Focusing Transducer Research Articles

Non-invasive estimation of temperature using diagnostic ultrasound during high intensity focused ultrasound therapy

The use of HIFU for thermal ablation of human tissues requires safe real-time monitoring of the lesion formation during the treatment to avoid damage of the surrounding healthy tissues and to control temperature rise. Besides MR imaging, several methods have been proposed for temperature imaging using diagnostic ultrasound, and echoshift estimation (using speckle tracking) is the most promising technique. It is based on the thermal dependence of the ultrasound echo that accounts for two different physical phenomena: local change in speed of sound and thermal expansion of the propagating medium due to changes in temperature. In our experiments we have used two separate transducers: HIFU exposure was performed using a 1.06 MHz single element focusing transducer of 64 mm aperture and 63.2mm focal length; the ultrasound diagnostic probe of 11 MHz operated in B-mode for image guidance. The temperature measurements were performed in an agar-based tissue-mimicking phantom complying with the specification given in IEC60601-2-5. To verify the obtained results, the numerical modeling of the acoustic and temperature fields was carried out using KZK and Pennes Bioheat equations. The comparison of the results simulated and measured, as well as measured with thermocouples, shows a rather good coincidence.

Advances in the ultrasound characterization of dry-cured meat products

In this work, the feasibility of using non-contact ultrasonic techniques (air-coupled and scanning acoustic microscopy, SAM) for characterizing different dry-cured meat products was assessed. Air-coupled ultrasonic measurements were performed on vacuum packaged sliced dry-cured ham, and compared with contact measurements. The average ultrasonic velocity in dry-cured ham was 1846±49m/s and 1842±42m/s for air-coupled and contact measurements, respectively. The deviation (1% relative error) between both techniques was related to the influence of the heterogeneous structure and composition of dry-cured ham and the transducer focusing. The SAM was used to characterize dry-cured ham and chorizo samples. B-scan images for dry-cured ham and chorizo showed two dominant reflections from the sample, linked to reflections in the lean and fatty tissues. The results indicate that contact ultrasonic measurements could be replaced by the air-coupled technique, reducing the measuring time and the material handling. On the other hand, SAM technique allows the microscopic characterization of dry-cured meat products.

An Improved Weighting Method for Inversely Determining Elastic Constants of Coating Layers from Dispersion Curves of Surface Acoustic Waves

This paper investigates the inversion of elastic constants of a coating layer from measured dispersion curves of a layered half-space sample. A systematic analysis on the sensitivity of dispersion curves is performed and the results provide important information in constructing a better inversion process. Numerical simulation has been given to demonstrate several possibilities for obtaining accurate elastic constants from the measured dispersion data over different frequency domains. Experiments have also been carried out using broadband Poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] focusing transducers and their surface wave measurement system over a wide frequency range of 4 ~ 120 MHz. Nickel coating layers electroplated on thick glass substrates are tested for the inversion. It is observed that the proposed inversion method, which carefully chooses the dispersion data at specific frequency ranges as well as adding some weightings to them, is indeed significantly improve the accuracy and convergence of the obtained elastic constants of the coated nickel layers. Young’s modulus of the coated nickel layers is also measured by a nano-indentation system and the results show good agreement with the data determined by the proposed inversion method.

Radiation force of an arbitrary acoustic beam on an elastic sphere in a fluid.

A theoretical approach is developed to calculate the radiation force of an arbitrary acoustic beam on an elastic sphere in a liquid or gas medium. First, the incident beam is described as a sum of plane waves by employing conventional angular spectrum decomposition. Then, the classical solution for the scattering of a plane wave from an elastic sphere is applied for each plane-wave component of the incident field. The net scattered field is expressed as a superposition of the scattered fields from all angular spectrum components of the incident beam. With this formulation, the incident and scattered waves are superposed in the far field to derive expressions for components of the radiation stress tensor. These expressions are then integrated over a spherical surface to analytically describe the radiation force on an elastic sphere. Limiting cases for particular types of incident beams are presented and are shown to agree with known results. Finally, the analytical expressions are used to calculate radiation forces associated with two specific focusing transducers.

The maturity characterization of orange fruit by using high frequency ultrasonic echo pulse method

In this present work, we develop a new ultrasonic echo pulse method in order to study the feasibility of maturity assessment of orange fruit. This study concerns two varieties of orange (Navel and Mandarin) which are the most harvested in the region of Souss-Massa-Drāa in Morocco. We worked in the range of high frequencies by the means of a focusing transducer with 20MHz as a central frequency. By taking into account the strong attenuation of the ultrasounds in the texture of fruits and vegetables, we limited our study only to the external layer of orange peel. This control is based mainly on the measure of the ultrasonic parameters eventually velocity and attenuation in order to check the aptitude of this technique to detect the maturity degree of the fruit without passing by penetrometric and biochemical measurements which are generally destructives but the mostly correlated with human perception concerning the firmness of the fruit.

