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Ultrasonics, Ferroelectrics, and Frequency Control, IEEE Transactions on

Issue 2 • Date Feb. 2005

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  • [Front cover]

    Page(s): c1 - c2
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  • [Inside front cover]

    Page(s): i
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  • [Inside back cover]

    Page(s): ii
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  • Table of Contents - Vol. 52 No. 2

    Page(s): iii - iv
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  • Information for contributors with multimedia addition

    Page(s): 149 - 153
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  • A multimedia example

    Page(s): 154
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  • Call for papers

    Page(s): 155
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  • Call for papers

    Page(s): 156
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  • Call for papers

    Page(s): 157
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  • Introduction to the special issue on coded waveforms

    Page(s): 158 - 159
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    Although coded waveforms were first explored in the field of Radar in the years following the Second World War, it was not until the very mid-seventies and early eighties that their application was discussed in the context of ultrasound imaging. Among the earliest discussions of coded waveforms in the field of nondestructive testing are those by Newhouse and Furgason in the mid 1970's using a random noise generator as a signal source and performing correlation on the received echo signals and a reference copy of the transmitted noise signal. A few years later, in 1980, Lee and Furgason described the use of Psuedo Random Binary Sequences (PRBS), including Golay codes, in the context of phased array-based flaw detection. About the same time, Takeuchi described the use of spread spectrum techniques for medical imaging. It is interesting to reflect on the fact that these early works were presented practically simultaneously with the introduction of phased array medical ultrasound imaging. However, many significant challenges impeded the introduction of coded waveforms in commercial ultrasound scanners. For example, short time-bandwidth products are necessary to avoid distortion of received echos due to dynamic received focusing. This distortion, and indeed any other form of distortion, typically results in elevated sidelobe levels following the pulse compression stage in the receiver. Additionally, at least in the case of medical imaging, one has to contend with phase aberrating effects due to sound velocity inhomegeneity, nonlinear propagation, differential rates of frequency dependent attenuation, limited available transducer bandwidth and target motion between successive pulses. During the 1980's, O'Donnell's group at General Electric Research further investigated coded imaging resulting in publications and patents describing in detail the parameters and design tradeoffs that must be taken into account when designing a coded waveform phased array digital scanner- However, it was not until the late 1990's that General Electric successfully introduced coded waveforms to diagnostic ultrasound scanners. This was largely a result of the efforts of a team led by Chiao and Thomas whose contributions are recorded in a series of related patents. This proof that coded imaging can make a significant contribution has proven to be a catalyst to renewed interest in the field of coded waveforms. Among more recent contributions are analyses of FM chirp-based waveforms (Misaridis and Jensen) and investigations of various other binary and continuous waveforms. This recent growth in activity, as indicated by the increasing number of coded waveforms papers submitted to the IEEE Transactions and to the IEEE Ultrasonics Symposium, were the motivation for organizing this Special Issue. View full abstract»

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  • Coded excitation for diagnostic ultrasound: a system developer's perspective

    Page(s): 160 - 170
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    Resolution and penetration are primary criteria for clinical image quality. Conventionally, high bandwidth for resolution was achieved with a short pulse, which results in a tradeoff between resolution and penetration. Coded excitation extends the bounds of this tradeoff by increasing signal-to-noise ratio (SNR) through appropriate coding on transmit and decoding on receive. Although used for about 50 years in radar, coded excitation was successfully introduced into commercial ultrasound scanners only within the last 5 years. This delay is at least partly due to practical implementation issues particular to diagnostic ultrasound, which are the focus of this paper. After reviewing the basics of biphase and chirp coding, we present simulation results to quantify tradeoffs between penetration and resolution under frequency-dependent attenuation, dynamic focusing, and nonlinear propagation. Next, we compare chirp and Golay code performance with respect to image quality and system requirements, then we show clinical images that illustrate the current applications of coded excitation in B-mode, harmonic, and flow imaging. View full abstract»

