A new system for real-time synthetic aperture ultrasonic imaging

The authors devised a way to generate in real time a cross-sectional image of an object with uniformly high resolution based on the synthetic aperture focusing technique (SAFT). A computer simulation was conducted to study the effects of essential parameters on the resulting images. An imaging system was built that produces a cross-sectional image composed of an assembly of line images of depth direction, i.e. processed A-scan images, and displays a scroll picture on a CRT (cathode ray tube) with no interruption regardless of the object size. It takes only 3 ms from the start of transmission of the ultrasonic wave to the completion of a line image reconstruction, and the framed image on a CRT is updated at the TV rate of 1/30 s. Imaging experiments were conducted using the system, and its expected performance was demonstrated.     

A nondestructive ultrasonic imaging system for detection of flawsin metal blocks

An ultrasonic imaging system is presented for the nondestructive evaluation of internal defects in metal blocks. The surfaces of the blocks are coarsely scanned by a single ultrasonic transducer to record multiple A-scans. The recorded A-scans are processed by a synthetic-aperture focusing technique (SAFT) algorithm to search for defects in a metal specimen. The processed information is then sent to a graphics package to be displayed on a CRT screen of a personal computer. The current linear imaging technique is presented and is compared to the SAFT algorithm. The theory and limitations of the SAFT algorithm as well as the hardware implementation are discussed. Images of defects in aluminum blocks are illustrated with varying operating parameters showing the performance of the imaging system     

A high-frame rate high-frequency ultrasonic system for cardiac imaging in mice

We report the development of a high-frequency (30-50 MHz), real-time ultrasonic imaging system for cardiac imaging in mice. This system is capable of producing images at 130 frames per second (fps) with a spatial resolution of less than 50 microm. A novel mechanical sector probe was developed that utilizes a magnetic drive mechanism and custom-built servo controller for high speed and accuracy. Additionally, a very light-weight (< 0.28 g), single-element transducer was constructed and used to reduce the mass load on the motor. The imaging electronics were triggered according to the angular position of the transducer in order to compensate for the varying speed of the sector motor. This strategy ensured the production of equally spaced scan lines with minimal jitter. Wire phantom testing showed that the system axial and lateral resolutions were 48 microm and 72 microm, respectively. In vivo experiments showed that high-frequency ultrasonic imaging at 130 fps is capable of showing a detailed depiction of a beating mouse heart.     

Linear system models for ultrasonic imaging: application to signal statistics

Linear equations for modeling echo signals from shift-variant systems forming ultrasonic B-mode, Doppler, and strain images are analyzed and extended. The approach is based on a solution to the homogeneous wave equation for random inhomogeneous media. When the system is shift-variant, the spatial sensitivity function-defined as a spatial weighting function that determines the scattering volume for a fixed point of time-has advantages over the point-spread function traditionally used to analyze ultrasound systems. Spatial sensitivity functions are necessary for determining statistical moments in the context of rigorous image quality assessment, and they are time-reversed copies of point-spread functions for shift variant systems. A criterion is proposed to assess the validity of a local shift-invariance assumption. The analysis reveals realistic situations in which in-phase signals are correlated to the corresponding quadrature signals, which has strong implications for assessing lesion detectability. Also revealed is an opportunity to enhance near- and far-field spatial resolution by matched filtering unfocused beams. The analysis connects several well-known approaches to modeling ultrasonic echo signals.     

Subaperture processing for ultrasonic imaging

Ultrasonic subaperture processing using aperture synthesis and beam space interpolation is presented. The number of beam lines scanning the image plane for a given transmit-receive subaperture combination is chosen according to the spatial sampling criteria for that combination. On each beam line, echo signals over the entire array are collected through electronic multiplexing of array channels, where the transmit subaperture at the transducer center is fired K successive times, with K equal to the number of nonoverlapping receive subapertures. For every receive subaperture, the number of beam lines is increased through digital interpolation using a linear filter with spatial frequency band associated with the subaperture. Interpolated beam lines from all receive subapertures are then added to obtain a high resolution sector image. The efficiency of subaperture processing for different system configurations is tested on experimental rf data acquired from two different phantoms using a 3.5 MHz, 128-element transducer array. The proposed subaperture processing reduces the number of firings for data acquisition, and thus allows real-time imaging with very low susceptibility to motion artifacts.     

Airborne ultrasonic vortex generation using flexible ferroelectrets

Cellular ferroelectrets exhibit interesting electromechanical- acoustical characteristics. Their recent appearance and remarkable properties open up new possibilities for the design and development of ultrasonic transducers. In particular, the feasibility of fabricating ultrasonic vortex generators using ferroelectret films is demonstrated in this work. To this end, a transducer prototype was built by gluing the material onto a tangential-helical surface (outer diameter: 40 mm, pitch: 3.45 mm). Experimental results agree well with the theoretical estimation of the pressure and phase of the acoustic field in the near field and far field, which corroborates the potential of ferroelectrets to customize special acoustic fields. Furthermore, the proposed fabrication procedure is inexpensive and represents a new alternative for exploring and analyzing the special characteristics of acoustical helical wavefronts.     

