[Show abstract][Hide abstract] ABSTRACT: We report on the passive measurement of time-dependent Green's functions in
the optical frequency domain with low-coherence interferometry. Inspired by
previous studies in acoustics and seismology, we show how the correlations of a
broadband and incoherent wave-field can directly yield the Green's functions
between scatterers of a complex medium. Both the ballistic and multiple
scattering components of the Green's function are retrieved. This approach
opens important perspectives for optical imaging and characterization in
complex scattering media.
[Show abstract][Hide abstract] ABSTRACT: This paper presents a detailed analysis of time-reversal experiments involving a moving point source that emits a pulse. Different configurations are addressed with full-aperture or partial-aperture time-reversal mirrors and with subsonic or supersonic sources. Doppler effects and lack of source-receiver reciprocity significantly affect the time-reversal refocusing when the velocity of the source becomes comparable as or larger than the speed of propagation. The main result is that refocusing can be enhanced when the velocity of the source becomes close to the speed of propagation compared to the classical diffraction-limited refocusing properties when the source does not move, and this super-resolution effect can be quantified by simple and explicit formulas.
[Show abstract][Hide abstract] ABSTRACT: Purpose: This work proposes shear wave elastography to quantify the elastic anisotropy of the cornea. Methods: Experiments were conducted on enucleated porcine eyeballs and anesthetized swine. We used the Supersonic Shear wave Imaging (SSI) method implemented on a dedicated 15 MHz rotating linear ultrasound array. This setup allows determining the shear wave speed variations for a set of radial propagation directions. Results: All the results showed a local anisotropy with one main direction of maximal stiffness. The influence of pulsatility was observed in vivo and ECG gating was consequently performed for all anesthetized swine. On ex vivo corneas, we performed N = 27 acquisitions in the limbus region, where the collagen fibrils are reported to run tangentially to the sclera. It was shown a good match between the direction of maximal stiffness and the expected direction of the collagen fibrils. Conclusions: This preliminary study demonstrates the potential of SSI for the assessment of corneal anisotropy in both ex vivo and in vivo conditions.
[Show abstract][Hide abstract] ABSTRACT: In this article we propose to use electronically tunable metasurfaces as spatial microwave modulators. We demonstrate that like spatial light modulators, which have been recently proved to be ideal tools for controlling light propagation through multiple scattering media, spatial microwave modulators can efficiently shape in a passive way complex existing microwave fields in reverberating environments with a non-coherent energy feedback. Unlike in free space, we establish that a binary-only phase state tunable metasurface allows a very good control over the waves, owing to the random nature of the electromagnetic fields in these complex media. We prove in an everyday reverberating medium, that is, a typical office room, that a small spatial microwave modulator placed on the walls can passively increase the wireless transmission between two antennas by an order of magnitude, or on the contrary completely cancel it. Interestingly and contrary to free space, we show that this results in an isotropic shaped microwave field around the receiving antenna, which we attribute again to the reverberant nature of the propagation medium. We expect that spatial microwave modulators will be interesting tools for fundamental physics and will have applications in the field of wireless communications.
[Show abstract][Hide abstract] ABSTRACT: We present an approach for two-dimensional (2D) imaging through a single
single-mode or multimode fiber without the need for scanners. A random
scattering medium placed next to the distal end of the fiber is used to encode
the collected light from every input spatial position with a different random
spectral signature. We demonstrate 2D imaging of objects illuminated by a
white-light fiber-coupled LED from a single measured spectrum. The technique is
insensitive to fiber bending, an advantage for endoscopic applications.
[Show abstract][Hide abstract] ABSTRACT: 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.
Physics in Medicine and Biology 09/2014; 59(19):L1. · 2.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In the manufacturing of cheese, the cutting of the curd is an essential step which depends on the firmness of the curds and significantly affects the yield of the cheese and its quality. In this work, we present a technique to measure elastic properties of the curd during coagulation that could be used to quantitatively determine the cutting time. The technique uses ultrasound to generate and measure shear waves. These waves do not propagate in liquids and their velocity of propagation depends on the viscoelastic characteristics of the medium. Hence, they can be used to identify the beginning of coagulation and subsequently to monitor the evolution of the coagulum firmness. Our results showed this technique is sensitive to changes of the medium structure during coagulation. It also proved reproducible and sensitive to different coagulation conditions. Therefore this technique can be used to develop a system suitable for the dairy industry.
Journal of Food Engineering 09/2014; 136:73–79. · 2.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The recent concept of metasurfaces is a powerful tool to shape waves by governing precisely the phase response of each constituting element through its resonance properties. While most efforts are devoted to realize reconfigurable metasurfaces that allow such complete phase control, for many applications a binary one is sufficient. Here, we propose and demonstrate through experiments and simulations a binary state tunable phase reflector based on the concept of hybridized resonators as unit cell for a possible metasurface. The concept presents the great advantages to be very general, scalable to all frequency domains and above all very robust to fluctuations induced by the tunable mechanism, as we prove it at microwave frequencies using electronically tunable patch reflectors.