Broadband focusing ultrasonic transducers based on dimpled LiNbO3 plate with inversion layer

A high-frequency broadband focusing transducer based on dimpled LiNbO(3) inversion layer plate has been fabricated and characterized. A spherical surface with a curvature radius of 6 mm is formed on the half-thickness LiNbO(3) inversion layer plate of Y36° cut orientation. The domain structure in the cross section is observed after a hydrofluoric acid etching process. For transducer fabrication, conductive epoxy is used as the backing material and polymer is deposited on the front face as the matching layer. The center frequency, bandwidth, and insertion loss of the focused transducer are measured to be 72 MHz, 136%, and -32 dB, respectively. The focused transducer has been successfully used for rabbit eyeball imaging and a better imaging capability compared with the planar transducer has been demonstrated. These promising results prove that the dimpled LiNbO(3) inversion layer plate has great potential for fabrication of high-frequency broadband focusing ultrasonic transducers.

A Study of High Intensity Focusing Ultrasonic Transducer

High intensity focused ultrasound (HIFU) is the fourth brand-new and efficient means to cure tumour acknowledged by the medical field. Study of ultrasonic transducer is a core part of HIFU technique, In order to ensure reliability and safety of treatment, it is a key for HIFU technique to realize accurate focusing of ultrasonic energy. In the thesis, ultrasonic focusing method, studies of current situations of cell and multiplex array focusing transducers and their existing problems are illustrated based on analyzing challenges faced by HIFU treatment at present. This study suggested that phased array was theoretically easy for realizing accurate control of computer， however, unbeneficial factors and engineering technical problems still exist; How to promote intensity of the focal spot of cell array focusing transducer, enlarge scope of the focal area and improve control way of the focal spot was a bottleneck problem for publicizing and applying cell array focusing transducer and one of urgent research topics for ensuring curative effect of HIFU and avoiding heat damages.

Simultaneous measurement of local longitudinal and transverse wave velocities, attenuation, density, and thickness of a layer by using point-focus ultrasonic spectroscopy

This paper presented an ultrasonic technique for simultaneous determination of the complete set of acoustical and geometrical properties of a layer embedded between two known materials using point-focus ultrasonic spectroscopy, which provides a high lateral resolution. The theoretical model of the two-dimensional spectrum Rt (θ, ω) of the layer is calculated as a function of six parameters of longitudinal and transverse velocities cl, ct, attenuation αl, αt, density ρ and thickness h, which fully determined the layer properties. The experimental spectrum Re (θ, ω) can be measured by V(z, t) technique. A two-step algorithm is presented to decompose the searching process of parameters from one six-dimensional to two three dimensional spaces. The sensitivity of the two-dimensional spectrum to individual properties and its stability against experimental noise are studied. The full set properties of a 250 μm thick stainless steel plate immersed in water are determined. The proposed technique used a point focus transducer, which makes the set-up simple and reliable. It allows measurement of the local properties of the layer and enables precision material characterization.

High frequency PMN–PT single crystal focusing transducer fabricated by a mechanical dimpling technique

High frequency (∼30MHz and ∼80MHz) focusing ultrasound transducers were fabricated using a PMN–0.28PT single crystal by a mechanical dimpling technique. The dimpled single crystal was used as an active element for the focusing transducer. Compared with a plane transducer, the focusing transducer fabricated with a dimpled active element exhibits much broader bandwidth and higher sensitivity. Besides, a high quality image can be obtained by the 30MHz focusing transducer, in which the −6dB axial and lateral resolution is 27μm and 139μm, respectively. These results prove that the dimpling technique is capable to fabricate the high frequency focusing transducers with excellent performance for imaging applications.

Development of a Practical Scanning Acoustic Microscopy

Scanning acoustic microscopy (SAM) is a powerful non-destructive testing tool used in electronic, material and medical testing area. Commercial SAM products are generally too expensive to be extended to common users. Therefore, a practical SAM system had been developed using high-frequency ultrasonic focus transducers, a wide-band pulse transmitter/receiver, a high-speed data acquisition card, and a high-precision motion system. The SAM system's precision and function can meet the requirement of practical test adequately, and the cost is much lower compared to commercial products. Several kinds of imaging method were introduced, and the SAM system has the ability to accomplish full-wave data acquisition.

