Clement Papadacci

3D ultrafast ultrasound imaging in vivo

Very high frame rate ultrasound imaging has recently allowed for the extension of the applications of echography to new fields of study such as the functional imaging of the brain, cardiac electrophysiology, and the quantitative imaging of the intrinsic mechanical properties of tumors, to name a few, non-invasively and in real time. In this study, we present the first implementation of Ultrafast Ultrasound Imaging in 3D based on the use of either diverging or plane waves emanating from a sparse virtual array located behind the probe. It achieves high contrast and resolution while maintaining imaging rates of thousands of volumes per second. A customized portable ultrasound system was developed to sample 1024 independent channels and to drive a 32  ×  32 matrix-array probe. Its ability to track in 3D transient phenomena occurring in the millisecond range within a single ultrafast acquisition was demonstrated for 3D Shear-Wave Imaging, 3D Ultrafast Doppler Imaging, and, finally, 3D Ultrafast combined Tissue and Flow Doppler Imaging. The propagation of shear waves was tracked in a phantom and used to characterize its stiffness. 3D Ultrafast Doppler was used to obtain 3D maps of Pulsed Doppler, Color Doppler, and Power Doppler quantities in a single acquisition and revealed, at thousands of volumes per second, the complex 3D flow patterns occurring in the ventricles of the human heart during an entire cardiac cycle, as well as the 3D in vivo interaction of blood flow and wall motion during the pulse wave in the carotid at the bifurcation. This study demonstrates the potential of 3D Ultrafast Ultrasound Imaging for the 3D mapping of stiffness, tissue motion, and flow in humans in vivo and promises new clinical applications of ultrasound with reduced intra--and inter-observer variability.

High-contrast ultrafast imaging of the heart

Noninvasive ultrafast imaging of intrinsic waves such as electromechanical waves or remotely induced shear waves in elastography imaging techniques for human cardiac applications remains challenging. In this paper, we propose ultrafast imaging of the heart with adapted sector size by coherently compounding diverging waves emitted from a standard transthoracic cardiac phased-array probe. As in ultrafast imaging with plane wave coherent compounding, diverging waves can be summed coherently to obtain high-quality images of the entire heart at high frame rate in a full field of view. To image the propagation of shear waves with a large SNR, the field of view can be adapted by changing the angular aperture of the transmitted wave. Backscattered echoes from successive circular wave acquisitions are coherently summed at every location in the image to improve the image quality while maintaining very high frame rates. The transmitted diverging waves, angular apertures, and subaperture sizes were tested in simulation, and ultrafast coherent compounding was implemented in a commercial scanner. The improvement of the imaging quality was quantified in phantoms and in one human heart, in vivo. Imaging shear wave propagation at 2500 frames/s using 5 diverging waves provided a large increase of the SNR of the tissue velocity estimates while maintaining a high frame rate. Finally, ultrafast imaging with 1 to 5 diverging waves was used to image the human heart at a frame rate of 4500 to 900 frames/s over an entire cardiac cycle. Spatial coherent compounding provided a strong improvement of the imaging quality, even with a small number of transmitted diverging waves and a high frame rate, which allows imaging of the propagation of electromechanical and shear waves with good image quality.

3-D ultrafast doppler imaging applied to the noninvasive mapping of blood vessels in Vivo

Ultrafast Doppler imaging was introduced as a technique to quantify blood flow in an entire 2-D field of view, expanding the field of application of ultrasound imaging to the highly sensitive anatomical and functional mapping of blood vessels. We have recently developed 3-D ultrafast ultrasound imaging, a technique that can produce thousands of ultrasound volumes per second, based on a 3-D plane and diverging wave emissions, and demonstrated its clinical feasibility in human subjects in vivo. In this study, we show that noninvasive 3-D ultrafast power Doppler, pulsed Doppler, and color Doppler imaging can be used to perform imaging of blood vessels in humans when using coherent compounding of 3-D tilted plane waves. A customized, programmable, 1024-channel ultrasound system was designed to perform 3-D ultrafast imaging. Using a 32 × 32, 3-MHz matrix phased array (Vermon, Tours, France), volumes were beamformed by coherently compounding successive tilted plane wave emissions. Doppler processing was then applied in a voxel-wise fashion. The proof of principle of 3-D ultrafast power Doppler imaging was first performed by imaging Tygon tubes of various diameters, and in vivo feasibility was demonstrated by imaging small vessels in the human thyroid. Simultaneous 3-D color and pulsed Doppler imaging using compounded emissions were also applied in the carotid artery and the jugular vein in one healthy volunteer.

