An ultrasonic array is a single transducer that contains a number of individually connected elements. Recent years have seen a dramatic increase in the use of ultrasonic arrays for non-destructive evaluation. Arrays offer great potential to increase inspection quality and reduce inspection time. Their main advantages are their increased flexibility over traditional single element transducer methods, meaning that one array can be used to perform a number of different inspections, and their ability to produce immediate images of the test structure. These advantages have led to the rapid uptake of arrays by the engineering industry. These industrial applications are underpinned by a wide range of published research which describes new piezoelectric materials, array geometries, modelling methods and inspection modalities. The aim of this paper is to bring together the most relevant published work on arrays for non-destructive evaluation applications, comment on the state-of the art and discuss future directions. There is also a significant body of published literature referring to use of arrays in the medical and sonar fields and the most relevant papers from these related areas are also reviewed. However, although there is much common ground, the use of arrays in non-destructive evaluation offers some distinctly different challenges to these other disciplines.

Ultrasonic arrays for non-destructive evaluation: A review

Processing of ultrasonic array data is traditionally based on having parallel transmission circuits that enable staggered firing of transmitter elements to produce the desired wavefront. This paper describes an alternative approach in which the full matrix of time domain signals from every transmitter–receiver pair is captured and post-processed. Various post-processing approaches are modelled and assessed in terms of their ability to image a point-like reflector. Experimental results are then presented which show good quantitative agreement with the modelled results. An advanced processing algorithm is also implemented which allows the array to be focused at every point in the target region in both transmission and reception. This approach is shown to offer significant performance advantages for NDE.

Post-processing of the full matrix of ultrasonic transmit-receive array data for non-destructive evaluation

Piezoelectric materials have dominated the ultrasonic transducer technology. Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as an alternative technology offering advantages such as wide bandwidth, ease of fabricating large arrays, and potential for integration with electronics. The aim of this paper is to demonstrate the viability of CMUTs for ultrasound imaging. We present the first pulse-echo phased array B-scan sector images using a 128-element, one-dimensional (1-D) linear CMUT array. We fabricated 64- and 128-element 1-D CMUT arrays with 100% yield and uniform element response across the arrays. These arrays have been operated in immersion with no failure or degradation in performance over the time. For imaging experiments, we built a resolution test phantom roughly mimicking the attenuation properties of soft tissue. We used a PC-based experimental system, including custom-designed electronic circuits to acquire the complete set of 128/spl times/128 RF A-scans from all transmit-receive element combinations. We obtained the pulse-echo frequency response by analyzing the echo signals from wire targets. These echo signals presented an 80% fractional bandwidth around 3 MHz, including the effect of attenuation in the propagating medium. We reconstructed the B-scan images with a sector angle of 90 degrees and an image depth of 210 mm through offline processing by using RF beamforming and synthetic phased array approaches. The measured 6-dB lateral and axial resolutions at 135 mm depth were 0.0144 radians and 0.3 mm, respectively. The electronic noise floor of the image was more than 50 dB below the maximum mainlobe magnitude. We also performed preliminary investigations on the effects of crosstalk among array elements on the image quality. In the near field, some artifacts were observable extending out from the array to a depth of 2 cm. A tail also was observed in the point spread function (PSF) in the axial direction, indicating the existence of crosstalk. The relative amplitude of this tail with respect to the mainlobe was less than -20 dB.

Capacitive micromachined ultrasonic transducers: next-generation arrays for acoustic imaging?

Capacitive micromachined ultrasonic transducers (CMUTs) have been subject to extensive research for the last two decades. Although they were initially developed for air-coupled applications, today their main application space is medical imaging and therapy. This paper first presents a brief description of CMUTs, their basic structure and operating principles. Our progression of developing several generations of fabrication processes is discussed with an emphasis on the advantages and disadvantages of each process. Monolithic and hybrid approaches for integrating CMUTs with supporting integrated circuits are surveyed. Several prototype transducer arrays with integrated front-end electronic circuits we developed and their use for 2D and 3D, anatomical and functional imaging, and ablative therapies are described. The presented results prove the CMUT as a micro-electro-mechanical systems technology for many medical diagnostic and therapeutic applications.