Angular measurement of acoustic reflection coefficients by the inversion of V(z, t) data with high frequency time-resolved acoustic microscopy

For inspection of mechanical properties and integrity of critical components such as integrated circuits or composite materials by acoustic methodology, it is imperative to evaluate their acoustic reflection coefficients, which are in close correlation with the elastic properties, thickness, density, and attenuation and interface adhesion of these layered structures. An experimental method based on angular spectrum to evaluate the acoustic coefficient as a function of the incident angle, θ, and frequency, ω, is presented with high frequency time-resolved acoustic microscopy. In order to achieve a high spatial resolution for evaluation of thin plates with thicknesses about one or two wavelengths, a point focusing transducer with a nominal center frequency of 25 MHz is adopted. By measuring the V(z, t) data in pulse mode, the reflection coefficient, R(θ, ω), can be reconstructed from its two-dimensional spectrum. It brings simplicity to experimental setup and measurement procedure since only single translation of the transducer in the vertical direction is competent for incident angle and frequency acquisition. It overcomes the disadvantages of the conventional methods requiring the spectroscopy for frequency scanning and/or ultrasonic goniometer for angular scanning. Two substrates of aluminum and Plexiglas and four stainless plates with various thicknesses of 100 μm, 150 μm, 200 μm, and 250 μm were applied. The acoustic reflection coefficients are consistent with the corresponding theoretical calculations. It opened the way of non-destructive methodology to evaluate the elastic and geometrical properties of very thin multi-layers structures simultaneously.

A Miniature Broadband High-Frequency Focused Transducer for Interventional Applications with the Competence of Both OCT and IVUS

A Miniature Broadband High-Frequency Focused Transducer for Interventional Applications with the Competence of Both OCT and IVUS

PMN–PT single crystal focusing transducer fabricated using a mechanical dimpling technique

A ∼5 MHz focusing PMN–PT single crystal ultrasound transducer has been fabricated utilizing a mechanical dimpling technique, where the dimpled crystal wafer was used as an active element of the focusing transducer. For the dimpled focusing transducer, the effective electromechanical coupling coefficient was enhanced significantly from 0.42 to 0.56. The dimpled transducer also yields a −6 dB bandwidth of 63.5% which is almost double the bandwidth of the plane transducer. An insertion loss of the dimpled transducer (−18.1 dB) is much lower than that of the plane transducer. Finite element simulation also reveals specific focused beam from concave crystal surface. These promising results show that the dimpling technique can be used to develop high-resolution focusing single crystal transducers.

Time reversed acoustics techniques for elastic imaging in reverberant and nonreverberant media: An experimental study of the chaotic cavity transducer concept

In view of emerging imaging technologies based on the combination of Time Reversed Acoustics (TRA) with Nonlinear Elastic Wave Spectroscopy (NEWS) for the detection and localization of micro-damage in solids, we have investigated the benefits of chirped source signal excitation, inverse filtering techniques, and the implementation of chaotic cavity transducers to improve the quality of energy focusing, especially for weakly reverberant media. Chaotic cavity transducer focusing is defined as the hardware-software combination of a piezoelectric ceramic glued on a cavity of chaotic shape on the one hand with the reciprocal Time Reversal (or Inverse Filter) technique on the other hand. Experimental data for reverberant and nonreverberant composite plates show that the use of chirps, inverse filtering and chaotic cavity transducers significantly enhances the focusing process, and enables focusing in a nonreverberant medium using only one transducer. As a potential exploitation, the application of the chaotic cavity transducer concept for synthetic imaging is examined, revealing several properties similar to phased arrays.

Evolution of high-intensity focusing systems for different applications in medicine (Review)

The evolution of high-intensity focusing systems for application in medicine has been outlined from their introduction (in the 1950s) to date. This work is in fact a review with two sections. The focusing systems for medical application discussed under the first section are based on using single focusing transducers with a radiating element shaped as a section of a spherical shell. Attention is mainly given to the devices developed at the Andreev Acoustics Institute in the 1970s–1980s; focusing systems developed abroad in the past few decades and most widely clinically used are also briefly discussed there. The second section of the review is devoted to focusing systems based on phased arrays which allow users not only to electronically steer the focus within a selected range, but also, especially importantly for various medical applications, to simultaneously create several foci. The main attention under the second section is given to the two-dimensional phased arrays with randomized location of elements on the surface, which the Acoustics Institute developed in active collaboration with others. It has been shown that irregularity in the location of elements either substantially improves the quality of spatial intensity distributions by reducing the level of secondary maxima of intensity in the array-induced fields or allows to considerably reduce the number of elements in the array with the same quality of distributions. The paper discusses principles of designing such arrays, methods of their calculations and possible applications.