Supersonic Shear Wave Imaging to Assess Arterial Nonlinear Behavior and Anisotropy: Proof of Principle via Ex Vivo Testing of the Horse Aorta

Supersonic shear wave imaging (SSI) is a noninvasive, ultrasound-based technique to quantify the mechanical properties of bulk tissues by measuring the propagation speed of shear waves (SW) induced in the tissue with an ultrasound transducer. The technique has been successfully validated in liver and breast (tumor) diagnostics and is potentially useful for the assessment of the stiffness of arteries. However, SW propagation in arteries is subjected to different wave phenomena potentially affecting the measurement accuracy. Therefore, we assessed SSI in a less complex ex vivo setup, that is, a thick-walled and rectangular slab of an excised equine aorta. Dynamic uniaxial mechanical testing was performed during the SSI measurements, to dispose of a reference material assessment. An ultrasound probe was fixed in an angle position controller with respect to the tissue to investigate the effect of arterial anisotropy on SSI results. Results indicated that SSI was able to pick up stretch-induced stiffening of the aorta. SW velocities were significantly higher along the specimens circumferential direction than in the axial direction, consistent with the circumferential orientation of collagen fibers. Hence, we established a first step in studying SW propagation in anisotropic tissues to gain more insight into the feasibility of SSI-based measurements in arteries.

A versatile and experimentally validated finite element model to assess the accuracy of shear wave elastography in a bounded viscoelastic medium

The feasibility of shear wave elastography (SWE) in arteries for cardiovascular risk assessment remains to be investigated as the arterys thin wall and intricate material properties induce complex shear wave (SW) propagation phenomena. To better understand the SW physics in bounded media, we proposed an in vitro validated finite element model capable of simulating SW propagation, with full flexibility at the level of the tissues geometry, material properties, and acoustic radiation force. This computer model was presented in a relatively basic set-up, a homogeneous slab of gelatin-agar material (4.35 mm thick), allowing validation of the numerical settings according to actual SWE measurements. The resulting tissue velocity waveforms and SW propagation speed matched well with the measurement: 4.46 m/s (simulation) versus 4.63 ± 0.07 m/s (experiment). Further, we identified the impact of geometrical and material parameters on the SW propagation characteristics. As expected, phantom thickness was a determining factor of dispersion. Adding viscoelasticity to the model augmented the estimated wave speed to 4.58 m/s, an even better match with the experimental determined value. This study demonstrated that finite element modeling can be a powerful tool to gain insight into SWE mechanics and will in future work be advanced to more clinically relevant settings.

Shear Wave Imaging of the heart using a cardiac phased array with coherent spatial compound

The concept of coherent compound for diverging waves is proposed to make Shear Wave Imaging at very high frame rate (up to 4000 images/sec) with a conventional cardiac phased array probe non invasively in a beating human heart. The first goal of this study was to demonstrate the improvement of the imaging performances based on spatial coherent compound with diverging wave. We show here that this technique allows tracking the shear waves with a good temporal resolution and a good signal to noise ratio which leads to a precise estimation of the local medium stiffness. The amelioration is quantified in gel and pig heart. Finally, the feasibility of this technique in vivo in a human heart is shown and shear wave velocity during diastole is estimated in the anterior wall of the left ventricle.

Imaging the dynamics of cardiac fiber orientation in vivo using 3D Ultrasound Backscatter Tensor Imaging

The assessment of myocardial fiber disarray is of major interest for the study of the progression of myocardial disease. However, time-resolved imaging of the myocardial structure remains unavailable in clinical practice. In this study, we introduce 3D Backscatter Tensor Imaging (3D-BTI), an entirely novel ultrasound-based imaging technique that can map the myocardial fibers orientation and its dynamics with a temporal resolution of 10 ms during a single cardiac cycle, non-invasively and in vivo in entire volumes. 3D-BTI is based on ultrafast volumetric ultrasound acquisitions, which are used to quantify the spatial coherence of backscattered echoes at each point of the volume. The capability of 3D-BTI to map the fibers orientation was evaluated in vitro in 5 myocardial samples. The helicoidal transmural variation of fiber angles was in good agreement with the one obtained by histological analysis. 3D-BTI was then performed to map the fiber orientation dynamics in vivo in the beating heart of an open-chest sheep at a volume rate of 90 volumes/s. Finally, the clinical feasibility of 3D-BTI was shown on a healthy volunteer. These initial results indicate that 3D-BTI could become a fully non-invasive technique to assess myocardial disarray at the bedside of patients.