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Capacitive micromachined ultrasonic transducers for medical imaging and therapy

Capacitive micromachined ultrasonic transducer (MUT) technology is a prime candidate for next generation imaging systems. Medical and underwater imaging and the nondestructive evaluation (NDE) societies have expressed growing interest in cMUTs over the years. Capacitive micromachined ultrasonic transducer technology is expected to make a strong impact on imaging technologies, especially volumetric imaging, and to appear in commercial products in the near future. This paper focuses on fabrication technologies for cMUTs and reviews and compares variations in the production processes. We have developed two main approaches to the fabrication of cMUTs: the sacrificial release process and the recently introduced wafer-bonding method. This paper gives a thorough review of the sacrificial release processes, and it describes the new wafer-bonding method in detail. Process variations are compared qualitatively and quantitatively whenever possible. Through these comparisons, it was concluded that wafer-bonded cMUT technology was superior in terms of process control, yield, and uniformity. Because the number of steps and consequent process time were reduced (from six-mask process to four-mask process), turn-around time was improved significantly.

Capacitive micromachined ultrasonic transducers: fabrication technology

An 8-channel, 10-bit pipeline analog-to-digital converter (ADC) designed for use in an integrated 3- dimensional ultrasound imaging system, has been implemented in a 0.25-µm CMOS technology. Two parallel sample-and-hold stages are employed to multiplex a total of 8 adjacent ultrasound channels, each sampled at 20 MHz. The sampled and multiplexed signals are fed into two parallel pipelines, each operating at 80 MHz. These two pipelines are subsequently multiplexed into a single pipeline operating at 160 MHz to conserve area. An experimental prototype of the proposed architecture occupies less than 4 mm2of silicon area and achieves a peak SNDR more than 54 dB for a 2.1-MHz input signal, while dissipating only 20 mW per each analog input channel from a 2.5-V power supply.

A multichannel, pipeline analog&#8211;to&#8211;digital converter for an integrated 3&#8211;D ultrasound imaging system

In this paper, the first experimental results of pulse-echo imaging employing a 1-D capacitive micromachined ultrasonic transducer array are presented. A 1-D array consisting of 16 elements is used in the experiment. A wire phantom consisting of seven steel wires immersed in vegetable oil is used as the imaging target. A B-scan image with a sector angle of 90 degrees and an image depth of 150 mm is reconstructed by employing RF beamforming and synthetic phase array approaches. The reconstructed image is displayed at different display dynamic ranges. The measured near and far side lobe levels are around /spl sim/25 dB and /spl sim/35 dB, respectively. The noise floor of the image is below /spl sim/50 dB.

Initial pulse-echo imaging results with one-dimensional capacitive micromachined ultrasonic transducer arrays

An 8-channel 10-bit pipeline analog-to-digital converter, designed for use in an integrated three-dimensional ultrasound imaging system, has been implemented in a 0.25-/spl mu/m CMOS technology. Two parallel multiplexing sample-and-hold stages are employed to multiplex a total of eight adjacent ultrasound channels, each sampled at 20 MHz. The sampled and multiplexed signals are fed into two parallel time-interleaved pipeline paths, each operating at 80 MHz. The two parallel pipelines are subsequently multiplexed into a single pipeline operating at 160 MHz to conserve area and reduce complexity. An experimental prototype of the proposed architecture occupies less than 4 mm/sup 2/ of active silicon area and shows a peak signal-to-noise-plus-distortion ratio more than 54 dB for a 2.1-MHz input signal, while dissipating only 20 mW of analog power per input channel from a 2.5-V supply.

/pdf/a-multichannel-pipeline-analog-to-digital-converter-for-an-3sz0lmcew8.pdf

K. Kaviani

Papers

Capacitive micromachined ultrasonic transducers: next-generation arrays for acoustic imaging?

A multichannel, pipeline analog–to–digital converter for an integrated 3–D ultrasound imaging system

Initial pulse-echo imaging results with one-dimensional capacitive micromachined ultrasonic transducer arrays

A multichannel pipeline analog-to-digital converter for an integrated 3-D ultrasound imaging system

K. Kaviani

Papers

Capacitive micromachined ultrasonic transducers: next-generation arrays for acoustic imaging?

A multichannel, pipeline analog&#8211;to&#8211;digital converter for an integrated 3&#8211;D ultrasound imaging system

Initial pulse-echo imaging results with one-dimensional capacitive micromachined ultrasonic transducer arrays

A multichannel pipeline analog-to-digital converter for an integrated 3-D ultrasound imaging system

A multichannel, pipeline analog–to–digital converter for an integrated 3–D ultrasound imaging system