[Show abstract][Hide abstract] ABSTRACT: Multiple scattering of waves in disordered media is a nightmare whether it be
for detection or imaging purposes. The best approach so far to get rid of
multiple scattering is optical coherence tomography. It basically combines
confocal microscopy and coherence time-gating to discriminate ballistic photons
from a predominant multiple scattering background. Nevertheless, the imaging
depth range remains limited to 1 mm at best in human soft tissues. Here we
propose a matrix approach of optical imaging to push back this fundamental
limit. By combining a matrix discrimination of ballistic waves and iterative
time-reversal, we show both theoretically and experimentally an extension of
the imaging-depth limit by at least a factor two compared to optical coherence
tomography. In particular, the reported experiment demonstrates imaging through
a strongly scattering layer from which only one reflected photon over 1000
billion is ballistic. This approach opens a new route towards ultra-deep tissue
[Show abstract][Hide abstract] ABSTRACT: Metamaterials, man-made composite media structured on a scale much smaller than a wavelength, offer surprising possibilities for engineering the propagation of waves. One of the most interesting of these is the ability to achieve superlensing--that is, to focus or image beyond the diffraction limit. This originates from the left-handed behavior--the property of refracting waves negatively--that is typical of negative index metamaterials. Yet reaching this goal requires the design of 'double negative' metamaterials, which act simultaneously on the permittivity and permeability in electromagnetics, or on the density and compressibility in acoustics; this generally implies the use of two different kinds of building blocks or specific particles presenting multiple overlapping resonances. Such a requirement limits the applicability of double negative metamaterials, and has, for example, hampered any demonstration of subwavelength focusing using left-handed acoustic metamaterials. Here we show that these strict conditions can be largely relaxed by relying on media that consist of only one type of single resonant unit cell. Specifically, we show with a simple yet general semi-analytical model that judiciously breaking the symmetry of a single negative metamaterial is sufficient to turn it into a double negative one. We then demonstrate that this occurs solely because of multiple scattering of waves off the metamaterial resonant elements, a phenomenon often disregarded in these media owing to their subwavelength patterning. We apply our approach to acoustics and verify through numerical simulations that it allows the realization of negative index acoustic metamaterials based on Helmholtz resonators only. Finally, we demonstrate the operation of a negative index acoustic superlens, achieving subwavelength focusing and imaging with spot width and resolution 7 and 3.5 times better than the diffraction limit, respectively. Our findings have profound implications for the physics of metamaterials, highlighting the role of their subwavelength crystalline structure, and hence entering the realm of metamaterial crystals. This widens the scope of possibilities for designing composite media with novel properties in a much simpler way than has been possible so far.
[Show abstract][Hide abstract] ABSTRACT: Arterial stiffness is related to age and collagen properties of the arterial wall and can be indirectly evaluated by the pulse wave velocity (PWV). Ultrafast ultrasound imaging, a unique ultrahigh frame rate technique (>10 000 images/s), recently emerged enabling direct measurement of carotid PWV and its variation over the cardiac cycle. Our goal was to characterize the carotid diastolic-systolic arterial stiffening using ultrafast ultrasound imaging in healthy individuals and in vascular Ehlers-Danlos syndrome (vEDS), in which collagen type III is defectuous.
Ultrafast ultrasound imaging was performed on common carotids of 102 healthy individuals and 37 consecutive patients with vEDS. Results are mean ± standard deviation.
Carotid ultrafast ultrasound imaging PWV in healthy individuals was 5.6 ± 1.2 in early systole and 7.3 ± 2.0 m/s in end systole, and correlated with age (r = 0.48; P < 0.0001 and r = 0.68; P < 0.0001, respectively). Difference between early and end-systole PWV increased with age independently of blood pressure (r = 0.54; P < 0.0001). In patients with vEDS, ultrafast ultrasound imaging PWV was 6.0 ± 1.5 in early systole and 6.7 ± 1.5 m/s in end systole. Carotid stiffness change over the cardiac cycle was lower than in healthy people (0.021 vs. 0.057 m/s per mmHg; P = 0.0035).