Anechoic sphere phantoms for estimating 3-D resolution of very-high-frequency ultrasound scanners

Two phantoms have been constructed for assessing performance of high-frequency ultrasound imagers. They also allow for periodic quality assurance tests and training technicians in the use of higher-frequency scanners. The phantoms contain eight blocks of tissue-mimicking material; each block contains a spatially random distribution of suitably small anechoic spheres having a small distribution of diameters. The eight mean sphere diameters are distributed from 0.10 to 1.09 mm. The two phantoms differ primarily in terms of the frequency dependence of the backscatter coefficient of the background material. Because spheres have no preferred orientation, all three (spatial) dimensions of resolution contribute to sphere detection on an equal basis; thus, the resolution is termed 3-D. Two high-frequency scanners are compared. One employs single-element (fixed focus) transducers (25 and 55 MHz), and the other employs variable focus linear arrays (20, 30, and 40 MHz). The depth range for detection of spheres of each size is determined corresponding to determination of 3-D resolution as a function of depth. As expected, the single-element transducers are severely limited in useful imaging depth ranges compared with the linear arrays. In this preliminary report, only one human observer analyzed images.

On the use of a chaotic cavity transducer in nonlinear elastic imaging

We discuss the advantages of “chaotic cavity transducer focusing” to enhance the localization of microdamage in conjunction with nonlinear elastic wave spectroscopy methods. Chaotic cavity transducer focusing is defined as the hardware-software combination of a piezoelectric ceramic glued on a cavity of chaotic shape on the one hand with the reciprocal time reversal (or inverse filter) technique on the other hand. Additional optimization through the use of sweeps and inverse filtering techniques are discussed as well. The technique is applied to image a crack in a steel sample.

Fabrication of poly(vinylidene fluoride-trifluoroethylene) ultrasound focusing transducers and measurements of elastic constants of thin plates

This paper investigates a modified method for fabricating focusing ultrasound transducers based on spin-coated poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer films. The ferroelectric hysteresis loops of the spin-coated P(VDF-TrFE) films are measured to establish the best fabrication parameters. The fabricated transducers are used for high-frequency wave velocity measurements and for non-destructive determination of elastic constants of thin isotropic plates. Both through-thickness longitudinal wave velocities and Rayleigh surface wave velocities of thin-plate samples with thicknesses ranging from 125 to 250 μm are measured, and hence their elastic constants can be determined in a straightforward and accurate way. The obtained results are in good agreement with the data reported by the manufacturers. Limitation of the proposed fabrication method and conventional method is compared and discussed in terms of signal intensity and frequency characteristics.

Double aperture focusing transducer for controlling microparticle motions in trapezoidal microchannels with surface acoustic waves

We present a method for controlling the motion of microparticles suspended in an aqueous solution, which fills in a microchannel fabricated into a piezoelectric substrate, using propagating surface acoustic waves. The cross-sectional shape of this microchannel is trapezoidal, preventing the formation of acoustic standing waves across the channel width and therefore allowing the steering of microparticles. The induced acoustic streaming transports these particles to eliminate the use of external pumps for fluid actuation.

Measurement of Surface Acoustic Wave Velocity Using a Variable-Line-Focus Polyurea Thin-Film Ultrasonic Transducer

This paper presents the novel measurement method of the surface acoustic wave velocity by the variable-line- focus transducer using a polyurea piezoelectric ultrasonic transducer. First, a multiresonant polyurea thin-film ultrasonic transducer is fabricated by the vapor deposition polymerization process using 2 monomers. Second, the measurement system of surface acoustic wave velocity modified from the V(z) curve method is established. The system uses the fabricated polyurea thin film as a variable-line-focus transducer at the 30-MHz resonance frequency. The focal length is changed by varying the radius of curvature of the film transducer. To estimate the surface acoustic wave velocities from the measured data theoretically, the photographs of the transducer bent shapes are taken by using a digital microscope, and the bent transducer curvature is modeled by the 7th-order polynomial. To examine the performances of the variable-line-focus transducer, the surface acoustic wave velocities of an aluminum and a synthesized silica glass specimen have been measured. The measured surface acoustic velocities showed good agreement with the reference values.