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  • Coded excitation for synthetic aperture ultrasound imaging

    Page(s): 171 - 176
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    Peak acoustic power limits the signal-to-noise ratio (SNR) of real-time ultrasound images. For most conventional scan formats, however, the average power is well below heating limits. This means the SNR can be significantly increased using coded excitation. A coded system transmits a broadband, temporally elongated excitation pulse with a finite time-bandwidth product. The received signal must be decoded to produce an imaging pulse with improved SNR resulting from the higher average power in the elongated excitation. Decoding can produce significant range side lobes, however, greatly reducing image quality. All practical coding designs, therefore, represent a trade-off between SNR gain and range side lobes. A specific coding scheme appropriate for synthetic aperture imaging is presented. A 14.5 dB SNR improvement with acceptable range side lobes is demonstrated on a forward-looking imaging system appropriate for intravascular applications. View full abstract»

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  • Use of modulated excitation signals in medical ultrasound. Part I: basic concepts and expected benefits

    Page(s): 177 - 191
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    This paper, the first from a series of three papers on the application of coded excitation signals in medical ultrasound, discusses the basic principles and ultrasound-related problems of pulse compression. The concepts of signal modulation and matched filtering are given, and a simple model of attenuation relates the matched filter response with the ambiguity function, known from radar. Based on this analysis and the properties of the ambiguity function, the selection of coded waveforms suitable for ultrasound imaging is discussed. It is shown that linear frequency modulation (FM) signals have the best and most robust features for ultrasound imaging. Other coded signals such as nonlinear FM and binary complementary Golay codes also have been considered and characterized in terms of signal-to-noise ratio (SNR) and sensitivity to frequency shifts. Using the simulation program Field II, it is found that in the case of linear FM signals, a SNR improvement of 12 to 18 dB can be expected for large imaging depths in attenuating media, without any depth-dependent filter compensation. In contrast, nonlinear FM modulation and binary codes are shown to give a SNR improvement of only 4 to 9 dB when processed with a matched filter. Other issues, such as depth-dependent matched filtering and use of filters other than the matched filter (inverse and Wiener filters) also are addressed. View full abstract»

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  • Use of modulated excitation signals in medical ultrasound. Part II: design and performance for medical imaging applications

    Page(s): 192 - 207
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    For pt.I, see ibid., vol.52, no.2, p.177-91 (2005). In the first paper, the superiority of linear FM signals was shown in terms of signal-to-noise ratio and robustness to tissue attenuation. This second paper in the series of three papers on the application of coded excitation signals in medical ultrasound presents design methods of linear FM signals and mismatched filters, in order to meet the higher demands on resolution in ultrasound imaging. It is shown that for the small time-bandwidth (TB) products available in ultrasound, the rectangular spectrum approximation is not valid, which reduces the effectiveness of weighting. Additionally, the distant range sidelobes are associated with the ripples of the spectrum amplitude and, thus, cannot be removed by weighting. Ripple reduction is achieved through amplitude or phase predistortion of the transmitted signals. Mismatched filters are designed to efficiently use the available bandwidth and at the same time to be insensitive to the transducer's impulse response. With these techniques, temporal sidelobes are kept below 60 to 100 dB, image contrast is improved by reducing the energy within the sidelobe region, and axial resolution is preserved. The method is evaluated first for resolution performance and axial sidelobes through simulations with the program Field II. A coded excitation ultrasound imaging system based on a commercial scanner and a 4 MHz probe driven by coded sequences is presented and used for the clinical evaluation of the coded excitation/compression scheme. The clinical images show a significant improvement in penetration depth and contrast, while they preserve both axial and lateral resolution. At the maximum acquisition depth of 15 cm, there is an improvement of more than 10 dB in the signal-to-noise ratio of the images. The paper also presents acquired images, using complementary Golay codes, that show the deleterious effects of attenuation on binary codes when processed with a matched filter, als- - o confirmed by the presented simulated images. View full abstract»

