Many measurement tasks in industrial automation, e.g. non-contact distance measurement, room surveillance, object identification and gas flow measurement can be accomplished by sensors using airborne ultrasound. Their performance is determined by the properties of the ultrasonic transducers. The authors compare electrostatic and piezoelectric transducers. Piezoceramic transducers combine a high efficiency factor and ruggedness. Composite transducers consisting of piezoceramic and polymer materials show a wider relative bandwidth of about 30% at the expense of efficiency. Ultrasonic transducers based on piezopolymer foils offer a variety of acoustic properties at low expenditure. Their bandwidth corresponds to that of composite transducers. Composite, as well as piezopolymer, foil transducers are suited to the design of phased arrays. Foil transducers have the additional advantage of very small crosstalk between neighbouring transducer elements. By electronically controlled deflection of the sound beam, lateral details and consequently the spatial structure of objects can also be detected.

http://iopscience.iop.org/article/10.1088/0957-0233/3/3/001/pdf

Ultrasonic transducers and transducer arrays for applications in air

Mechanical properties of the arterial walls are significantly altered by atherosclerosis, and various studies have been recently conducted to measure the regional elastic properties (radial strain) of the arterial wall. We have developed a phase-sensitive correlation-based method, namely, the phased-tracking method, to measure the regional radial strain. On the other hand, the measurement of blood flow is an important practical routine in the diagnosis of atherosclerosis. It would be useful if the regional strain of the arterial wall as well as blood flow could be assessed simultaneously. Such measurement would require a high frame rate of several kilohertz. In this study, acquisition of ultrasonic RF echoes at a high frame rate (about 3500 Hz) was achieved using parallel beamforming in which plane waves were transmitted only 3 times and receive beamforming created 24 beams for each transmit beam. The accuracy in measurement of the minute radial strain was evaluated by a basic experiment using a cylindrical phantom. The error of the measured strain from the theoretical strain profile and its standard deviation were 4.8% and 9.5%, respectively. Furthermore, the radial strain of a carotid arterial wall and blood flow were simultaneously imaged in vivo.

/pdf/simultaneous-imaging-of-artery-wall-strain-and-blood-flow-by-4f7t0rut5m.pdf

Simultaneous imaging of artery-wall strain and blood flow by high frame rate acquisition of RF signals

The past decades have witnessed an increasing interest in developing advanced polymerization techniques subjected to external fields. Various physical modulations, such as temperature, light, electricity, magnetic field, ultrasound, and microwave irradiation, are noninvasive means, having superb but distinct abilities to regulate polymerizations in terms of process intensification and spatial and temporal controls. Gas as an emerging regulator plays a distinctive role in controlling polymerization and resembles a physical regulator in some cases. This review provides a systematic overview of seven types of external-field-regulated polymerizations, ranging from chain-growth to step-growth polymerization. A detailed account of the relevant mechanism and kinetics is provided to better understand the role of each external field in polymerization. In addition, given the crucial role of modeling and simulation in mechanisms and kinetics investigation, an overview of model construction and typical numerical methods used in this field as well as highlights of the interaction between experiment and simulation toward kinetics in the existing systems are given. At the end, limitations and future perspectives for this field are critically discussed. This state-of-the-art research progress not only provides the fundamental principles underlying external-field-regulated polymerizations but also stimulates new development of advanced polymerization methods.

Role of External Field in Polymerization: Mechanism and Kinetics.

Wood is a biological growth medium, and given that standing trees adapt themselves in their growth to environmental conditions, their material properties vary with age. These changes result in variations that are far more complex than anisotropy. Wood quality and intraspecific variability can thus be studied to gain an understanding of the development mechanisms of trees, and this can be useful for clonal selection and the management of tree communities. A number of techniques are available to determine wood properties in standing trees, but the signal-processing approaches currently used are not always robust and do not always provide the image resolution needed in the particular cases of acoustic or ultrasonic tomography. This review paper thus aims to present important aspects that should be taken into account when using tomography techniques and addresses a number of open problems. A brief review of current non-destructive wood imaging techniques is initially presented followed by a comparison of the protocols, methods and models used in acoustic and ultrasonic tomography. The devices cited were studied in terms of measurement systems and signal processing. The analysis aimed to highlight and analyze the advantages and disadvantages of each device and describe challenges and trends. The effect of various parameters is discussed: frequency, signal-to-noise ratio, number of sensors and inversion algorithm. General conclusions are then drawn in relation to future signal-processing work in the acoustic and ultrasonic tomography of standing trees.

