Nicolas Sénégond

Fast time-domain modeling of fluid-coupled cMUT cells: from the single cell to the 1-D linear array element

We report a fast time-domain model of fluid-coupled cMUTs developed to predict the transient response-i.e., the impulse pressure response-of an element of a linear 1-D array. Mechanical equations of the cMUT diaphragm are solved with 2-D finite-difference schemes. The time-domain solving method is a fourth-order Runge-Kutta algorithm. The model takes into account the electrostatic nonlinearity and the contact with the bottom electrode when the membrane is collapsed. Mutual acoustic coupling between cells is introduced through the numerical implementation of analytical solutions of the impulse diffraction theory established in the case of acoustic sources with rectangular geometry. Processing times are very short: they vary from a few minutes for a single cell to a maximum of 30 min for one element of an array. After a description of the model, the impact of the nonlinearity and the pull-in/pull-out phenomena on the dynamic behavior of the cMUT diaphragm is discussed. Experimental results of mechanical displacements obtained by interferometric measurements and the acoustic pressure field are compared with simulations. Different excitation signals-high-frequency bandwidth pulses and toneburst excitations of varying central frequency-were chosen to compare theory with experimental results.

Characterization of cMUT by Dynamic Holography Microscopy

The comparison of numerical simulations to real cMUT behavior is essential to assess and optimize the process. For a complete characterization multiple devices (FIB, SEM, impedancemetry, laser interferometry, ...) are needed. In this context, we propose to use a full-field measurement device, a Digital Holography Microscopy (DHM Lynceetec®), which acquires static and dynamic in-plane and out-of-plane information of membranes population in a single acquisition and then provides most of required parameters. This paper presents the principle of this device, its performances comparing to other ones and its limits.

A strategy to predict and reduce baffle effects in linear array of CMUTs

This paper presents a cMUT array model based on matrix formulation like for the theory of linear systems. The model is compared with experimental data from which the baffle effect phenomenon in cMUT arrays is clearly identified. We propose to use the eigen-mode decomposition of linear matrices to understand and interpret origins of these phenomena. The impact of normal modes on the radiation directivity of cMUT array is clearly analyzed. The role of some geometric parameters on the baffle effect is studied too.

Non-linear dynamic response of cMUTs population: modeling and characterization

This paper presents a time domain model for simulating cMUT in fluid. Non linearity is taking into account and the collapse / snapback phenomena are modeled with a ¿thrust¿ model. Confrontation with another simulation tool is performed and comparison with experimental results is done.

Dual mode cMUTs fabricated with LPCVD sacrificial release process

This paper aims to develop an integrated dual mode ultrasonic transducers (based on cMUTs technology) for applications of targeted drug delivery dedicated to small animal experiments. Two functions are designed on the same transducer: one for high frequency imaging and the other for thermal activation of liposomes. Tests and performances evaluation are reported.

Control and monitoring of dynamic collapse event in cMUTs arrays for therapeutic applications

In this paper, the displacement behavior of the cMUT is analyzed thanks to a time domain model. The nonlinear components in the response spectrum with different excitation parameters are compared. A method based on the addition of a series impedance to the cMUT, which permits to decrease the harmonic components and enhance the maximum displacement swept by the diaphragm, is applied in a therapeutic context. The benefits of such a method are pointed out and discussed. After that, experimental measurements are performed to confirm these advantages, and discussed.

Preliminary investigation of dual mode CMUT probe for ultrasound image guided HIFU therapy

This preliminary study focuses on Capacitive Micro-machined Ultrasound Transducers (CMUTs) for dualmode ultrasound therapy and imaging. A planar probe design is being investigated in the context of Ultrasound-guided High Intensity Focused Ultrasound (USgHIFU) designed for endocavitary HIFU tissue ablation of the prostate gland. The hypothesis is that this CMUT design could offer potential advantages over currently used piezoelectric probes such as ease of miniaturization, increased elements for improved imaging and more control over electronic focusing allowing focal therapy. A planar dual mode CMUT probe consisting of a 3 MHz, 64-element annular array for therapy and 7 MHz, 256-element linear array for imaging were investigated in simulation and experimentally. Numerical models of the pressure field and tissue heating simulations were performed and compared with an existing piezo-based geometrically focused annular array HIFU probe (16 rings) with a hole in the center that possesses a curvilinear imaging array (128 elements). To validate the modelling and the focusing capabilities, pressure field hydrophone measurements were performed. Simulations showed that the planar annular CMUT design could dynamically focus from 3–7cm which matched well with the experimentally obtained pressure field data. Radiation force balance measurements confirmed the desired 5 W/cm2 surface acoustic intensity. This CMUT prototype will require further investigation for determination of the robustness of this technology and the ability to create lesions, yet according to the modelling and experimental validation thus far, this CMUT design shows promise for improved USgHIFU treatment strategies.

Assessment of imaging capability of 16×16 2D cMUT transducer arrays

Among the different transducers technologies suitable for medical imaging, the cMUTs are known to offer an attractive alternative to conventional piezoelectric ones, with features including wider bandwidth, wider directivity, and readiness for integration with electronics. This last point is of particular interest for 2D matrix arrays. Here, we present the acoustic and imaging performances of these devices initially dedicated for external cardiac imaging (frequency range [1.5–4 MHz]).

Design, characterization and test of a multi-transmitters, multi-receivers probe based on cMUTs for cortical bone evaluation

This paper presents the conception of a cMUTs probe for the cortical bone evaluation. A specific topology was designed in order to match the cMUTs technology to the requirements of the axial transmission measurement. We report here all the steps of the probe manufacturing from the design to the first postprocess characterization of the cMUTs. The first results of axial transmission measurements performed on a bone mimicking plate are presented and compared with the ones obtained with the same probe in PZT technology.