Dry-contact technique for high-resolution ultrasonic imaging

To accomplish a high-resolution ultrasonic imaging without wetting a sample, the efficiency of the dry-contact ultrasonic transmission is discussed. In this study, a dry-contact interface is formed on a sample by inserting a thin film between water and a sample, and the pressure is working on the interface by evacuating the air between the film and the sample. A model of dry-contact ultrasonic transmission is presented to assess the signal loss accompanied with the transmission. From the determination of the signal loss caused by the transmission using various films, it was found that the higher frequency ultrasound is transmitted effectively into the sample by selecting an optimum film, which can keep the displacement continuity between the film and the sample during ultrasonic transmission. Finally, ultrasonic imaging with the sufficient signal-to-noise ratio (SNR) and high lateral resolution was performed on the delamination in a package and the jointing interface of the ball-grid-array package without wetting the packages.     

三维超声地震模型实时成像系统

室内水槽实验是研究地质沉积过程及其演化规律的重要手段,高精度获取沉积过程中的地质体的变化是这类模拟实验非常关键的环节。文章介绍了新研制的三维超声地震模型实时成像系统的主要组成及关键技术。该系统用于模拟海上地震,可以在沉积实验后通过快速测量及对数据的实时偏移处理与成像可以获取变化的多层复杂地质模型动态图像,极大地提高了实验效率和成像精度。该系统具有良好的实时性、成像质量以及探测范围,在对研究地质沉积、海洋地质以及三维地震模型研究等方面有着广泛应用前景。     

An FPGA-based ultrasound imaging system using capacitive micromachined ultrasonic transducers

We report the design and experimental results of a field-programmable gate array (FPGA)-based real-time ultrasound imaging system that uses a 16-element phased-array capacitive micromachined ultrasonic transducer fabricated using a fusion bonding process. The imaging system consists of the transducer, discrete analog components situated on a custom-made circuit board, the FPGA, and a monitor. The FPGA program consists of five functional blocks: a main counter, transmit and receive beamformer, receive signal pre-processing, envelope detection, and display. No dedicated digital signal processor or personal computer is required for the imaging system. An experiment is carried out to obtain the sector B-scan of a 4-wire target. The ultrasound imaging system demonstrates the possibility of an integrated system-in-a-package solution.     

Airborne lidar imaging of salmon

Lidar images of adult salmon are presented. The lidar system is built around a pulsed green laser and a gated intensified CCD camera. The camera gating is timed to collect light scattered from the turbid water below the fish to produce shadows in the images. Image processing increases the estimated contrast-to-noise ratio from 3.4 in the original image to 16.4 by means of a matched filter.     

Rotary encoding for intravascular ultrasonic imaging systems

Images produced with an intravascular ultrasound system (IVUS) can be distorted because of uncertainty in the instantaneous angular position of a rotating ultrasonic transducer. A rotary encoder placed in proximity to the transducer is required to detect the problem; however, size constraints make a conventional electromechanical or optomechanical encoder difficult to implement. Measurements that test the feasibility of a software-derived encoder, based of the rate of decorrelation of ultrasonic RF lines with angle, are reported. Provided that the instantaneous angular velocity of the transducer can be measured, adjustments can be made to the pulse rate of the transducer, which would eliminate the image distortion.     

Experimental results with a real-time adaptive ultrasonic imaging system for viewing through distorting media

An online adaptive phased-array ultrasonic imaging system capable of markedly improving the detectability of targets viewed through inhomogeneous media is described. An online adaptive phase correction technique implemented on a research phased-array scanner is described. The theoretical basis for this technique is presented by describing the relationship between the magnitude of phase aberrations and the regional brightness of speckle and pointlike targets. The system currently generates a corrected image in approximately 0.1 s and utilizes no prior knowledge of the aberrating media or the target. The adaptive imaging algorithm uses regional target brightness as a quality factor. The results of in vitro tests with this system using electronic and physical aberrators for both diffuse and pointlike targets are presented.     

High-accuracy data acquisition architectures for ultrasonic imaging

This paper proposes a novel architecture for a data acquisition system intended to support the next generation of ultrasonic imaging instruments operating at or above 100 MHz. Existing systems have relatively poor signal-to-noise ratios and are limited in terms of their maximum data sampling rate, both of which are improved by a combination of embedded averaging and embedded interleaved sampling. "On-the-fly" pipelined operation minimizes control overheads for signal averaging. A two-clock sampling timing system provides for effective sampling rates that are a factor of 20 or more above the basic sampling rate of the analog-to-digital converter (ADC). The system uses commercial field-programmable gate array devices operated at clock frequencies commensurable with the ADC clock. Implementation is via the Xilinx Xtreme digital signal processing development kit, available at low cost. Sample rates of up to 2160 MHz have been achieved in combination with up to 16384 coherent averages using the above-mentioned off-the-shelf hardware.     

