Rémi Dufait

Piezocomposite 30MHz linear array for medical imaging: design challenges and performances evaluation of a 128 elements array

The range of applications demanding the development of high frequency ultrasound imaging is very broad, from dermatology and ophthalmology to intravascular and small animals imaging, to improve the resolution of the images over a small penetration depth. However the manufacture of such imaging systems remains very challenging from both transducer manufacture (small dimensions of the elements, thin layers materials properties control) and associated electronic system perspectives. This article extends the current state of the art limited at 20 MHz for fully operational array devices and presents the acoustical design, manufacture and evaluation of a 128 elements 30 MHz ultrasound array based on the 1-3 piezocomposite technology, with a 100 /spl mu/m pitch and a 2 mm elevation aperture. Electro-acoustical characterisation of the full array is reported (in terms of bandwidth, pulse duration and homogeneity performance) as well as electrical characterisation (impedance and cross-coupling measurements).

High frequency piezo-composite transducer array designed for ultrasound scanning applications

A 20 MHz high density linear array transducer is presented in this paper, This array has been developed using an optimized ceramic-polymer composite material. The electro-mechanical behaviour of this composite, especially designed for high frequency applications, is characterised and the results are compared to theoretical predictions. To support this project, a new method of transducer simulation has been implemented. This simulation software takes into account the elementary boundary phenomena and allows prediction of inter-element coupling modes in the array. The model also yields realistic computed impulse responses of transducers, A miniature test device and water tank have been constructed to perform elementary acoustic beam pattern measurements. It is equipped with highly accurate motion controls and a specific needle-shaped target has been developed. The smallest displacement available in the three main axes of this system is 10 microns. The manufacturing of the array transducer has involved high precision dicing and micro interconnection techniques. The flexibility of the material provides us with the possibility of curving and focusing the array transducer. Performance of this experimental array are discussed and compared to the theoretical predictions. The results demonstrate that such array transducers will allow high quality near field imaging. This work presents the efforts to extend the well known advantages of composite piezoelectric transducers to previously unattainable frequencies.

Piezocomposite and CMUT arrays assessment through in vitro imaging performances

Piezoelectric and capacitive micromachined ultrasound transducers (CMUT) are usually measured and compared in regards to acoustic and electro-acoustic performances. This paper is focused on the imaging performances of such transducers and propose a quantitative imaging assessment of B-mode images. In this purpose, fully integrated CMUT and piezocomposite-based probes were manufactured. Transducers were designed with close features (geometries, center frequency, interconnect and packaging) and plug on a clinical ultrasound system with a research interface. Major imaging performances (speckle to noise ratio, resolutions and contrast) of the probes are presented. Despite an environment dedicated to piezoelectric transducers, CMUT probe exhibits comparable image quality as compared to a state-of-the-art piezocomposite probe. Indeed, close resolutions are calculated, field of view is improved in phased-array imaging, and contrast is significantly improved.

Tissue harmonic imaging with CMUTs

In this work, we report on the characterization of a CMUT probe (Capacitive Micromachined Ultrasound Transducer) for Tissue Harmonic Imaging (THI). The intrinsic nonlinear behavior of the CMUT probe was first investigated. Matched electrical waveforms were transmitted to limit the impact of the transmit response distortion. With the implemented method, we demonstrated higher performances through in-vitro harmonic imaging.

Three-dimensional real-time synthetic aperture imaging using a rotating phased array transducer

Current 3D real-time imaging is done either with sparse 2D arrays, or with mechanically moved phased arrays. The former results in a poor resolution and contrast due to a limited amount of elements. The latter has the disadvantage of low frame rates due to the sequential acquisition of the volume line-by-line and plane-by-plane. This paper describes an approach which combines mechanically moved phased array with synthetic transmit aperture imaging, resulting in high volume acquisition rates without a trade-off in image quality. The scan method uses a conventional fully populated 64 element phased array, which is rotated over the volume of interest. The data is acquired using coded signals and synthetic transmit aperture imaging. Only one group of elements transmits at a time. The delays are set such as to form a cylindrical wave. The back-scattered signal carries information not only from the plane located directly below the transducer, but also from neighboring planes. A complete dataset for all elements for the whole rotation is acquired and stored. The volume is then focused from this complete data set in order to obtain dynamic transmit and receive focusing in all directions.

