Keyhole acceleration for magnetic resonance acoustic radiation force imaging (MR ARFI)

MR ARFI measures the displacement induced by the ultrasonic radiation force and provides the location of the focal spot without significant heating effects. Displacements maps obtained with MR ARFI provide an indirect estimation of the acoustic beam intensity at the target. This measure is essential for dose estimation prior to focused ultrasound treatments (FUS) and adaptive focusing procedures of MR-guided transcranial and transribs FUS. In the latter case, the beam correction is achieved by maximizing the displacement at focus. A significant number of serial MR ARFI images are required and thus, a partial k-space updating method, such as keyhole appears as a method of choice. The purpose of this work is to demonstrate via simulations and experiments the efficiency of the keyhole technique combined with a two-dimensional spin-echo MR ARFI pulse sequence. The method was implemented in an ex vivo calf brain taking advantage of the a priori knowledge of the focal spot profile. The coincidence of the phase-encoding axis with the longest axis of the focal spot makes the best use of the technique. Our approach rapidly provides the focal spot localization with accuracy, and with a substantial increase to the signal-to-noise ratio, while reducing ultrasound energy needed during MR-guided adaptive focusing procedures.     

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.     

Limits of Near‐Coloring of Sparse Graphs

Let be nonnegative integers. A graph G is ‐colorable if its vertex set can be partitioned into sets such that the graph induced by has maximum degree at most d for , while the graph induced by is an edgeless graph for . In this article, we give two real‐valued functions and such that any graph with maximum average degree at most is ‐colorable, and there exist non‐‐colorable graphs with average degree at most . Both these functions converge (from below) to when d tends to infinity. This implies that allowing a color to be d‐improper (i.e., of type ) even for a large degree d increases the maximum average degree that guarantees the existence of a valid coloring only by 1. Using a color of type (even with a very large degree d) is somehow less powerful than using two colors of type (two stable sets).     

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.     

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.     

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.     

Experimental demonstration of noninvasive transskull adaptive focusing based on prior computed tomography scans

Developing minimally invasive brain surgery by high-intensity focused ultrasound beams is of great interest in cancer therapy. However, the skull induces strong aberrations both in phase and amplitude, resulting in a severe degradation of the beam shape. Thus, an efficient brain tumor therapy would require an adaptive focusing, taking into account the effects of the skull. In this paper, we will show that the acoustic properties of the skull can be deduced from high resolution CT scans and used to achieve a noninvasive adaptive focusing. Simulations have been performed with a full 3-D finite differences code, taking into account all the heterogeneities inside the skull. The set of signals to be emitted in order to focus through the skull can thus be computed. The complete adaptive focusing procedure based on prior CT scans has been experimentally validated. This could have promising applications in brain tumor hyperthermia but also in transcranial ultrasonic imaging.     

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.     

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.