Optimal focusing by spatio-temporal inverse filter. II. Experiments. Application to focusing through absorbing and reverberating media

To focus ultrasonic waves in an unknown heterogeneous medium using a phased array, one has to calculate the optimal set of signals to be applied on the transducers of the array. (In most applications of ultrasound, medical imaging, medical therapy, nondestructive testing, the first step consists of focusing a broadband ultrasound beam deeply inside the medium to be investigated.) Focusing in a homogeneous medium simply requires to compensate for the varying focus-array elements geometrical distances. Nevertheless, heterogeneities in the medium, in terms of speed of sound, density, or absorption, may strongly degrade the focusing. Different techniques have been developed in order to correct such aberrations induced by heterogeneous media (time reversal, speckle brightness, for example). In the companion to this paper, a new broadband focusing technique was investigated: the spatio-temporal inverse filter. Experimental results obtained in various media, such as reverberating and absorbing media, are presented here. In particular, intraplate echoes suppression and high-quality focusing through a human skull, as well as hyper-resolution in a reverberating medium, will be shown. It is important to notice that all these experiments were performed with fully programmable multichannel electronics whose use is required to fully exploit the spatio-temporal technique.     

Synthesis of a (+)-anatoxin-a analogue for monoclonal antibodies production

Anatoxin-a analogue was prepared by resolution of aminoacid 1 using chiral (R)-4-phenyl-oxazolidin-2-thione as derivatizing agent. X-ray diffraction of a diastereomer allowed us to determine its absolute configuration. The synthesis could then be completed in few steps followed by introduction of an amide linker bearing a terminal alkyne in order to attach a carrier protein for monoclonal antibodies production.     

Black foam films from aqueous solutions of a mixture of phospholipids and a permeation enhancer

The influence of a permeation enhancer on the properties of phospholipid black foam films has been studied through the combination of three complementary techniques: surface tension measurements, X-ray reflectivity, and the "diminishing bubble" method. This permeation enhancer is said to optimize the delivery of active ingredients into or through the stratum corneum: the 4-decyl oxazolidin-2-one. We made films of a complex phospholipid mixture that mimic the behavior of the enhancer in a membrane cell. Mixed phospholipids/4-decyl oxazolidin-2-one/NaCl solutions were studied with various 4-decyl oxazolidin-2-one concentrations. Stable black films were obtained and their thicknesses examined. The evolution of the coefficient of gas permeability with 4-decyl oxazolidin-2-one concentration is also addressed.     

Maturation of the [FeFe]-Hydrogenase: Direct Transfer of the (κ3-cysteinate)FeII(CN)(CO)2 Complex B from HydG to HydE

[FeFe]-hydrogenases efficiently catalyze the reversible oxidation of molecular hydrogen. Their prowess stems from the intricate H-cluster, combining a [Fe₄S₄] center with a binuclear iron center ([2Fe]_H). In the latter, each iron atom is coordinated by a CO and CN ligand, connected by a CO and an azadithiolate ligand. The synthesis of this active site involves a unique multiprotein assembly, featuring radical SAM proteins HydG and HydE. HydG initiates the transformation of L-tyrosine into cyanide and carbon monoxide to generate complex B, which is subsequently transferred to HydE to continue the biosynthesis of the [2Fe]_H-subcluster. Due to its instability, complex B isolation for structural or spectroscopic characterization has been elusive thus far. Nevertheless, the use of a biomimetic analogue of complex B allowed circumvention of the need for the HydG protein during in vitro functional investigations, implying a similar structure for complex B. Herein, we used the HydE protein as a nanocage to encapsulate and stabilize the complex B product generated by HydG. Using X-ray crystallography, we successfully determined its structure at 1.3 Å resolution. Furthermore, we demonstrated that complex B is directly transferred from HydG to HydE, thus not being released into the solution post-synthesis, highlighting a transient interaction between the two proteins.     