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  • Use of modulated excitation signals in medical ultrasound. Part III: high frame rate imaging

    Page(s): 208 - 219
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    For pt.II, see ibid., vol.52, no.2, p.192-207 (2005). This paper, the last from a series of three papers on the application of coded excitation signals in medical ultrasound, investigates the possibility of increasing the frame rate in ultrasound imaging by using modulated excitation signals. Linear array-coded imaging and sparse synthetic transmit aperture imaging are considered, and the trade-offs between frame rate, image quality, and SNR are discussed. It is shown that FM codes can be used to increase the frame rate by a factor of two without a degradation in image quality and by a factor of 5, if a slight decrease in image quality can be accepted. The use of synthetic transmit aperture imaging is also considered, and it is here shown that Hadamard spatial encoding in transmit with FM emission signals can be used to increase the frame rate by 12 to 25 times with either a slight or no reduction in signal-to-noise ratio and image quality. By using these techniques, a complete ultrasound-phased array image can be created using only two emissions. View full abstract»

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  • Spatio-temporal coding in complex media for optimum beamforming: the iterative time-reversal approach

    Page(s): 220 - 230
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    Spatio-temporal encoding in transmit and receive modes is of major importance in the development of ultrasound imaging devices. Classically, the assumption of constant sound speed in the medium allows one to restrict the beamforming process to the application of a cylindrical time-delay law on the elements of a multiple-transducer array. Here, an iterative time-reversal method capable of taking into account all the heterogeneities of the medium, concerning density, speed of sound, and absorption variations is proposed. It is shown that this iterative focusing process converges toward a spatio-temporal inverse filter focusing, the first step of the process being a time-reversal focusing on the targeted point. This method can be seen as a calibration process and has been successfully applied to transskull focusing and intraplate echoes suppression. It is leading the way to promising applications such as high-resolution ultrasonic brain imaging and high-resolution focusing through complex reverberating media, in nondestructive testing and telecommunications. This work highlights the advantages of using spatio-temporal coding to focus through complex media. Such codes require the use of fully programmable, multichannel electronics to implement this technique in real time. View full abstract»

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  • Coded pulse excitation for ultrasonic strain imaging

    Page(s): 231 - 240
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    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 (SNR/sub d/), local modulation transfer function (LMTF), and contrast-to-noise ratio for strain (CNR/sub /spl epsi//). 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. View full abstract»

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  • Harmonic chirp imaging method for ultrasound contrast agent

    Page(s): 241 - 249
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    Coded excitation is currently used in medical ultrasound to increase signal-to-noise ratio (SNR) and penetration depth. We propose a chirp excitation method for contrast agents using the second harmonic component of the response. This method is based on a compression filter that selectively compresses and extracts the second harmonic component from the received echo signal. Simulations have shown a clear increase in response for chirp excitation over pulse excitation with the same peak amplitude. This was confirmed by two-dimensional (2-D) optical observations of bubble response with a fast framing camera. To evaluate the harmonic compression method, we applied it to simulated bubble echoes, to measured propagation harmonics, and to B-mode scans of a flow phantom and compared it to regular pulse excitation imaging. An increase of approximately 10 dB in SNR was found for chirp excitation. The compression method was found to perform well in terms of resolution. Axial resolution was in all cases within 10% of the axial resolution from pulse excitation. Range side-lobe levels were 30 dB below the main lobe for the simulated bubble echoes and measured propagation harmonics. However, side-lobes were visible in the B-mode contrast images. View full abstract»

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  • Design and preliminary tests of a family of adaptive waveforms to measure blood vessel diameter and wall thickness