Towards backscatter tensor imaging (BTI): Analysis of the spatial coherence of ultrasonic speckle in anisotropic soft tissues

The assessment of fiber architecture is of major interest in the progression of myocardial disease. Recent techniques such as MR Diffusion Tensor Imaging or Ultrasound Elastic Tensor Imaging (ETI) can derive the fiber directions by measuring the anisotropy of water diffusion or tissue lasticity, but these techniques present severe limitations in clinical setting. In this study, we measure the spatial coherence of ultrasonic speckle in skeletal muscles and myocardial tissues, in order to determine the fibers directions and compare it to ETI. Acquisitions were performed using a linear transducer array mounted on a rotation device to image ex vivo bovine skeletal muscle and porcine LV myocardial samples. At each angle, multiple plane waves were transmitted and the backscattered echoes recorded. The coherence factor was measured as the integral of the correlation function. In skeletal muscle, maximal/minimal coherence factor was found for the probe parallel/perpendicular to the fibers. In myocardium, the coherence was assessed across the entire myocardial thickness, and the position of maxima and minima varied transmurally due to the complex fibers distribution. The shear wave speed variation with the probe angle was found to follow the coherence variation. Spatial correlation can thus reveal the anisotropy of the ultrasonic speckle in skeletal muscle and myocardium. BTI could be used on any type of ultrasonic scanner for non invasive evaluation of myocardial fibers.

Supersonic shear wave imaging to assess arterial anisotropy: Ex-vivo testing of the horse aorta

Supersonic shear wave imaging (SSI) has recently emerged as a reliable technique for soft tissue characterization in bulk tissues (e.g. in the context of breast and liver cancer diagnostics). Another promising application of SSI is arterial stiffness assessment, though challenged by complex shear wave (SW) propagation phenomena in this thin-walled setting such as guided waves, dispersion, reflection and refraction on the arterial walls. Therefore, we investigated the sensitivity of SSI to (i) stretch-induced stiffening and (ii) the arterial fiber organization in a simpler ex-vivo arterial setup based on equine aortic tissue, where the SW propagation is deprived of dispersion and guided-wave effects. For this purpose, we conducted simultaneous dynamic mechanical testing of the tissue along with SSI measurements. The probe was rotated around its axis relative to the tissue to investigate whether SSI is able to determine the dominant collagen fiber direction in the tissue. The cyclic behavior of the SW velocities as a response to the dynamic mechanical testing demonstrated the ability of SSI to detect stretch-induced stiffening, though mainly in the circumferential direction. Furthermore, SW velocities were lower when the probe was positioned away from the circumferential direction of the tissue, which could be explained due to the uni-axial testing, the arterial anisotropy and the progressive recruitment of collagen fibers in the circumferential direction. The elasticity modulus assessed from the SSI measurements and the mechanical testing demonstrated the feasibility of SSI to detect the increase in E-modulus as expected from the measured stress-strain curve (factor 2.1 versus 2.3 increase for SSI and mechanical testing respectively). Future work will include performing histology on the investigated tissue to confirm these findings and clarify the link between SSI measurements and the actual fiber orientation.

4-D ultrafast ultrasound imaging: in vivo quantitative imaging of blood flows and tissue viscoelastic properties

Very high frame rate ultrasound imaging has enabled the development of novel imaging modalities such as the functional imaging of the brain, cardiac electrophysiology, and the quantitative real-time imaging of the intrinsic mechanical properties of tumors, to name a few. We present here the extension of Ultrafast Ultrasound Imaging in three dimensions based on the use of either diverging or plane waves emanating from a sparse virtual array located behind the probe. High contrast and resolution were achieved while maintaining imaging rates of thousands of volumes per second. A customized portable ultrasound system was developed to sample 1024 independent channels and to drive a 32X32 matrix-array probe. Its capability to track in 3D transient phenomena occurring in the millisecond range within a single ultrafast acquisition was demonstrated for 3-D Shear-Wave Imaging, 4-D Ultrafast Doppler Imaging, and 4D vector flow imaging. These results demonstrate the potential of 4-D Ultrafast Ultrasound Imaging for t...