Pulse-echo System Research Articles

Linear System Models for Ultrasonic Imaging: Intensity Signal Statistics.

Despite a great deal of work characterizing the statistical properties of radio frequency backscattered ultrasound signals, less is known about the statistical properties of demodulated intensity signals. Analysis of intensity is made more difficult by a strong nonlinearity that arises in the process of demodulation. This limits our ability to characterize the spatial resolution and noise properties of B-mode ultrasound images. In this paper, we generalize earlier results on two-point intensity covariance using a multivariate systems approach. We derive the mean and autocovariance function of the intensity signal under Gaussian assumptions on both the object scattering function and acquisition noise, and with the assumption of a locally shift-invariant pulse-echo system function. We investigate the limiting cases of point statistics and a uniform scattering field with a stationary distribution. Results from validation studies using simulation and data from a real system applied to a uniform scattering phantom are presented. In the simulation studies, we find errors less than 10% between the theoretical mean and variance, and sample estimates of these quantities. Prediction of the intensity power spectrum (PS) in the real system exhibits good qualitative agreement (errors less than 3.5 dB for frequencies between 0.1 and 10 cyc/mm, but with somewhat higher error outside this range that may be due to the use of a window in the PS estimation procedure). We also replicate the common finding that the intensity mean is equal to its standard deviation (i.e., signal-to-noise ratio = 1) for fully developed speckle. We show how the derived statistical properties can be used to characterize the quality of an ultrasound linear array for low-contrast patterns using generalized noise-equivalent quanta directly on the intensity signal.

전영역 펄스-에코 초음파전파영상화 시스템의 CN-235의 도색된 샌드위치 조종면 In-situ 비파괴평가 기술

전영역 펄스-에코 초음파전파영상화 시스템의 CN-235의 도색된 샌드위치 조종면 In-situ 비파괴평가 기술

Defect visualization of aircraft UHF antenna radome using full-field pulse-echo ultrasonic propagation imaging system

Most of aircraft antennas usually have various types of radome made of composite materials for protecting antenna structures. However, these antenna radome structures, which are installed on the outside of airplane, are easy to be damaged by external forces such as drag, foreign object, bird strike and others. In this study, full-field pulse-echo ultrasonic propagation imaging (PE UPI) system is proposed as the non-destructive inspection technique to visualize manufacturing defects in composite antenna radome. Based on the results of the sample case study, it is shown that the ultrasonic wave propagation imaging (UWPI) that is generated by the proposed full-field PE UPI system is able to highlight the intact internal condition of antenna structure and its defect area. Additional damage visualization techniques like ultrasonic energy mapping (UEM), variable time window amplitude map (VTWAM) and also ultrasonic spectral imaging (USI) algorithms are applied to improve the reliability of the damage visualization. It can be concluded that the proposed PE UPI system is an effective non-destructive inspection technique for the composite radome structures.

Enhanced resolution pulse-echo imaging with stabilized pulses.

Many pulse-echo imaging systems use focused beams to improve lateral resolution. The beam width is determined by the choice of source and apodization function, the frequency, and the physics of focusing. Postprocessing strategies to improve lateral resolution can be limited by the need for conditioning the mathematics of inverse filtering, due to instabilities. We present an analysis that defines key constraints on sampled versions of lateral beampatterns. Within these constraints are useful symmetric beampatterns, which, when properly sampled, can have a stable inverse filter. A framework for analysis and processing is described and applied to phantoms and tissues to demonstrate the improvements that can be realized.

Adaptive Light Modulation for Improved Resolution and Efficiency in All-Optical Pulse-Echo Ultrasound.

In biomedical all-optical pulse-echo ultrasound systems, ultrasound is generated with the photoacoustic effect by illuminating an optically absorbing structure with a temporally modulated light source. Nanosecond range laser pulses are typically used, which can yield bandwidths exceeding 100 MHz. However, acoustical attenuation within tissue or nonuniformities in the detector or source power spectra result in energy loss at the affected frequencies and in a reduced overall system efficiency. In this work, a laser diode is used to generate linear and nonlinear chirp optical modulations that are extended to microsecond time scales, with bandwidths constrained to the system sensitivity. Compared to those obtained using a 2-ns pulsed laser, pulse-echo images of a phantom obtained using linear chirp excitation exhibit similar axial resolution (99 versus 92 μm, respectively) and signal-to-noise ratios (SNRs) (10.3 versus 9.6 dB). In addition, the axial point spread function (PSF) exhibits lower sidelobe levels in the case of chirp modulation. Using nonlinear (time-stretched) chirp excitations, where the nonlinearity is computed from measurements of the spectral sensitivity of the system, the power spectrum of the imaging system was flattened and its bandwidth broadened. Consequently, the PSF has a narrower axial extent and still lower sidelobe levels. Pulse-echo images acquired with time-stretched chirps as optical modulation have higher axial resolution (64 μm) than those obtained with linear chirps, at the expense of a lower SNR (6.8 dB). Using a linear or time-stretched chirp, the conversion efficiency from optical power to acoustical pressure improved by a factor of 70 or 61, respectively, compared to that obtained with pulsed excitation.

