H. Calas

Air-coupled ultrasonic resonant spectroscopy for the study of the relationship between plant leaves' elasticity and their water content

Air-coupled wideband ultrasonic piezoelectric transducers are used in the frequency range 0.3 to 1.3 MHz to excite and sense first-order thickness resonances in the leaves of four different tree species at different levels of hydration. The phase and magnitude spectra of these resonances are measured, and the inverse problem solved; that is, leaf thickness and density, ultrasound velocity, and the attenuation coefficient are obtained. The elastic constant in the thickness direction (c33) is then determined from density and velocity data. The paper focuses on the study of c33, which provides a unique, fast, and noninvasive ultrasonic method to determine leaf elasticity and leaf water content.

Acoustic field modeling for physiotherapy ultrasound applicators by using approximated functions of measured non-uniform radiation distributions

The strongest therapeutic effects in ultrasonic physiotherapy are mainly produced at the first centimeters, i.e. close to the applicator surface and, in general, only in the near-field zone. The acoustic field produced in practice by this type of transducers differs from the classical models because the vibration distribution on the real transducer surfaces is non-uniform. However, neither models using uniform distribution, nor those using typical non-uniform distribution patterns for the source accurately represent the radiation of this kind of transducers. Although this therapy is widely used and many efforts have been made in experimentally studying the patterns of ultrasound radiation produced during physiotherapy applications (IEC-61689, 1998), additional modeling researches still would be needed in order to achieve improved models giving field patterns closer to the measured ultrasonic results. In this paper, acoustic patterns produced from two source radiation functions are proposed and evaluated for field modeling of physiotherapy applicators. Both the functions are approximations to the pressure distribution measured close to the emitting surface and they are based on the modulation of the classical simply-supported function using either sinusoidal or Bessel-type distributions. The simply-supported function is accounted for the radiator-fixing condition and the modulation function simulates the complex vibration distribution of this kind of transducer. The modulator Bessel function is based on reports about Bessel-type vibration distributions found in piezoelectric disk resonators. The use of a selected sinusoidal segment represents another analytical option for obtaining an approximated behavior of the measured data in a real applicator. Both the field models are implemented using the finite element method (FEM) to obtain the numerical solution of wave equation at each point in the radiated space. The solution is reached by considering axisymmetric radiation in attenuation-free media. The results indicate the viability of applying an adequate model for acoustic field calculation by simulating the radiating distribution on the emitting surface as either sinusoidal or Bessel-modulated functions. Models using both the functions describe reasonably real behaviors, but those based on Bessel functions are better correlated with the measurements. The results for three commercial applicators indicate the possibility of representing, with adequate verisimilitude, the acoustic field radiated by physiotherapy ultrasound transducers using linear combinations of Bessel profiles describing the radiation source.

Limited-diffraction wave generation by approaching theoretical X-wave electrical driving signals with rectangular pulses

The main interest of using limited diffracting waves is motivated by their potential applications in the enlargement of the field depth in acoustic imaging systems, under collimated conditions. In this work, an approach for simplifying the experimental arrangement, needed to generate limited diffracting waves, is proposed. The main idea is to approximate the theoretical X-wave electrical excitations by means of simple driving rectangular pulses. In order to optimize these driving signals in each array annulus, the L2 curve criterion is applied. The differences between theoretical X-wave signals and approximate driving pulses, related to their excitation effects, were minimized by using the time widths and amplitudes of the rectangular pulses as fitting parameters. The good agreement of the source vibration signals and resulting field distributions, provided by the drastic simplification presented here, with those obtained from the classical X-wave excitations, can be justified by the filtering effects induced by the transducer elements in frequency domain. These results suggest the possibility of achieving limited-diffraction waves with relatively simple driving waveforms, which can be implemented with a moderate cost in analogical electronics.

Vibration modes in ultrasonic Bessel transducer

Diagnosis ultrasonic transducers with uniformly poled can not satisfy the condition of improved lateral resolution and a large depth of the generated ultrasonic field. Bessel transducers appear as a good alternative to improve simultaneously both requirements. In this work a 2 MHz circular Bessel transducer was constructed with three concentric electrode rings, which were poled following a Bessel function. The results show a well focused beam and a depth of field as large as /spl sim/20 times the beam width (6 dB). Moreover, it is verified that the fundamental and some harmonic of the radial mode are suppressed, under an in phase excitation of the rings.

Estimation of PSD Shifts for High-Resolution Metrology of Thickness Micro-Changes with Possible Applications in Vessel Walls and Biological Membrane Characterization

Achieving accurate measurements of inflammation levels in tissues or thickness changes in biological membranes (e.g., amniotic sac, parietal pleura) and thin biological walls (e.g., blood vessels) from outside the human body, is a promising research line in the medical area. It would provide a technical basis to study the options for early diagnosis of some serious diseases such as hypertension, atherosclerosis or tuberculosis. Nevertheless, achieving the aim of non-invasive measurement of those scarcely-accessible parameters on patient internal tissues, currently presents many difficulties. The use of high-frequency ultrasonic transducer systems appears to offer a possible solution. Previous studies using conventional ultrasonic imaging have shown this, but the spatial resolution was not sufficient so as to permit a thickness evaluation with clinical significance, which requires an accuracy of a few microns. In this paper a broadband ultrasonic technique, that was recently developed by the authors to address other non-invasive medical detection problems (by integrating a piezoelectric transducer into a spectral measuring system), is extended to our new objective; the aim is its application to the thickness measurement of sub-millimeter membranes or layers made of materials similar to some biological tissues (phantoms). The modeling and design rules of such a transducer system are described, and various methods of estimating overtones location in the power spectral density (PSD) are quantitatively assessed with transducer signals acquired using piezoelectric systems and also generated from a multi-echo model. Their effects on the potential resolution of the proposed thickness measuring tool, and their capability to provide accuracies around the micron are studied in detail. Comparisons are made with typical tools for extracting spatial parameters in laminar samples from echo-waveforms acquired with ultrasonic transducers. Results of this advanced measurement spectral tool are found to improve the performance of typical cross-correlation methods and provide reliable and high-resolution estimations.

