stiffness: Stunned Versus Infarcted Myocardium

published_or_final_versio

Mapping Myocardial Fiber Orientation Using Echocardiography-Based Shear Wave Imaging

The assessment of disrupted myocardial fiber arrangement may help to understand and diagnose hypertrophic or ischemic cardiomyopathy. We hereby proposed and developed shear wave imaging (SWI), which is an echocardiography-based, noninvasive, real-time, and easy-to-use technique, to map myofiber orientation. Five in vitro porcine and three in vivo open-chest ovine hearts were studied. Known in physics, shear wave propagates faster along than across the fiber direction. SWI is a technique that can generate shear waves travelling in different directions with respect to each myocardial layer. SWI further analyzed the shear wave velocity across the entire left-ventricular (LV) myocardial thickness, ranging between 10 (diastole) and 25 mm (systole), with a resolution of 0.2 mm in the middle segment of the LV anterior wall region. The fiber angle at each myocardial layer was thus estimated by finding the maximum shear wave speed. In the in vitro porcine myocardium (n=5), the SWI-estimated fiber angles gradually changed from +80° ± 7° (endocardium) to +30° ± 13° (midwall) and-40° ± 10° (epicardium) with 0° aligning with the circumference of the heart. This transmural fiber orientation was well correlated with histology findings (r2=0.91± 0.02, p<0.0001). SWI further succeeded in mapping the transmural fiber orientation in three beating ovine hearts in vivo. At midsystole, the average fiber orientation exhibited 71° ± 13° (endocardium), 27° ± 8° (midwall), and-26° ± 30° (epicardium). We demonstrated the capability of SWI in mapping myocardial fiber orientation in vitro and in vivo. SWI may serve as a new tool for the noninvasive characterization of myocardial fiber structure. © 2012 IEEE.published_or_final_versio

Non-invasive biomechanical characterization of intervertebral discs by shear wave ultrasound elastography: a feasibility study.

PublishedJournal ArticleThe final publication is available at Springer via http://dx.doi.org/10.1007/s00330-014-3382-8OBJECTIVES: Although magnetic resonance is widely spread to assess qualitatively disc morphology, a simple method to determine reliably intervertebral disc status is still lacking. Shear wave elastography is a novel technique that allows quantitative evaluation of soft-tissues' mechanical properties. The aim of this study was to assess preliminary the feasibility and reliability of mechanical characterization of cervical intervertebral discs by elastography and to provide first reference values for asymptomatic subjects. METHODS: Elastographic measurements were performed to determine shear wave speed (SWS) in C6-C7 or C7-T1 disc of 47 subjects; repeatability and inter-operator reproducibility were assessed. RESULTS: Global average shear wave speed (SWS) was 3.0 ± 0.4 m/s; measurement repeatability and inter-user reproducibility were 7 and 10%, respectively. SWS was correlated with both subject's age (p = 1.3 × 10(-5)) and body mass index (p = 0.008). CONCLUSIONS: Shear wave elastography in intervertebral discs proved reliable and allowed stratification of subjects according to age and BMI. Applications could be relevant, for instance, in early detection of disc degeneration or in follow-up after trauma; these results open the way to larger cohort studies to define the place of this technique in routine intervertebral disc assessment. KEY POINTS: A simple method to obtain objectively intervertebral disc status is still lacking. Shear wave elastography was applied in vivo to assess intervertebral discs. Elastography showed promising results in biomechanical disc evaluation. Elastography could be relevant in clinical routine for intervertebral disc assessment.ParisTech BiomecAM chair programParisTechYves Cotrel FoundationsSociété GénéraleProteorCove

Intervertebral disc characterization by shear wave elastography: An in vitro preliminary study.

Published onlineJOURNAL ARTICLEAuthor's accepted (post-print) manuscriptThe final version of record is available at http://dx.doi.org/10.1177/0954411914540279Patient-specific numerical simulation of the spine is a useful tool both in clinic and research. While geometrical personalization of the spine is no more an issue, thanks to recent technological advances, non-invasive personalization of soft tissue's mechanical properties remains a challenge. Ultrasound elastography is a relatively recent measurement technique allowing the evaluation of soft tissue's elastic modulus through the measurement of shear wave speed. The aim of this study was to determine the feasibility of elastographic measurements in intervertebral disc. An in vitro approach was chosen to test the hypothesis that shear wave speed can be used to evaluate intervertebral disc mechanical properties and to assess measurement repeatability. In total, 11 oxtail intervertebral discs were tested in compression to determine their stiffness and apparent elastic modulus at rest and at 400 N. Elastographic measurements were performed in these two conditions and compared to these mechanical parameters. The protocol was repeated six times to determine elastographic measurement repeatability. Average shear wave speed over all samples was 5.3 ± 1.0 m/s, with a repeatability of 7% at rest and 4.6% at 400 N; stiffness and apparent elastic modulus were 266.3 ± 70.5 N/mm and 5.4 ± 1.1 MPa at rest, respectively, while at 400 N they were 781.0 ± 153.8 N/mm and 13.2 ± 2.4 MPa, respectively. Correlations were found between elastographic measurements and intervertebral disc mechanical properties; these preliminary results are promising for further in vivo application.ParisTech BiomecAM chair programProteorParisTechYves Cotrel Foundation

Multi-scale mapping along the auditory hierarchy using high-resolution functional UltraSound in the awake ferret

A major challenge in neuroscience is to longitudinally monitor whole brain activity across multiple spatial scales in the same animal. Functional UltraSound (fUS) is an emerging technology that offers images of cerebral blood volume over large brain portions. Here we show for the first time its capability to resolve the functional organization of sensory systems at multiple scales in awake animals, both within small structures by precisely mapping and differentiating sensory responses, and between structures by elucidating the connectivity scheme of top-down projections. We demonstrate that fUS provides stable (over days), yet rapid, highly-resolved 3D tonotopic maps in the auditory pathway of awake ferrets, thus revealing its unprecedented functional resolution (100/300µm). This was performed in four different brain regions, including very small (1–2 mm3 size), deeply situated subcortical (8 mm deep) and previously undescribed structures in the ferret. Furthermore, we used fUS to map long-distance projections from frontal cortex, a key source of sensory response modulation, to auditory cortex