Ultrafast ultrasound imaging can evaluate carotid PWV and its variation over the cardiac cycle. This allowed to demonstrate the age-induced increase of the arterial diastolic-systolic stiffening in healthy people and a lower stiffening in vEDS, both characterized by arterial complications. We believe that this easy-to-use technique could offer the opportunity to go beyond the diastolic PWV to better characterize arterial stiffness change with age or other collagen alterations.
No preview · Article · Sep 2015 · Journal of Hypertension
[Show abstract][Hide abstract] ABSTRACT: Electromagnetic cavities are used in numerous domains of applied and fundamental physics, from microwave ovens and electromagnetic compatibility to masers, quantum electrodynamics (QED), and quantum chaos. The wave fields established in cavities are statically fixed by their geometry, which are usually modified by using mechanical parts like mode stirrers in reverberation chambers or screws in masers and QED. Nevertheless, thanks to integral theorems, tailoring the cavity boundaries theoretically permits us to design at will the wave fields they support. Here, we show in the microwave domain that it is achievable dynamically simply by using electronically tunable metasurfaces that locally modify the boundaries, switching them in real time from Dirichlet to Neumann conditions. We prove that at a high modal density, counterintuitively, it permits us to create wave patterns presenting hot spots of intense energy. We explain and model the physical mechanism underlying the concept, which allows us to find a criterion ensuring that modifying parts of a cavity’s boundaries turn it into a completely different one. We finally prove that this approach even permits us, in the limiting case where the cavity supports only well-separated resonances, to choose the frequencies at which the latter occur.
[Show abstract][Hide abstract] ABSTRACT: Thanks to a Multiple Scattering Theory algorithm, we present a way to focus
energy at the deep subwavelength scale, from the far-field, inside a cubic
disordered bubble cloud by using broadband Time Reversal (TR). We show that the
analytical calculation of an effective wavenumber performing the Independant
Scattering Approximation (ISA) matches the numerical results for the focal
extension. Subwavelength focusings of lambda/100 are reported for simulations
with perfect bubbles (no loss). A more realistic case, with viscous and thermal
losses, allows us to obtain a $\lambda/14$ focal spot, with a low volume
fraction of scatterers (phi = 0.01). Bubbly materials could open new
perspective for acoustic actuation in the microfluidic context.
No preview · Article · Jun 2015 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: In the field of shear wave
elastography, a specific technique called Supersonic Shear Imaging (SSI) was developed since almost 15 years. This technique is based on two concepts: By means of the acoustic radiation pressure phenomena shear waves are generated directly within tissues. Then, shear wave propagation is caught in real time by using an ultrafast ultrasound device (up to 20,000 frames/s). As shear wave speed is directly related to stiffness of tissues, such a concept allows to recover elastic maps of organs. Nevertheless, stiffness is not always only sufficient to better understand organs
pathologies and behaviors. So, there is a need to add new parameters for a better characterization. In this context, SSI technique can be extended in order to reach new mechanical parameters, which can potentially help physicians. By looking at the shear wave dispersion, viscosity of tissues can be retrieved by using the right rheological model. Elastic anisotropy is recovered by rotating the probe at the surface of the investigated organ. For each position, the shear wave speed is calculated allowing to deduce orientation of fibers. At last, the change in tissue stiffness as a function of the pressure applied over medium, also called acoustoelasticity theory, allows the assessment of the nonlinear elastic properties. The combination of all these new parameters, viscosity, anisotropy, and nonlinearity, with stiffness offer new possibilities of diagnosis for physicians to better understand organs
No preview · Article · Apr 2015 · The Journal of the Acoustical Society of America
[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.
Full-text · Article · Jan 2015 · Physical Review Letters
[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.
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.
All the results showed a local anisotropy with one main direction of maximal stiffness. The influence of pulsatility was observed in vivo, and electrocardiogram (ECG) gating was consequently performed for all anesthetized swine. On ex vivo corneas, n = 27 acquisitions were performed in the limbus region, where the collagen fibrils are reported to run tangentially to the sclera. A good match was shown between the direction of maximal stiffness and the expected direction of the collagen fibrils.
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.
Full-text · Article · Oct 2014 · Scientific Reports
[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.
Full-text · Article · Sep 2014 · Physics in Medicine and Biology
[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.