Parallel beamforming using synthetic transmit beams

Parallel beamforming is frequently used to increase the acquisition rate of medical ultrasound imaging. However, such imaging systems will not be spatially shift invariant due to significant variation across adjacent beams. This paper investigates a few methods of parallel beam-forming that aims at eliminating this flaw and restoring the shift invariance property. The beam-to-beam variations occur because the transmit and receive beams are not aligned. The underlying idea of the main method presented here is to generate additional synthetic transmit beams (STB) through interpolation of the received, unfocused signal at each array element prior to beamforming. Now each of the parallel receive beams can be aligned perfectly with a transmit beam--synthetic or real--thus eliminating the distortion caused by misalignment. The proposed method was compared to the other compensation methods through a simulation study based on the ultrasound simulation software Field II. The results have been verified with in vitro experiments. The simulations were done with parameters similar to a standard cardiac examination with two parallel receive beams and a transmit-line spacing corresponding to the Rayleigh criterion, wavelength times f-number (lambda x f#). From the results presented, it is clear that straightforward parallel beamforming reduces the spatial shift invariance property of an ultrasound imaging system. The proposed method of using synthetic transmit beams seems to restore this important property, enabling higher acquisition rates without loss of image quality.     

Third harmonic transmit phasing for tissue harmonic generation

Generation of tissue harmonic signals during acoustic propagation is based on the combined effect of two different spectral interactions of the transmit signal. One produces harmonic whose frequency is the sum of transmit frequencies. The other results in harmonic at difference frequency of the transmit signals. Both the frequency-sum component and the frequency-difference component are sensitive to the phase of their constitutive spectral signals. In this study, a novel approach for modifying the amplitude of tissue harmonic signal is proposed based on phasing these two components to achieve either harmonic enhancement or suppression. Both experiments and simulations were performed to investigate the effects of 3f0 transmit phasing on tissue harmonic generation. Results indicate that the relative phasing between the frequency-sum component and the frequency-difference component markedly changes the amplitude of the second harmonic signal. For harmonic enhancement, approximate 6 dB increase of second harmonic amplitude can be achieved while the lateral harmonic beam pattern also is improved as compared to conventional situations in which only the frequency-sum component is considered. For harmonic suppression, the second harmonic signal also could be significantly reduced by about 11 dB when the frequency-difference component is out of phase with the frequency-sum component. Hence, the method of 3f0 transmit phasing has potentials for both improving signal-to-noise ratio in tissue harmonic imaging and enhancing image contrast in contrast-agent imaging by suppression of tissue harmonic background.     

Directional velocity estimation using a spatio-temporal encoding technique based on frequency division for synthetic transmit aperture ultrasound

This paper investigates the possibility of flow estimation using spatio-temporal encoding of the transmissions in synthetic transmit aperture imaging (STA). The spatial encoding is based on a frequency division approach. In STA, a major disadvantage is that only a single transmitter (denoting single transducer element or a virtual source) is used in every transmission. The transmitted acoustic energy will be low compared to a conventional focused transmission in which a large part of the aperture is used. By using several transmitters simultaneously, the total transmitted energy can be increased. However, to focus the data properly, the signals originating from the different transmitters must be separated. To do so, the pass band of the transducer is divided into a number of subbands with disjoint spectral support. At every transmission, each transmitter is assigned one of the subbands. In receive, the signals are separated using a simple filtering operation. To attain high axial resolution, broadband spectra must be synthesized for each of the transmitters. By multiplexing the different waveforms on different transmitters over a number of transmissions, this can be accomplished. To further increase the transmitted energy, the waveforms are designed as linear frequency modulated signals. Therefore, the full excitation amplitude can be used during most of the transmission. The method has been evaluated for blood velocity estimation for several different velocities and incident angles. The program Field II was used. A 128-element transducer with a center frequency of 7 MHz was simulated. The 64 transmitting elements were used as the transmitting aperture and 128 elements were used as the receiving aperture. Four virtual sources were created in every transmission. By beamforming lines in the flow direction, directional data were extracted and correlated. Hereby, the velocity of the blood was estimated. The pulse repetition frequency was 16 kHz. Three different setups were investigated with flow angles of 45, 60, and 75 degrees with respect to the acoustic axis. Four different velocities were simulated for each angle at 0.10, 0.25, 0.50, and 1.00 m/s. The mean relative bias with respect to the peak flow for the three angles was less than 2%, 2%, and 4%, respectively.     