Beam transformation techinques for ultrasonic medical imaging

Data acquisition rates in pulsed ultrasound scanners are limited by the speed of sound in the human body. This poses severe limitations to the design of future ultrasound equipment, such as 3-D imaging scanners. The authors describe a technique for higher data acquisition rates based on the simultaneous transmission of multiple beams. By using a linear combination of the received beams, interbeam interference due to the sidelobe energy of the transmitting beams can be significantly reduced. The transformation coefficients are found by using a least squares minimization criterion. A simulation environment used for the evaluation of the authors' methodologies and various simulation results are presented.     

Coded pulse excitation for ultrasonic strain imaging

Decorrelation strain noise can be significantly reduced in low echo-signal-to-noise (eSNR) conditions using coded excitation. Large time-bandwidth-product (>30) pulses are transmitted into tissue mimicking phantoms with 2.5-mm diameter inclusions that mimic the elastic properties of breast lesions. We observed a 5-10 dB improvement in eSNR that led to a doubling of the depth of focus for strain images with no reduction of spatial resolution. In high eSNR conditions, coded excitation permits the use of higher carrier frequencies and shorter correlation windows to improve the attainable spatial resolution for strain relative to that obtained with conventional short pulses. This paper summarizes comparative studies of strain imaging in noise-limited conditions obtained by short pulses and four common aperiodic codes (chirp, Barker, suboptimal, and Golay) as a function of attenuation, eSNR and applied strain. Imaging performance is quantified using SNR for displacement (SNRd), local modulation transfer function (LMTF), and contrast-to-noise ratio for strain (CNRepsilon). We found that chirp and Golay codes are the most robust for imaging soft tissue deformation using matched filter decoding. Their superior performance is obtained by balancing the need for low-range lobes, large eSNR improvement, and short-code duration.     

Piezo-polymer transducers for ultrasonic imaging in air

Airborne polyvinylidenefluoride transducers have been designed for robotic applications in air. Characteristics of transducer prototypes are: working frequencies from 61 kHz to 86 kHz, quality factor Q from 4 to 6, and two-way insertion loss of about 90 dB. The small dimension, the lightness, and the low-cost fabrication technology allow the development of arrays or matrices for ultrasonic imaging systems in air. In this work two different image reconstruction algorithms are proposed: the first carries out a combined spectral and aperture synthesis for detecting isolated scatterers with a spatial resolution of about 2 mm; the second is based on an accurate ranging algorithm with sub-millimeter resolution at distances up to 50 cm. Finally, this work's application to the reconstruction of three-dimensional object profiles is discussed.     

一种用于相控阵B超成像的数字波束合成器的设计

本文介绍了一种能沿任意波束指向全程动态聚焦的数字波束合成器的设计，此合成器用于相控阵Ｂ超成像系统。回声信号的延时量分解为“指向延时”和“聚焦延时”两部分，分别用产生相控发射激励的时序逻辑电路和一个“动态聚焦延时量表”实现。通过对Ａ／Ｄ采样时钟的控制及对Ａ／Ｄ采样时钟、地址计数时钟和存储器写时钟的时序配合，实现了同相位数据点采样及无冗余数据的缓冲存储器。所设计的数字相控波束合成器只用廉价的高速数字电路即可实现，成本极低。实验结果验证了该方案的可行性。     

Post-beamforming second-order Volterra filter for pulse-echo ultrasonic imaging

We present a new algorithm for deriving a second-order Volterra filter (SVF) capable of separating linear and quadratic components from echo signals. Images based on the quadratic components are shown to provide contrast enhancement between tissue and ultrasound contrast agents (UCAs) without loss in spatial resolution. It is also shown that the quadratic images preserve the low scattering regions due to their high dynamic range when compared with standard B-mode or harmonic images. A robust algorithm for deriving the filter has been developed and tested on real-time imaging data from contrast and tissue-mimicking media. Illustrative examples from image targets containing contrast agent and tissue-mimicking media are presented and discussed. Quantitative assessment of the contrast enhancement is performed on both the RF data and the envelope-detected log-compressed image data. It is shown that the quadratic images offer levels of enhancement comparable or exceeding those from harmonic filters while maintaining the visibility of low scattering regions of the image.     

Surface micromachined capacitive ultrasonic transducer for underwater imaging

Abstract

This study presents the primary design, fabrication process and device measurement of a Capacitive Micromachined Ultrasonic Transducer (CMUT) for underwater acoustic imaging. Theoretical analysis and computer simulations of the CMUT are performed. The CMUT fabrication uses the full surface micromachining techniques of the Micro Electro Mechanical System (MEMS). These techniques are Low Pressure Chemical Vapor Deposition (LPCVD), photolithography, Reactive Ion Etching System (RIE) dry etching, sacrificial layer wet etching, metal thermal evaporation coating and Plasma‐Enhanced Chemical Vapor Deposition (PECVD). Several important issues regarding fabrication are discussed. The measured input impedance of the CMUT is in agreement with the theoretical prediction. The received signal has a 35 dB signal‐to‐noise ratio indicating that practical applications of the immersion CMUT are feasible and that the radiation pattern measurement of the CMUT array has good beamforming characteristics for underwater imaging.