A. Ramos

Influence of thresholding procedures in ultrasonic grain noise reduction using wavelets

Wavelet transform based techniques are used for signal-to-noise ratio (SNR) enhancement in ultrasonic non-destructive testing and evaluation of strong sound scattering materials. The overall denoising performance of a wavelet signal processor is conditioned by several processing parameters, including the type of wavelet, thresholding method, and threshold selection rules. Different thresholding procedures and threshold selection rules are analysed in this paper using the discrete wavelet transform and decomposition level dependent thresholds. Global performance is evaluated by means of the SNR enhancement using synthetic grain noise registers with an incrusted flaw signal, with different values of the input SNR, and experimental ultrasonic traces acquired from a carbon fibre reinforced plastic composite block.

Improvement in transient piezoelectric responses of NDE transceivers using selective damping and tuning networks

NDE ultrasonic applications for quality control purposes, based on piezoelectric devices working as pulsed ultrasonic probes, usually include some tuning circuit either across the pulser output connectors or close to the piezoelectric probe electrodes. Classic criteria to determine tuning parameters in ultrasonic transducers do not perform very well in transient regime under the typical electrical conditions and peculiar output impedances of the required high-voltage pulsers. In most practical situations, the tuning values are manually adjusted in specific circuits for each NDE case, testing each channel of a multiprobe inspection system to find the best sensitivity. In this paper, the positive influences of certain selective damping and tuning networks on the time and frequency behaviors of NDE piezoelectric transceivers are analyzed in detail. Different waveforms and spectra of received ultrasonic signals are comparatively evaluated for different real tuning conditions, after passing through two ultrasonic processes linked in cascade: a transmitter of industrial type and a broad-band ultrasonic receiver. Piezoelectric transducer impedances, transduction transfer functions, and HV output spikes from a piezoelectrically loaded NDE pulser, are computed, to identify separately the influence of each stage. In order to improve amplitude and axial resolution in the received transient responses, relative contributions from emission and reception tuning networks are individually evaluated for a particular NDE two-stage piezoelectric channel. Also shown are the final experimental results relative to the ultrasonic test pulse and detected in a through-transmission NDE configuration, gradually improved by using distinct tuning levels.

Dependence of pulser driving responses on electrical and motional characteristics of NDE ultrasonic probes

Acoustic performance in ultrasonic transmitters can be improved by means of a suitable electrical driving response and matching/tuning networks. It is important to predict this electrical response, but doing so is not easy because it departs notably from the nominal pattern with the loading probes. In practice, the analysis of HV pulser spikes in NDE applications requires fairly complex models in the transient regime and, in addition, non-linear problems could arise, especially in the case of tuned transmitters. In this paper, the most relevant influences of loading characteristics of NDT ultrasonic probes on the pulser electrical driving responses are evaluated in time and frequency domains. Conventional pulse generators and typical NDE pulsers are considered. Driving responses are analysed across commercial ultrasonic probes and, alternatively, across similar purely electrical loads. Distinct influences on pulser responses from electrical and motional sections of the probes are identified. All these aspects are studied on the basis of experimental and computer results.

Electrical matching effects on the piezoelectric transduction performance of a through-transmission pulsed process

Abstract Many applications of non destructive testing employ piezoelectric transducers operated in through-transmission mode, to perform C-scan ultrasonic imaging. Normally a high efficiency and a pulsed regime for the ultrasonic process are required. The enlargement of both the amplitude and bandwidth in the testing signals permits to improve the resolution and sensitivity of the imaging systems. This can be done by means of external electric circuits. In this paper, the effects of some electrical matching components over the global emitter-receiver piezoelectric transduction performance, are analyzed under transient operating conditions, considering the signal waveforms and frequency spectra. In particular, a broadband configuration with inductive tuning circuits individualized for both transmitter and receiver subsystems is evaluated, by means of equivalent circuit analysis, computer simulation, and experimental assessment.