Preliminary analysis and simulation of the influence of radiations from the rim of therapeutic transducers in the irradiated acoustic field

In this work, the radiation behavior of two ultrasonic therapy applicators (working around 1 MHz range) is investigated when they are radiating into water media having acoustic impedance similar to that being encountered in biological tissues. Several experimental plots of field patterns generated by both therapy transducers are gate-detected and accurately evaluated in laboratory. One very serious problem observed in many real ultrasonic patterns emitted at the very near field is the influence of non-ideal housing radiation in the effectively radiated acoustic energy patterns. This measuring fact is shown in some detail and its causes are theoretically analyzed here. These additional radiation effects perturb the desired spatial uniformity in the emitted ultrasonic field and could change the beam path causing that therapeutic radiation outside the treatment field should be heating neighbor tissues. A preliminary analysis to check in the practice these possible real radiation problems, associated with such type of ultrasonic therapeutic devices, is presented. And a computer simulation, using Zemanek method, of the influence of anomalous & unexpected radiations, originated by aperture rim and housing of a typical therapy transducer, in the effective acoustic field, is presented here, comparing it with experimental results.

Possible application of spectral analysis techniques on ultrasonic echo-traces improved for studying changes in blood vessel walls

Parametric algorithms previously developed by authors for spectral evaluation of biological multi-echo waveforms are adapted and improved here in order to achieve a more elevated frequency resolution. These results permit to undertake viability studies around the possible application of a new auto-regressive spectral technique to estimate physical properties like wall thickness changes in blood vessels, with accuracy enough. These difficult and sophisticated measurements in vessels have an increasing interest as tools to estimate basic parameters for calculating elastic properties in the vessel walls. Laboratory thickness data obtained for a latex phantom (mimicking vessel properties) are shown, giving a promising expectative for this improved estimation technique in blood vessels characterization, a diagnostic tool nowadays of growing attention by the researches. In fact, the results suggest clear improvement in spatial resolution, over the classic cross-correlation and non-parametric techniques, to estimate delays between pulsed signals. But, still further efforts and rigorous analyses of ultrasonic echo-signals acquired from well-controlled sanguineous tissues phantoms are needed in order to optimize the potential resolution of this new thickness measurement procedure and also to evaluate its possible clinic limitations.

Laboratory characterization of electromechanical behaviour of Bessel array-transducer annuli for detection & imaging in biological media

X wave annular arrays, properly driven, could be extensively applied for medical imaging and detection. The main utility of such a device is to permit the achieving of good collimation in the ultrasonic beam maintained along certain depth. In order to generate these kinds of waves, it is necessary to precisely excite a radiating aperture in a Bessel-transducer way. The work aim is the characterization of the behavior of array annuli in a 2 MHz piezoelectric Bessel transducer. The analysis, in function of the frequency, of a basic electrical characterization, measuring the electrical complex admittance and the cross-coupling of the array elements, as well as the evaluation of the annuli emission impulse responses, are performed by employing an installation of own design (Videus-CSIC lab), reciently developed for: pulsed acoustic signals acquisition, efficient array driving and advanced processing. Knowing the response of each one of elements in this type of array, it is already possible the achieving of a correct design of the especial multichannel pulsed electrical driving needed to generate limited diffraction X-waves applicable in medicine.

Design of Bessel transducers based on circular piezoelectric composites for cranial Doppler detection

A Bessel transducer is constructed based on a ceramic with two external electrodes and an internal structure of alternated active and inactive annular piezoelectric zones. This type of transducers allows obtaining a good relationship between the depth of field and the lateral resolution of the acoustic field. This behavior of the acoustic field becomes the Bessel transducers in appropriate devices for applications in intra-cranial Doppler detection. In this report, a study of lateral contact between the rings of several Bessel transducers is presented. Four Bessel transducers were constructed and grouped in two sets. The first set contains annular composites combining PZT-5 and epoxy. The second set includes non-homogeneously polarized PZT-5 discs. The influence of the structure in the radiation pattern and the vibration modes is obtained through measurements of the input electric impedance in transducers.

Visualization of lamb wave propagation in uncured CFRP and curved surfaces using air-coupled ultrasound

Propagation of Lamb waves, generated and detected using air-coupled piezoelectric transducers (0.1-1.0 MHz), is visualized. Hence phase and group velocities are obtained. The technique is first tested on plates (aluminum and carbon fiber reinforced polymers-CFRP-plates). Then it has been applied to un-cured CFRPs plates and curved surfaces: steel pipes and vessels and to the curved section of CFRP beams. Two different experimental set-ups are proposed: 1) use of monolithic transducers and mechanical scans along the direction of propagation, 2) use of a phased array linear transducer and an electronic scan along the direction of propagation.