[Show abstract][Hide abstract] ABSTRACT: Although conventional pulse-wave Doppler has proved to be a valuable diagnostic method for many vascular pathologies, it is hampered by issues related to repeatability as well as problems associated with quantification and system-dependent variability. These limitations are due to intrinsic spectral broadening on the Doppler spectrum, resulting from the directivity pattern of the ultrasound focused beam. Here, we develop a new spatial statistical technique, Doppler frequency spatial analysis (DFSA), which is based on ultrafast plane-wave imaging. Similar to standard pulse-wave Doppler, which is commonly used by sonographers, it yields a two-dimensional output (frequency versus time), while dramatically reducing the presence of intrinsic spectral broadening on the Doppler spectra. Therefore, the technique is much more sensitive to the velocity profile and turbulences than the standard pulse-wave Doppler. The proposed technique could improve diagnosis of vascular diseases, including arterial plaque characterization. Moreover, by summarizing all main information contained in the ultrafast Doppler acquisition, it permits a direct visualization of the data within the velocity profile. Here, we have compared our novel statistical technique to the standard pulse-wave Doppler approach during in vivo imaging of the human carotid artery. Notably, we achieved a greater than 4-fold reduction in intrinsic spectral broadening.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 08/2014; 61(8):1396-1408. · 1.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The assessment of fiber architecture is of major interest in the progression of myocardial disease. Recent techniques such as magnetic resonance diffusion tensor imaging (MR-DTI) or ultrasound elastic tensor imaging (ETI) can derive the fiber directions by measuring the anisotropy of water diffusion or tissue elasticity, but these techniques present severe limitations in a clinical setting. In this study, we propose a new technique, backscatter tensor imaging (BTI), which enables determination of the fiber directions in skeletal muscles and myocardial tissues, by measuring the spatial coherence of ultrasonic speckle. We compare the results to ultrasound ETI. Acquisitions were performed using a linear transducer array connected to an ultrasonic scanner mounted on a motorized rotation device with angles from 0?? to 355?? by 5?? increments to image ex vivo bovine skeletal muscle and porcine left ventricular myocardial samples. At each angle, multiple plane waves were transmitted and the backscattered echoes recorded. The coherence factor was measured as the ratio of coherent intensity over incoherent intensity of backscattered echoes. 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 because of the complex fibers distribution. In ETI, the shear wave speed variation with the probe angle was found to follow the coherence variation. Spatial coherence can thus reveal the anisotropy of the ultrasonic speckle in skeletal muscle and myocardium. BTI could be used on any type of ultrasonic scanner with rotating phased-array probes or 2-D matrix probes for noninvasive evaluation of myocardial fibers.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 06/2014; 61(6):986-996. · 1.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Imaging the electrical activity of the body is central to the diagnosis of and treatment planning for some of our most pressing healthcare challenges, including heart and brain diseases. The acoustoelectric effect has recently been shown to provide contrast directly from current densities by measuring ultrasound-modulated electrical impedance in the heart. While promising, these approaches, based on focused emission at low frequency, result in limited signal-to-noise ratios (SNR), and temporal and spatial resolutions. In this study, we developed Ultrafast Acoustoelectric Tomography (UAT), based on plane wave emissions, which provides high frame rates and uniformly high spatial and temporal resolutions. We developed a novel reconstruction approach for UAT and demonstrated its feasibility in phantom experiments at current density levels similar to the ones occurring naturally in vivo, indicating that UAT could become a unique technique to map current density distributions in tissues and image their propagation at very high frame rates.
2014 IEEE 11th International Symposium on Biomedical Imaging (ISBI 2014); 04/2014
[Show abstract][Hide abstract] ABSTRACT: In vivo mapping of the full vasculature dynamics based on Ultrafast Doppler is showed noninvasively in the challenging case of the neonatal brain. Contrary to conventional pulsed-wave (PW) Doppler Ultrasound limited for >40 years to the estimation of vascular indices at a single location, the ultrafast frame rate (5,000 Hz) obtained using plane-wave transmissions leads to simultaneous estimation of full Doppler spectra in all pixels of wide field-of-view images within a single cardiac cycle and high sensitivity Doppler imaging. Consequently, 2D quantitative maps of the cerebro-vascular resistivity index (RI) are processed and found in agreement with local measurements obtained on large arteries of healthy neonates using conventional PW Doppler. Changes in 2D resistivity maps are monitored during recovery after therapeutic whole-body cooling of full-term neonates treated for hypoxic ischemic encephalopathy. Arterial and venous vessels are unambiguously differentiated on the basis of their distinct hemodynamics. The high spatial (250 × 250 μm(2)) and temporal resolution (<1 ms) of Ultrafast Doppler imaging combined with deep tissue penetration enable precise quantitative mapping of deep brain vascular dynamics and RI, which is far beyond the capabilities of any other imaging modality.Journal of Cerebral Blood Flow & Metabolism advance online publication, 26 March 2014; doi:10.1038/jcbfm.2014.49.