Channel correlation of transmit diversity FSO systems with partially coherent optical beams

Spatial diversity, including transmit diversity, has been proven to be a promising technique to mitigate turbulence-induced signal fading. However, due to the limitation of size in compact FSO (free-space optical) terminals, the transmit antennas cannot always be separated far enough from each other, resulting in increased channel correlation and deteriorated diversity performance. In this paper, we focus on the channel correlation statistics of transmit diversity FSO systems with two transmit beams. Specifically, we consider partially coherent optical beams, which reduce scintillation over fully coherent optical beams. We perform theoretical analysis and obtain some interesting numerical results, such as the degree of channel correlation which changed mainly with source correlation radius from 0.001 to 0.1 m. These results can be helpful for designing transmit diversity FSO systems with partially coherent beams.     

Producing bowtie limited diffraction beams with synthetic arrayexperiment

Limited diffraction beams have a large depth of field and could have applications in medical ultrasound and other wave related areas such as electromagnetics and optics. However, these beams have higher sidelobes than conventional focused beams at their focuses. Recently, a new type of beam, called bowtie limited diffraction beams, was developed. These beams can achieve both low sidelobes and a large depth of field in medical imaging. In this paper, the production of bowtie beams in water with a synthetic array experiment is reported. A broad-band PZT ceramic/polymer composite transducer of about 1 mm diameter and 2.5 MHz central frequency was scanned in a raster format and placed at the centers of elements of an equivalent two-dimensional array of 50 mm diameter aperture. A polyvinylidene fluoride (PVDF) needle hydrophone of 0.5 mm diameter was used to receive the waves produced by the transducer. Proper weighting functions were applied to the received signals to produce various beams. Results show that the bowtie beams produced with the synthetic array experiment are in good agreement with those derived from theory and obtained by computer simulations. The depth of field of these beams is about 216 mm and sidelobes of a tenth derivative bowtie X wave in pulse-echo imaging are about 30 dB lower than those of rotary symmetric limited diffraction beams such as the zeroth-order X wave discovered previously     

Single-pulse, two-line temperature-measurement technique using KrF laser-induced O(2) fluorescence

A new single-pulse, two-line laser-induced O(2) fluorescence (LIF) temperature-measurement technique was demonstrated. The fluorescence spectrum obtained with multichannel detection following simultaneous excitation of two coincident transitions in the 0-6 and the 2-7 bands of the B(3)Σ(-)(u)-X(3)Σ(-)(g) Schumann-Runge system was used to determine the gas temperature. The rms error of 100-pulse average LIF temperature measurements, referenced to their corresponding thermocouple measurements, was 1.3% over a temperature range of 1300-1800 K in atmospheric air. Photon shot noise was found to be the primary source of uncertainty for these measurements in a quiescent environment. Single-pulse temperature-measurement uncertainties (1 σ) ranged from approximately 13% at 1300 K to 7% at 1800 K.     

Single-pulse measurement of wind velocities using an Er:Yb:glass coherent laser radar

Many wind-field mapping applications require range-resolved atmospheric velocity measurements at long range and/or with a temporal resolution sufficient to investigate turbulence. We argue that this capability can be achieved only by coherent laser radar systems that transmit energetic (>1 mJ) pulses. We describe such a system and describe single-pulse measurement of the range-resolved line-of-sight velocities, and show that the instrument-limited reproducibility of the measurements is 0.4 ms(-1).     

Reflectometer using opposing light beams

A novel optical circuit and algorithm for operation of the circuit are proposed; together they make it possible to perform highly precise absolute measurements of the reflection factor and transmission coefficient of different optical elements, including fiber-optic elements as well as elements with reflection asymmetry and nanostructured film coatings.     

Surface analysis using proton beams

Characteristic X-rays, back-scattered protons and Auger electrons are emitted from the surface region of solids under bombardment by protons. Energy analysis of these emitted particles yields data which permit identification of the atoms present in the surface region; in addition, quantitative surface composition information has been obtained using appropriate calibrations. Results from analyses using the techniques of (1) proton-induced X-rays (PIX), (2) Rutherford back-scattering (RBS) and (3) Auger electron spectroscopy (AES) for bombardments by mono-energetic 100–400 keV protons are collated and discussed. These previously published results are reviewed here to illustrate the types of compositional information obtainable and the advantages and disadvantages of each of the techniques with regard to elemental sensitivities, depth resolutions, reaction cross sections and complex targets. Finally, the complementary nature of the three techniques is discussed with emphasis on the possibilities for simultaneous analyses.     