Literature review of acoustic and ultrasonic tomography in standing trees

Tissue mimicking phantoms have been widely reported to be an important tool for development, optimisation and performance testing of ultrasound-based diagnostic techniques. In particular, modern applications of tissue mimicking phantoms often include characterisation of the nonlinear behaviour of experimental ultrasound contrast agents. In such cases, the tissue-mimicking materials should be chosen not only based on the values of their density, speed of sound and attenuation coefficient, but also considering their effect on the appearance of "native harmonics" due to nonlinear distortion of ultrasound signal during propagation. In a previous paper it was demonstrated that a cellulose-based hydrogel is suitable to simulate nonlinear acoustical behaviour of liver tissue for thicknesses up to 8 cm. In this paper we present the experimental characterisation of the nonlinear acoustical behaviour of a different polyethylene glycol diacrylate (PEGDA)-based hydrogel, in order to assess whether and how it can improve the performances and overcome some limitations of the cellulose-based hydrogel as liver tissue-mimicking material. Samples of pig liver tissue, cellulose-based hydrogel and PEGDA-based hydrogel were insonified in a through-transmission set-up, employing 2.25-MHz pulses with different mechanical index (MI) values. Second harmonic and first harmonic amplitudes were extracted from the spectra of received signals and their difference was then used to compare sample behaviours. Obtained results show how a new more accurate and combined experimental model of linear and nonlinear acoustical behaviour of liver tissue is feasible. In fact, a further confirmation of the cellulose-based hydrogel effectiveness to precisely simulate the liver tissue for penetration depths up to 8 cm was provided, and it was also shown that the employment of the PEGDA-based hydrogel can extend the range of useful tissue-mimicking material thicknesses up to 11 cm, moreover allowing a considerable improvement of the time stability and behaviour reliability of the corresponding manufactured phantoms.

Full experimental modelling of a liver tissue mimicking phantom for medical ultrasound studies employing different hydrogels

The ultrasound computed tomography (CT) method presented in this paper is an improvement of conventional back projection imaging methods for wooden pillars. The sound velocity in wood changes with the direction of the ultrasound propagation path; therefore, the properties of a medium must be considered in the imaging method. We obtained the 36 time-of-flight (TOF) data of tangents on the same circumference for various diameters. We determined an irregular pathway for each diameter, if the sound velocity of the pathway is slower than 92% of the average velocity of 36 pathways. TOF data for 270 paths were measured in total by manual scanning for one CT image. Although a hole of 25 mm diameter and a slit-type (3×33 mm2) hole are not detected by the conventional back projection method, these artificial defects were detected by our method. Two slit-shaped defects were also detected by our method. We found some knots and cracks in the X-ray image in a wood specimen. These knots were detected by our method; however, it was impossible to distinguish the knots and holes from each other using the CT image.

Development of Ultrasonic Time-of-Flight Computed Tomography for Hard Wood with Anisotropic Acoustic Property

Holographic sonar, which incorporates a transducer array, is capable of high speed data acquisition. We have proposed a high speed imaging system using transmitting pulses modulated with Walsh functions ’. Walsh functions have a number of merits with respect to modulating the functions of acoustical imaging systems; ① they are easy to generate and process because of their binary properties, ② the duration time of pulses becomes short, and ③ an image can be reconstructed by means of a single transmitting and receiving process (single shot imaging).

A Multiple Shots 3-D Holographic Sonar Using a Set of Orthogonalized Modulating Signals

The aim of this work is to develop an acoustical imaging system which permits high speed data acquisition with simple hardware. In the previous works [1,2], we have introduced the holographic sonar using transmitting pulses modulated by the system of orthogonal functions. The imaging system transmits uncorrelated pulses from multiple transmitters simultaneously, and detects the echo wave-front by the array of receivers. With the recorded waveforms of echo, beam-forming is accomplished indirectly.

Holographic Sonar Using Orthogonal Transmitting Pulses

The “Encoding Array (EA)”, proposed by the authors1, represents a new concept in image sensors. With this EA, an image can be reconstructed without resort to either mechanical or electrical scanning—something that we, the authors, have already proposed the benefits of in previous papers2–5.

2-CH M-Sequence Encoding Array Transducer System for Acoustical Imaging

Dynamic ultrasound three-dimensional (3D) imaging using coded transmission is described. A sequence of 3D images of moving objects is reconstructed for each transmission of pulses modulated by Walsh functions. Then, the dynamic structure of the objects is extracted from the image sequence. In this paper, we discuss the relationship of the system performance to the parameters of the transmitting code such as duration time, repetition period, and total transmission length. A Doppler shift estimation and motion compensated image reconstruction method is described. The computer simulation for the motion compensation image reconstruction was carried out. Ultrasound 3D stroboscopic photography was demonstrated by a preliminary experiment utilizing the evaluation system in a 3D medical diagnostic scanner which has 32 transmitters and 32 receivers.

Yasutaka Tamura

Papers

Development of Ultrasonic Time-of-Flight Computed Tomography for Hard Wood with Anisotropic Acoustic Property

A Multiple Shots 3-D Holographic Sonar Using a Set of Orthogonalized Modulating Signals

Holographic Sonar Using Orthogonal Transmitting Pulses

2-CH M-Sequence Encoding Array Transducer System for Acoustical Imaging

Observation of Dynamic Structure Using Ultrasound 3D Imaging System with Encoded Wave Front