Journal of cerebral blood flow and metabolism: official journal of the International Society of Cerebral Blood Flow and Metabolism 03/2014; · 5.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Imaging with optical resolution through and inside complex samples is a
difficult challenge with important applications in many fields. The fundamental
problem is that inhomogeneous samples, such as biological tissues, randomly
scatter and diffuse light, impeding conventional image formation. Despite many
advancements, no current method enables to noninvasively image in real-time
using diffused light. Here, we show that owing to the memory-effect for speckle
correlations, a single image of the scattered light, captured with a standard
high-resolution camera, encodes all the information that is required to image
through the medium or around a corner. We experimentally demonstrate
single-shot imaging through scattering media and around corners using
incoherent light and various samples, from white paint to dynamic biological
samples. Our lensless technique is simple, does not require laser sources,
wavefront-shaping, nor time-gated detection, and is realized here using a
camera-phone. It has the potential to enable imaging in currently inaccessible
[Show abstract][Hide abstract] ABSTRACT: The optical diffraction limit has stood for a long time in the way of achieving higher optical resolution in far-field imaging, photolithography, and optical data storage. We present here a simple and original concept for broadband far-field imaging in the visible and ultraviolet range that beats this limit. A finite-sized ultrathin metallic slab is used to encode subwavelength details of the broadband field radiated by an object. This field excites a set of surface plasmon modes on the finite-sized slab that radiates in the far field. A numerical time reversal imaging process is applied to reconstruct the image of the object with a resolution smaller than λ/6 for a gold slab and λ/8 for a silver slab. With these structures, the highest spatial frequencies are no longer limited by the pitch of the array of the subwavelength resonators as in classical metalenses. We show that the resolution depends mainly on the intrinsic properties of the metal but can be slightly controlled by the geometry design of the slab. Thanks to advances in the control of light in space and time, this concept would provide a promising alternative for high-resolution imaging techniques in the visible and ultraviolet range.
Physical Review B 02/2014; 89(11). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 02/2014; 61(2):288-301. · 1.80 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this Letter we propose to use subwavelength diffraction gratings as very good semitransparent mirrors for electromagnetic waves to design open cavities. To do so, we replace part of the walls of a cavity by such a grating. We numerically and analytically link the grating characteristics to the spectral properties of the realized open cavity. Then we demonstrate that the eigenmodes of the cavity can be transmitted perfectly through the grating to the exterior, thereby turning a point source inside the cavity into a very directive source. We investigate the effect of disorder, which leads to isotropic radiation patterns, and perform experiments in the microwave domain in order to support our claims. Finally, we present an example of application of the concept in fundamental physics, by measuring from outside the eigenmodes of a disordered microwave cavity.
[Show abstract][Hide abstract] ABSTRACT: Although the use of ultrasonic plane-wave transmissions rather than line-per-line focused beam transmissions has been long studied in research, clinical application of this technology was only recently made possible through developments in graphical processing unit (GPU)-based platforms. Far beyond a technological breakthrough, the use of plane or diverging wave transmissions enables attainment of ultrafast frame rates (typically faster than 1000 frames per second) over a large field of view. This concept has also inspired the emergence of completely novel imaging modes which are valuable for ultrasound-based screening, diagnosis, and therapeutic monitoring. In this review article, we present the basic principles and implementation of ultrafast imaging. In particular, present and future applications of ultrafast imaging in biomedical ultrasound are illustrated and discussed.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 01/2014; 61(1):102-119. · 1.80 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Elasticity maps of tissue have proved to be particularly useful in providing complementary contrast to ultrasonic imaging, e.g., for cancer diagnosis at the millimeter scale. Optical coherence tomography (OCT) offers an endogenous contrast based on singly backscattered optical waves. Adding complementary contrast to OCT images by recording elasticity maps could also be valuable in improving OCT-based diagnosis at the microscopic scale. Static elastography has been successfully coupled with full-field OCT (FF-OCT) in order to realize both micrometer-scale sectioning and elasticity maps. Nevertheless, static elastography presents a number of drawbacks, mainly when stiffness quantification is required. Here, we describe the combination of two methods: transient elastography, based on speed measurements of shear waves induced by ultrasonic radiation forces, and FF-OCT, an en face OCT approach using an incoherent light source. The use of an ultrafast ultrasonic scanner and an ultrafast camera working at 10,000 to 30,000 images/s made it possible to follow shear wave propagation with both modalities. As expected, FF-OCT is found to be much more sensitive than ultrafast ultrasound to tiny shear vibrations (a few nanometers and micrometers, respectively). Stiffness assessed in gel phantoms and an ex vivo rat brain by FF-OCT is found to be in good agreement with ultrasound shear wave elastography.
Journal of Biomedical Optics 12/2013; 18(12):121514. · 2.75 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this article, we investigate composite media which present both a local resonance and a periodic structure. We numerically and experimentally consider the case of a very academic and simplified system that is a quasi-one dimensional split ring resonator medium. We modify its periodicity to shift the position of the Bragg bandgap relative to the local resonance one. We observe that for a well-chosen lattice constant, the local resonance frequency matches the Bragg frequency thus opening a single bandgap which is at the same time very wide and strongly attenuating. We explain this interesting phenomenon by the dispersive nature of the unit cell of the medium, using an analogy with the concept of white light cavities. Our results provide new ways to design wide and efficient bandgap materials.