Oxidation processes using ion beams

Recent work demonstrates that reactive ion beams can be used to grow compound layers on metal and semiconductor surfaces. Ion beams are characterized by easily controlled ion flux and energy, with a narrow energy spread. Also, the ion species are delivered to the sample in a low pressure environment. These advantages allow greater control and simpler analysis of compound formation processes than other techniques do. Ion beam oxidation is reviewed and compared with thermal, plasma and r.f. oxidation and with oxidation by ion implantation. The ion energy range of several electronvolts to hundreds of electronvolts is suitable for depositing the oxidizing species in the metal in an active state, and simultaneous sputtering can produce a self-limiting oxide thickness when the sputtering yield and oxidation rate are in proper balance. The process of ion beam oxidation is also discussed in light of the etching behavior of metals under combined inert gas and oxygen ion bombardment and of the related technique of reactive ion beam sputter deposition using ion bombardment of a growing film. Additional examples are drawn from the use of other reactive ion beam species, including nitrogen and hydrogen.     

Ellipsometer measurements using focused and masked beams

While optical spectroscopic measurements using ellipsometry may be made in air and are non-destructive, the relatively large (> 2 mm) spot size has limited their use to surface regions greater than 2 mm in lateral extent. Recent developments in focusing instruments have made spot sizes on the order of 20 to 25 μm possible. The work to be presented explores the use of the 25 μm spot size to probe non-uniform nanostructured thin films. Measurements were performed on a highly non-uniform film (0 to 2 μm in thickness across 4 mm in lateral dimension) using such a 25 μm spot. Further reduction of the spot size is possible using mechanical masking with a slit. Measurements have been made to the range of a few microns in width. The practical resolution limits of beam masking may be decreased by increasing incident light intensity, improving slit alignment, and improving detection methods.     

Strengthening of concrete beams using fiber-reinforced platics

One application of composite materials in civil engineering is examined: the strengthening of a reinforced concrete beamin situ by externally-bonded fiber reinforced plastic (FRP). Studies of the mechanical properties of the interface and the rheological behaviour of composite materials are very important to design. For the experimental determination of the mechanical properties of the concrete/glue/plate interface, a new test is suggested. An iterative analytical model capable of simulating the bond-slop and the material non-linearity, based on the compatibility of deformations and the equilibrium of forces, is developed in order to predict the ultimate forces and deflections. A new equation is proposed to anticipate the maximal shear and normal stresses at the interface goal to anticipate the failure mode due to the debonding of the plate. Finally, a series of large-scale beams strengthened with fiber reinforced plastic is tested up to failure; load-deflection curves are measured and compared with the predicted values to study the efficiency of the externally-bonded plate and to verify the theoretical method.

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Résumé Nous examinons un cas d'application des matériaux composites pour le génie civil: le renforcement par placage de tissus composites de poutres en béton armé. La connaissance des propriétés d'adhérence béton-composites et du comportement rhéologique de ces matériaux est indispensable. Pour cela, un nouvel essai a été proposé pour déterminer les propriétés mécaniques de l'interface béton/colle/plaque. Pour dimensionner les ouvrages, nous proposons une méthode d'analyse itérative capable de simuler la non-linéarité des matériaux et le glissement des plaques afin d'évaluer les niveaux de portance de ces structures. Une nouvelle equation a été développée pour prévoir la contrainte maximum de l'interface et finalement une série de poutres renforcées par placage de tissus composites a été testée jusqu' à rupture pour étudier l'efficacité de la méthode de placage et vérifier la méthode de calcul.

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Editorial note Prof. Patrice Hamelin is a RILEM Senior Member.     

Surface modification using lasers and ion beams

Abstract

The influences of ion beam and laser treatment on the thermal oxidation and sulphidation of metallic materials and their particular features, advantages, and disadvantages are separately described and compared with conventional alloy addition. Examples of the various mechanisms involved and the associated problems are also presented. The ion beam technique considerably improves high temperature resistance, despite the shallow depths involved and ion beam deposited coatings are already in commercial production. Laser beams produce thicker coatings and have considerable industrial potential.