Noninvasive Temperature Estimation in Oncology Hyperthermia Using Phase Changes in Pulse–Echo Ultrasonic Signals

An accurate and noninvasive temperature measurement inside phantoms and tissues is important to improve hyperthermia treatment. The use of ultrasonic echo-pulses for this purpose is a promising line of research. In this paper, we propose an alternative method for processing echo-signals received from inner regions in phantoms. Ultrasonic estimation of temperature is carried out using a phase demodulation technique, which gauges the indirect effects in the phase domain of the previously reported time-shifts in the echo position. The potential performance and high sensitivity of this procedure is explained and verified with several simulated examples. Finally, the proposed procedure is applied to the experimental assessment of phase changes with temperature in ultrasonic echoes successively acquired from different milieus. In all cases, the results show good performance and high sensitivity. This is an interesting alternative to measurements in waveforms of the very small delays (nanoseconds) associated with this ultrasonic temperature estimation.

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.

Models for Piezoelectric Transducers Used in Broadband Ultrasonic Applications

Piezoelectric transducers are key elements of many broadband ultrasonic systems, either pulse-echo or through-transmission, used for imaging and detection purposes. In ultrasonic broadband applications such as medical imaging, or non-destructive testing, piezoelectric transducers should generate/receive ultrasonic signals with good efficiency over a large frequency range. This implies the use of piezoelectric transducers with high sensitivity, broad bandwidth and short-duration impulse responses. High sensitivity provides large signal amplitudes which determine a good dynamic range for the system and the short duration of the received ultrasonic signal provides a good axial resolution.

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.

Shift Invariant Wavelet Denoising of Ultrasonic Traces

Basic wavelet denoising techniques rely on a thresholding of the discrete wavelet transform (DWT) coefficients of the noisy signal. Some improvements in noise reduction efficiency can be obtained by the use of shift-invariant undecimated wavelet transforms (UWT). Ultrasonic grain noise is one of the most usual types of noise present in ultrasonic non-destructive evaluation. It comes from reflections in the material structure, and occupies a frequency band very similar to that of the echosignals of interest. In this work, new advances in the application of redundant wavelet transforms to ultrasonic grain noise reduction are presented. Wavelet denoising is applied to several sets of synthetic ultrasonic traces, which are obtained from a model that includes frequency dependent attenuation for both grain and flaw backscattered echoes, frequency dependent scattering from the grains, and an accurate model for the pulse-echo frequency response of the piezoelectric ultrasonic transducer. Two processors based on traditional DWT and alternative a trous UWT denosing have been implemented and compared, using level dependent thresholds (appropriate for correlated noise), soft thresholding, and Universal, SURE and Minimax threshold selection rules. The performances of the two processors are analyzed in terms of the mean value and standard deviation of the signal-to-noise ratio (SNR) of different sets of ultrasonic traces before and after denoising. It is shown that a trous UWT processing provides better results than DWT with a general tendency to higher quality of the resulting traces and greater robustness of the processing. It is also shown that the better performance of the UWT is mainly related to the redundancy of the representation, since there are not significant variations between the threshold values obtained with each processor.

Some non-linear aspects of the electronic stages in time-domain modelling of NDE pulse-echo ultrasonic systems.

Electronics interfacing with NDE probes frequently include non-linear switching devices and semiconductor networks, which influence the excitation pulses and detected echo signals. Classical approaches to modelling a pulse-echo process use ideal assumptions for the electronics and do not consider these influences on the echoes, which can be very relevant in HF cases. This paper proposes new ways to consider these non-linear effects in a time-domain simulation process, extending previous approaches by including new elements in the modelling. Specific electrical models covering the pulse-echo process are applied in the evaluation of echo-graphic signals. They include semiconductor devices and other non-ideal elements. From these models, and using SPICE as a simulation tool, strong non-linear effects on pulsed responses, computed for both E/R stages of typical NDE transceivers, are analysed.