    Page(s): 250 - 260
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    In this article, we consider the adaptive design of waveforms to be used in vascular ultrasound. The advantage of these waveforms, when used with the proposed processing scheme, is that their application results in increased reflected energy, especially when compared with more conventional methods such as a short-gated sinusoid. This increase in reflected energy has potential to permit inferences to be made about wall thickness and vessel diameter from deeper vessels than possible with more traditional techniques. Here, the use of waveforms of the type A(t)e(j(kt/sup 2/)), 0/spl les/t/spl les/b, where A(t) is a specially designed envelope and k a sweep frequency, are proposed. Theorems are proved that describe how to choose an A(t) which results in either a maximum of reflected energy signal-to-noise ratio (SNR), or range resolution. The design of the waveform is adaptive in that both A(t) and k are derived in consideration of a specific blood vessel whose transfer function has been obtained experimentally. Numerical simulations illustrate the advantages of using these waveforms as well as the effects of the parameters. A simple experimental implementation of the methodology is presented on a brachial artery. The measurement of the impulse response of the artery is presented in this context. Results indicate that a processing gain in SNR over the instantaneous values obtained from the raw echo waveforms of 11 dB to 14 dB can be obtained via this methodology. View full abstract»

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  • ICARUS: imaging pulse compression algorithm through remapping of ultrasound

    Page(s): 261 - 279
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    In this work, we tackle the problem of applying to echographic imaging, those synthetic aperture focusing techniques (SAFT) in the frequency domain commonly used in the field of synthetic aperture radars (SAR). The aim of this research is to improve echographic image resolution by using chirp transmit signals, and by performing pulse compression in both dimensions (depth and lateral). The curved geometry present in the unfocused radio-frequency (RF) ultrasonic image is the main cause of inaccuracy in the direct application of frequency domain SAFT algorithms to echographic imaging. The focusing method proposed in this work, after pulse compression in the depth dimension, performs lateral focusing in the mixed depth-lateral spatial frequency domain by means of a depth variant remapping followed by lateral pulse compression. This technique has the advantage of providing a resolution that is uniform in nonfrequency selective attenuation media, and improved with respect to conventional time domain SAFT, without requiring the acquisition and processing of channel data necessary for the most advanced synthetic transmit aperture techniques. Therefore, the presented method is suitable for easy real-time implementation with current generation hardware. View full abstract»

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  • Noncontact, high-resolution ultrasonic imaging of wood samples using coded chirp waveforms

    Page(s): 280 - 288
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    A noncontact ultrasonic inspection technique has been developed to study the properties of wood samples in air. The system makes use of two broad bandwidth capacitive transducers, combined with signal processing techniques. A coded chirp signal was used in the current application to provide a waveform that could be postprocessed to provide sufficient sensitivity for transmission across samples of wood. It is shown in this paper that the signal-to-noise ratio (SNR) can be greatly improved using two signal recovery techniques, namely pulse compression and swept frequency multiplication (SFM). A simulation of both techniques is presented and compared to experimental data. As seen from the experimental results, it is possible to perform noncontact ultrasonic experiments to extract a range of useful information such as ring density and the presence of microcracks. View full abstract»

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  • The clock model and its relationship with the Allan and related variances

    Page(s): 289 - 296
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    The clock errors are modeled by stochastic differential equations (SDE) and the relationships between the diffusion coefficients used in SDE and the Allan variance, a typical tool used to estimate clock noise, are derived. This relationship is fundamental when a mathematical clock model is used, for example in Kalman filter, noise estimation, and clock prediction activities. View full abstract»

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  • Coupled determination of gravimetric and elastic effects on two resonant chemical sensors: Love wave and microcantilever platforms

    Page(s): 297 - 303
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    The objective of this paper is to couple theoretical and experimental results from microcantilevers and Love-wave acoustic devices in order to identify and separate mass loading effects from elastic effects. This is important in the perspective of sensing applications. For that, a thin-film polymer is deposited on both resonant platforms. It is demonstrated that microcantilevers are essentially mass sensitive. They allow one to determine the polymer layer thickness, which is validated by optical profilometry measurements. Then, taking into account this thickness, theoretical modeling and experimental measurements with Love-wave devices permit one to estimate an equivalent elastic shear modulus of the thin-film polymer at high frequency. Results are interesting if one is to fully understand and optimize (bio)chemical sensor responses. View full abstract»