MST/1073     

Creating biological nanomaterials using synthetic biology

Synthetic biology is a new discipline that combines science and engineering approaches to precisely control biological networks. These signaling networks are especially important in fields such as biomedicine and biochemical engineering. Additionally, biological networks can also be critical to the production of naturally occurring biological nanomaterials, and as a result, synthetic biology holds tremendous potential in creating new materials. This review introduces the field of synthetic biology, discusses how biological systems naturally produce materials, and then presents examples and strategies for incorporating synthetic biology approaches in the development of new materials. In particular, strategies for using synthetic biology to produce both organic and inorganic nanomaterials are discussed. Ultimately, synthetic biology holds the potential to dramatically impact biological materials science with significant potential applications in medical systems.     

Detecting SNPs using a synthetic nanopore

We have discovered a voltage threshold for permeation through a synthetic nanopore of dsDNA bound to a restriction enzyme that depends on the sequence. Molecular dynamic simulations reveal that the threshold is associated with a nanonewton force required to rupture the DNA-protein complex. A single mutation in the recognition site for the restriction enzyme, i.e., a single nucleotide polymorphism (SNP), can easily be detected as a change in the threshold voltage. Consequently, by measuring the threshold voltage in a synthetic nanopore, it may be possible to discriminate between two variants of the same gene (alleles) that differ in one base.     

Production of Raman laser beams using injection-locking technique

We produced two Raman-laser beams with a frequency offset of 9.2 GHz by injection-locking of a master diode-laser to a slave diode-laser. The master laser was phase-modulated at 9.2 GHz while the laser beam passed through an electrooptic modulator. The phase-modulated beam was injected into the slave laser that was oscillating around one of the side-bands of the master laser. The relative linewidth of the two lasers was less than 10 Hz. Using these laser beams, coherent population trapping resonance of cesium atoms was observed for the purpose of testing the phase-coherence of the laser beams.     

Characterization of thin films using heavy ion beams

Thin films are used in many technological applications. The characterization of thin films requires compositional information as a function of sample depth. Ion beam analysis techniques, such as Rutherford backscattering spectrometry and elastic recoil detection (ERD), can provide this information in a uniform way for all elements and as absolute concentrations without relying on standards. These techniques can fully be exploited when projectile beams of heavy ions such as Si or Au are used. This improves the elemental resolution and the depth resolution when compared with standard He ion beam analysis. The use of gas ionization detectors increases detection efficiency and minimizes the beam exposure of the samples, so that the analysis is essentially nondestructive. The sampling depth of a few micrometers makes these techniques ideal for the stoichiometric analysis of the surface region of homogeneous materials and, in particular, thin surface films.     

Measurements in radiotherapy beams using on-line MOSFET detectors

When acquiring data to characterise radiation beams for radiotherapy treatment planning measurements in steep dose gradients such as beam penumbra or dose build-up are often required. A metal oxide semiconductor field effect transistor (MOSFET) with its inherent high spatial resolution was used for penumbra measurements in a 120 kVp X ray beam. The customised MOSFET system features a pulsed readout that allows the acquisition of data points in user defined time intervals of less than 1 s. Using a modified scanning beam data acquisition system the penumbra was acquired on-line in 0.1 mm steps. Measurements were made at different distances behind the beam collimator. From the extrapolation of the penumbra width to a location directly under the block the spatial resolution of the MOSFET system can be estimated to be better than 0.1 mm. This excellent spatial resolution has many potential applications in radiotherapy dosimetry, including the characterisation of multileaf collimator systems.     

Recent developments in nanofabrication using focused ion beams

Focused ion beam (FIB) technology has become increasingly popular in the fabrication of nanoscale structures. In this paper, the recent developments of the FIB technology are examined with emphasis on its ability to fabricate a wide variety of nanostructures. FIB-based nanofabrication involves four major approaches: milling, implantation, ion-induced deposition, and ion-assisted etching of materials; all these approaches are reviewed separately. Following an introduction of the uniqueness and strength of the technology, the ion source and systems used for FIB are presented. The principle and specific techniques underlying each of the four approaches are subsequently studied with emphasis on their abilities of writing structures with nanoscale accuracy. The differences and uniqueness among these techniques are also discussed. Finally, concluding remarks are provided where the strength and weakness of the techniques studied are summarized and the scopes for technological improvement and future research are recommended.