Solid-phase synthesis of alpha-Gal epitopes: on-resin analysis of solid-phase oligosaccharide synthesis with 19F NMR spectroscopy

A route for solid-phase synthesis of the alpha-Gal epitopes Gal(alpha1-3)Gal(beta1-4)Glc and Gal(alpha1-3)Gal(beta1-4)GlcNAc is described. These trisaccharide antigens are responsible for hyperacute rejection in xenotransplantation of porcine organs. Optimization of the solid-phase synthesis relied on use of fluorinated protective groups for the carbohydrate building blocks and use of a fluorinated linker. This allowed convenient on-resin analysis of the reactions with gel-phase (19)F NMR spectroscopy. Conditions were established which allowed reductive ring-opening of 4,6-O-benzylidene acetals to be performed on the solid phase with high regioselectivity to furnish the corresponding 6-O-benzyl ethers. It was found that glycosylations could be conveniently carried out by using thioglycosides as donors with N-iodosuccinimide and trifluoromethanesulfonic acid as the promoter system. With use of these conditions a challenging alpha-glycosidic linkage was successfully installed with complete stereoselectivity in the final glycosylation. It was also established that fluorinated benzoates, benzyl ethers, and benzylidene acetals display almost identical chemical properties as their nonfluorinated counterparts, a finding that is essential for future use of fluorinated protective groups in solid-phase oligosaccharide synthesis.     

Rotational intermittency and turbulence induced lift experienced by large particles in a turbulent flow

The motion of a large, neutrally buoyant, particle freely advected by a turbulent flow is determined experimentally. We demonstrate that both the translational and angular accelerations exhibit very wide probability distributions, a manifestation of intermittency. The orientation of the angular velocity with respect to the trajectory, as well as the translational acceleration conditioned on the spinning velocity, provides evidence of a lift force acting on the particle.     

Sensitivity of school-aged children to pitch-related cues

Two experiments investigated the ability of 17 school-aged children to process purely temporal and spectro-temporal cues that signal changes in pitch. Percentage correct was measured for the discrimination of sinusoidal amplitude modulation rate (AMR) of broadband noise in experiment 1 and for the discrimination of fundamental frequency (F0) of broadband sine-phase harmonic complexes in experiment 2. The reference AMR was 100 Hz as was the reference F0. A child-friendly interface helped listeners to remain attentive to the task. Data were fitted using a maximum-likelihood technique that extracted threshold, slope, and lapse rate. All thresholds were subsequently standardized to a common d' value equal to 0.77. There were relatively large individual differences across listeners: eight had relatively adult-like thresholds in both tasks and nine had higher thresholds. However, these individual differences did not vary systematically with age, over the span of 6-16 yr. Thresholds were correlated across the two tasks and were about nine times finer for F0 discrimination than for AMR discrimination as has been previously observed in adults.     

Acoustoelasticity in soft solids: assessment of the nonlinear shear modulus with the acoustic radiation force

The assessment of viscoelastic properties of soft tissues is enjoying a growing interest in the field of medical imaging as pathologies are often correlated with a local change of stiffness. To date, advanced techniques in that field have been concentrating on the estimation of the second order elastic modulus (mu). In this paper, the nonlinear behavior of quasi-incompressible soft solids is investigated using the supersonic shear imaging technique based on the remote generation of polarized plane shear waves in tissues induced by the acoustic radiation force. Applying a theoretical approach of the strain energy in soft solid [Hamilton et al., J. Acoust. Soc. Am. 116, 41-44 (2004)], it is shown that the well-known acoustoelasticity experiment allowing the recovery of higher order elastic moduli can be greatly simplified. Experimentally, it requires measurements of the local speed of polarized plane shear waves in a statically and uniaxially stressed isotropic medium. These shear wave speed estimates are obtained by imaging the shear wave propagation in soft media with an ultrafast echographic scanner. In this situation, the uniaxial static stress induces anisotropy due to the nonlinear effects and results in a change of shear wave speed. Then the third order elastic modulus (A) is measured in agar-gelatin-based phantoms and polyvinyl alcohol based phantoms.     

Revisiting iterative time reversal processing: application to detection of multiple targets

The iterative time reversal processing represents a high speed and easy way to self-focus on the strongest scatterer in a multitarget medium. However, finding weaker scatterers is a more difficult task that can be solved by computing the eigenvalue and eigenvector decomposition of the time reversal operator, the so-called DORT method. Nevertheless, as it requires the measurement of the complete interelements response matrix and time-consuming computation, the separation of multiple targets may not be achieved in real time. In this study, a new real time technique is proposed for multitarget selective focusing that does not require the experimental acquisition of the time reversal operator. This technique achieves the operator decomposition using a particular sequence of filtered waves propagation instead of computational power. Due to its simplicity of implementation, this iterative process can be achieved in real time. This high speed selective focusing is experimentally demonstrated by detecting targets through a heterogeneous medium and in a speckle environment. A theoretical analysis compares this technique to the DORT formalism.