Nicolas Felix

Exploitation of capacitive micromachined transducers for nonlinear ultrasound imaging

Capacitive micromachined ultrasonic transducers (CMUTs) present advantages such as wide frequency bandwidth, which could be further developed for nonlinear imaging. However, the driving electrostatic force induces a nonlinear behavior of the CMUT, thus generating undesirable harmonic components in the generated acoustic signal. Consequently, the use of CMUT for harmonic imaging (with or without contrast agents) becomes challenging. This paper suggests 2 compensation approaches, linear and nonlinear methods, to cancel unwanted nonlinear components. Furthermore, nonlinear responses from contrast agent were evaluated using CMUT in transmit before and after compensation. The results were compared with those obtained using a PZT transducer in transmit. Results showed that CMUT nonlinear behavior is highly influenced by the excitation to bias voltage ratio. Measurements of output pressure very close to the CMUT surface allow the estimation of optimal parameters for each compensation approach. Both methods showed a harmonic reduction higher than 20 dB when one element or several elements are excited. In addition, the study demonstrates that nonlinear approach seems to be more efficient because it is shown to be less sensitive to interelement variability and further avoids fundamental component deterioration. The results from contrast agent measurements showed that the responses obtained using CMUT elements in transmit with compensation were similar to those from PZT transducer excitation. This experimental study demonstrates the opportunity to use CMUT with traditional harmonic contrast imaging techniques.

Investigation of cross-coupling in 1-3 piezocomposite arrays

Plate waves inside the piezoelectric layer are much involved in the element cross-coupling in transducer arrays for medical imaging. In this work, such waves are analyzed in 1-3 piezocomposite materials on the basis of conventional guided modes formalism in which the piezocomposite is considered as a homogeneous medium. Cross-coupling measurements have been made on two different transducer arrays using a network analyzer and a laser interferometric probe. It is shown how the analysis in terms of symmetrical Lamb waves gives an interesting qualitative interpretation, explaining most of the cross-coupling amplitude variations with frequency. Results show that the 0th and 3rd symmetrical Lamb waves are mainly involved in coupling inside composite plates. The S/sub 0/ mode is responsible for the inter-element coupling, whereas the S/sub 3/ mode widens the effective width of the excited element.

Experimental investigation of cross-coupling and its influence on the elementary radiation pattern in 1D ultrasound arrays

Cross-coupling mechanisms in ultrasound arrays have deep influences on their performances. Here, to investigate them, the surface displacement of a 1D linear array is measured in water with a laser interferometer. Displacement data are then analyzed in both Time-Space and in Time frequency-Spatial frequency domains. Radiation pattern is then simulated from this data and compared to the radiation pattern of a plane piston and to hydrophone measurements. Cross-coupling influence on the 1D array element behavior is finally compared and discussed. Laser interferometry is shown to be a useful tool for ultrasound array optimization as it allows the identification of vibration modes, the determination of the elementary effective aperture and the radiation pattern of elements.

Effect of acoustical properties of a lens on the pulse-echo response of a single element transducer

The purpose of this study is to compare two methods for calculating the electroacoustic response of a transducer at the focal point through an acoustic focusing lens. The theoretical conditions depend on the degrees of freedom taken into account as well as on the boundary conditions for the propagation. The most widely used conditions, i.e. longitudinal vibration and rigid baffle boundary condition are considered here. The theoretical approach consists in calculating the electroacoustic response using each electroacoustic model, both of these results being then propagated with the Rayleigh integral for a focused source. The first section consists in a modeling of the electroacoustic response based either on the modified classical KLM electrical equivalent circuit (1D) or on the finite element method (FEM) ATILA (axisymmetric 2D). Both models are applied to an axisymmetric configuration, and a classical propagation operation is used to obtain the response at the focal point. The second part deals with the experimental verification of the modeling methods. Measurements on a series of 10 MHz focused transducers are compared to theoretical curves, and results are discussed as a function of geometry and properties of the acoustical lens.