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  • Response of fiber Bragg gratings to longitudinal ultrasonic waves

    Page(s): 304 - 312
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    In the last years, fiber optic sensors have been widely exploited for several sensing applications, including static and dynamic strain measurements up to acoustic detection. Among these, fiber Bragg grating sensors have been indicated as the ideal candidate for practical structural health monitoring in the light of their unique advantages over conventional sensing devices. Although this class of sensors has been successfully tested for static and low-frequency measurements, the identification of sensor performances for high-frequency detection, including acoustic emission and ultrasonic investigations, is required. To this aim, the analysis of feasibility on the use of fiber Bragg grating sensors as ultrasonic detectors has been carried out. In particular, the response of fiber Bragg gratings subjected to the longitudinal ultrasonic (US) field has been theoretically and numerically investigated. Ultrasonic field interaction has been modeled, taking into account the direct deformation of the grating pitch combined with changes in local refractive index due to the elasto-optic effect. Numerical results, obtained for both uniform and Gaussian-apodized fiber Bragg gratings, show that the grating spectrum is strongly influenced by the US field in terms of shape and central wavelength. In particular, a key parameter affecting the grating response is the ratio between the US wavelength and the grating length. Normal operation characterized by changes in the wavelength of undistorted Bragg peak is possible only for US wavelengths longer than the grating length. For US wavelengths approaching the grating length, the wavelength change is accompanied by subpeaks formation and main peak amplitude modulation. This effect can be attributed to the nonuniformity of the US perturbation along the grating length. At very high US frequencies, the grating is not sensitive any longer. The results of this analysis provide useful tools for the design of grating-based ultrasound sensors fo- - r meeting specific requirements in terms of field intensity and frequencies. View full abstract»

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  • Ultrasonic data compression via parameter estimation

    Page(s): 313 - 325
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    Ultrasonic imaging in medical arid industrial applications often requires a large amount of data collection. Consequently, it is desirable to use data compression techniques to reduce data and to facilitate the analysis and remote access of ultrasonic information. The precise data representation is paramount to the accurate analysis of the shape, size, and orientation of ultrasonic reflectors, as well as to the determination of the properties of the propagation path. In this study, a successive parameter estimation algorithm based on a modified version of the continuous wavelet transform (CWT) to compress and denoise ultrasonic signals is presented. It has been shown analytically that the CWT (i.e., time/spl times/frequency representation) yields an exact solution for the time-of-arrival and a biased solution for the center frequency. Consequently, a modified CWT (MCWT) based on the Gabor-Helstrom transform is introduced as a means to exactly estimate both time-of-arrival and center frequency of ultrasonic echoes. Furthermore, the MCWT also has been used to generate a phase/spl times/bandwidth representation of the ultrasonic echo. This representation allows the exact estimation of the phase and the bandwidth. The performance of this algorithm for data compression and signal analysis is studied using simulated arid experimental ultrasonic signals. The successive parameter estimation algorithm achieves a data compression ratio of (1 5N/J), where J is the number of samples and N is the number of echoes in the signal. For a signal with 10 echoes and 2048 samples, a compression ratio of 96% is achieved with a signal-to-noise ratio (SNR) improvement above 20 dB. Furthermore, this algorithm performs robustly, yields accurate echo estimation, and results in SNR enhancements ranging from 10 to 60 dB for composite signals having SNR as low as -10 dB. View full abstract»

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Aims & Scope

IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control focuses on the theory, design, and application on generation, transmission, and detection of bulk and surface mechanical waves.

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Editor-in-Chief
Steven Freear
s.freear@leeds.ac.uk