Single crystal-based phased array for transoesophagial ultrasound probe

Lead-based piezoelectric single crystals such as Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PZN-PT) and Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PMN-PT) have been extensively studied during the last years as their outstanding piezoelectric properties (d/sub 33/ as high as 2000pC/N and k/sub 33/ > 90%) make them valuable for high-end ultrasound transducers. Nevertheless the specific nature of these materials, as compared with standard piezoceramics, requires to develop new transducer micro-machining and manufacturing processes to enhance the final properties of the transducers and to improve both sensitivity and bandwidth of these probes. This paper presents the acoustical design, fabrication and evaluation of a 64 elements ultrasound phased array for transoesophagial probe based on single crystal materials. The achieved phased array is a 5MHz configuration with a circular aperture. The whole transducer is included within diameter 10mm and 2mm height. Acoustical design and final results will show the improvements achieved with single crystals for this configuration by comparing these results to those obtained on standard piezoceramic phased array. Electro-acoustic characterisation is performed on the whole array and homogeneity performances will also be presented.

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.

Improving ultrasound imaging with integrated electronics

This paper presents an integrated electronic preamplifier design based on discrete components and evaluates its impact on image performances. The electronic, located close to the transducer, incorporates all useful functions to ensure compatible direct connection with most ultrasound scanners available on the market today. People who has worked on this subject know that numerous challenges and problems have to be overcome: high voltage bypass, preamplifier protection cells, miniaturization, power dissipation, electronic stability and many other constraints. We will discuss different unavoidable tradeoffs, starting with electrical performances and then with practical aspects. Our electronic solution has been evaluated with different probe configurations, namely a 5 MHz Phased Array and a 9.5 MHz Linear Array probe. Images have been acquired and analysis of signal to noise ratio (SNR) performed to quantify the gain in image quality.

Integrated mechanism based multiplane/3D ultrasonic imaging probes

Implementation of 3D capabilities on ultrasonic imaging systems tantalizingly proves the high interest for this diagnosing modality. However, to become a clinical tool, 3D ultrasound has to spend further technological efforts in acquisition performance and probe size to deliver on the fly, quality volumetric images as well as current functionalities. Nowadays, numerous 3D designs have been reported such as: hand-moving imaging probes equipped with position sensors, rotating/sweeping probe including array transducer or more complex 2D array transducer. The first type of device reveals to be inaccurate in position, the second is often heavy and exhibits excessive volume while, although of compact size, the third type is unreliable and excessive in manufacturing cost that limits commercial widespread use. In this paper, the simultaneous use of slim-line high density phased array transducer and multilayer actuator based ultrasonic motor devices enables imaging probes to exhibit both multiplane capability and high frame rate 3D rendering. The high level of integration allows the probe volume to be comparable to conventional 1D apparatus. Phased-array fabrication and interconnect problems have been addressed during this study as well as subsystem integration concerns.

Comparison of 3D synthetic aperture imaging and explososcan using phantom measurements

In this paper, initial 3D ultrasound measurements from a 1024 channel system are presented. Measurements of 3D Synthetic aperture imaging (SAI) and Explososcan are presented and compared. Explososcan is the ‘gold standard’ for real-time 3D medical ultrasound imaging. SAI is compared to Explososcan by using tissue and wire phantom measurements. The measurements are carried out using a 1024 element 2D transducer and the 1024 channel experimental ultrasound scanner SARUS. To make a fair comparison, the two imaging techniques use the same number of active channels, the same number of emissions per frame, and they emit the same amount of energy per frame. The measurements were performed with parameters similar to standard cardiac imaging, with 256 emissions to image a volume spanning 90°×90° and 150mm in depth. This results in a frame rate of 20 Hz. The number of active channels is set to 316 from the design of Explososcan. From wire phantom measurements the point spread functions of both techniques were measured. At 40mm depth Explososcan achieves a main lobe width (FWHM) of 2.5mm while SAIs FWHM is 2.2 mm. At 80mm the FWHM is 5.2mm for Explososcan and 3.4mm for SAI, which is a difference of 35 %. Another metric used on the PSF is the cystic resolution, which expresses the ability to detect anechoic cysts in a uniform scattering media. SAI improved the cystic resolution, R20dB, at 40mm depth from 4.5mm to 1.7 mm, compared to Explososcan. The speckle pattern looked better for SAI compared to Explososcans spatial shift variant speckle pattern.