3F-6 Ultra-Wide Bandwidth Array for New Imaging Modalities

A dual frequency probe composed of two outer low frequency arrays and one inner high frequency array is designed through a simulation stage, manufactured and fully characterized. Electroacoustical, acoustical and electrical properties suited the imaging modalities requiring an ultra wide bandwidth. The presented array transducer is a 128 elements linear array for superficial imaging with targeted frequency range from 2 MHz to 10 MHz. Electroacoustic and acoustic performances measured are close to those simulated. The device provides an ultra wide bandwidth of 130% improving superharmonic imaging.

A 3 MHz two dimensional array based on piezocomposite for medical imaging

2-D array transducers present a major interest for ultrasound volumetric imaging as they allow data acquisition and visualization in real time. However their manufacture remains a technological challenge because of the very high number of elements with reduced lateral dimensions close to a half wavelength pitch. During this work, we developed and set up an industrial process to manufacture fully connected array through an innovative rear material, combining the backing function and the high density interconnect. This process does not affect the acoustical behavior of each elements and the connection of all elements is performed in a single operation. In addition, for the acoustical design, specific backing and matching layers structure have been implemented to reduce cross-talk and to increase the acceptance angle. A 2D fully populated array, based on this design, with a 3 MHz center frequency, 300 microns pitch, containing, 4096 elements (64/spl times/64) is presented. The evaluation of the performances over the array includes electroacoustical characterizations (pulse echo waveforms and spectrum), electrical measurements (electrical impedance and cross-talk) and acceptance angle data, obtained by hydrophone setup techniques. This design can be applied to many configurations, over a wide range of pitch and frequency, from fully populated to sparse array.

1D ultrasound array: performance evaluation and characterization by laser interferometry

Nowadays, ultrasound medical imaging is more and more demanding with respect to transducer performance evaluation and optimization. Conventional tools and setups are limited in the complete evaluation and characterization of ultrasound imaging transducer, for example in the analysis of cross coupling (radiating and nonradiating). A vibration measurement method for ultrasonic array transducer using laser interferometry is presented. This method allows array designers to perfectly characterize the acoustic transducer, and contributes to the advanced optimization of bandwidth, cross coupling and sensitivity of ultrasound imaging array transducers.

Characterization of transducer arrays by laser interferometry: influence of acousto-optic interactions on displacement measurements in water

This paper deals with the influence of acoustooptic interactions on displacement measurements over transducer array with laser interferometer. Changes on the popular x-t and /spl omega/-k diagram of displacement due to interactions between acoustic and laser beams are discussed. A theoretical analysis of acousto-optic phenomenon, based on the plane wave decomposition of radiated field by the array is developed. Theoretical and experimental results are compared, showing first that waves with phase velocity near the one of the fluid are greatly amplified. Second, the interaction of laser beam with edge wave produced by the vertical size of elements induces a parasitic temporal pulse on the x-t diagram and so interference pattern in the /spl omega/-k diagram. Several possibilities of filtering are analyzed.

P2P-8 Characterization Standard of CMUT Devices Based on Electrical Impedance Measurements

A procedure for an electrical impedance characterization standard in air of CMUTs was reported. The electromechanical coupling factor was calculated by the low and high frequency capacity and the resonance and antiresonance frequencies measurements. The CMUT will be able to analyze and compared in terms of effective spring and mass with 1D model. The 1D model described the general behavior of the CMUT quite well, as long as first resonance/antiresonance mode and tensions below collapse were concerned

Inverse calculation method for piezocomposite materials characterization

The determination of elastic, dielectric and piezoelectric homogenised constants of piezocomposite materials is required for the optimisation and the performance evaluation of the material. This paper presents an inverse method enabling the determination of the effective electro-acoustic tensor |C, e, /spl epsiv/|. Data are displacement field measurements. The model used for the forward problem is presented. The calibration of the inversion process is carried out. A piezocomposite plate device is fully characterized. The determined parameters from the inverse problem are validated using other data, the acoustic transmission coefficient of the plate.