A computerized tomography system for transcranial ultrasound imaging.

Hardware for tomographic imaging presents both challenge and opportunity for simplification when compared with traditional pulse-echo imaging systems. Specifically, point diffraction tomography does not require simultaneous powering of elements, in theory allowing just a single transmit channel and a single receive channel to be coupled with a switching or multiplexing network. In our ongoing work on transcranial imaging, we have developed a 512-channel system designed to transmit and/or receive a high voltage signal from/to arbitrary elements of an imaging array. The overall design follows a hierarchy of modules including a software interface, microcontroller, pulse generator, pulse amplifier, high-voltage power converter, switching mother board, switching daughter board, receiver amplifier, analog-to-digital converter, peak detector, memory, and USB communication. Two pulse amplifiers are included, each capable of producing up to 400Vpp via power MOSFETS. Switching is based around mechanical relays that allow passage of 200V, while still achieving switching times of under 2ms, with an operating frequency ranging from below 100kHz to 10MHz. The system is demonstrated through ex vivo human skulls using 1MHz transducers. The overall system design is applicable to planned human studies in transcranial image acquisition, and may have additional tomographic applications for other materials necessitating a high signal output.

A computerized tomography system for transcranial ultrasound imaging

Hardware for tomographic imaging presents both challenge and opportunity for simplification when compared with traditional pulse-echo imaging systems. Specifically, point diffraction tomography does not require simultaneous powering of elements, in theory allowing just a single transmit channel and a single receive channel to be coupled with a switching or multiplexing network. In our ongoing work on transcranial imaging, we have developed a 512-channel system designed to transmit and/or receive a high voltage signal from/to arbitrary elements of an imaging array. The overall design follows a hierarchy of modules including a software interface, microcontroller, pulse generator, pulse amplifier, high-voltage power converter, switching mother board, switching daughter board, receiver amplifier, analog-to-digital converter, peak detector, memory, and USB communication. Two pulse amplifiers are included, each capable producing up to 400 Vpp via power MOSFETS. Switching is based around mechanical relays that allow passage of 200 V, while still achieving switching times of under 2 ms, with an operating frequency ranging from below 100 kHz to 10 MHz. The system is demonstrated through ex vivo human skulls using 1 MHz transducers. The overall system design is applicable to planned human studies in transcranial image acquisition, and may have additional tomographic applications for other materials necessitating a high signal output. [Work was supported by NIH R01 EB014296.]

Power MOSFET–Diode–Based Limiter for High-Frequency Ultrasound Systems

The purpose of the limiter circuits used in the ultrasound imaging systems is to pass low-voltage echo signals generated by ultrasonic transducers while preventing high-voltage short pulses transmitted by pulsers from damaging front-end circuits. Resistor-diode-based limiters (a 50 Ω resistor with a single cross-coupled diode pair) have been widely used in pulse-echo measurement and imaging system applications due to their low cost and simple architecture. However, resistor-diode-based limiters may not be suited for high-frequency ultrasound transducer applications since they produce large signal conduction losses at higher frequencies. Therefore, we propose a new limiter architecture utilizing power MOSFETs, which we call a power MOSFET-diode-based limiter. The performance of a power MOSFET-diode-based limiter was evaluated with respect to insertion loss (IL), total harmonic distortion (THD), and response time (RT). We compared these results with those of three other conventional limiter designs and showed that the power MOSFET-diode-based limiter offers the lowest IL (-1.33 dB) and fastest RT (0.10 µs) with the lowest suppressed output voltage (3.47 Vp-p) among all the limiters at 70 MHz. A pulse-echo test was performed to determine how the new limiter affected the sensitivity and bandwidth of the transducer. We found that the sensitivity and bandwidth of the transducer were 130% and 129% greater, respectively, when combined with the new power MOSFET-diode-based limiter versus the resistor-diode-based limiter. Therefore, these results demonstrate that the power MOSFET-diode-based limiter is capable of producing lower signal attenuation than the three conventional limiter designs at higher frequency operation.

Influence of polarization voltage on piezoelectric performance of polypropylene piezoelectret films

Piezoelectrets are a kind of space-charged electret material with a void-structure. The piezoelectric effect in such a material is related with its microstructure and space charge. In this paper the micro-structure of polypropylene (PP) films is first modified by using a pressed gas expansion process to enhance the charging capability of the films, and then a direct contact charging is carried out to polarize the expanded films. The relationship between the applied voltage and the space charge density, and the influence of the applied voltage on the piezoelectric performance of PP films are investigated. Results show that for 100 m thick modified PP films, the critical voltage necessary for the build-up of the macro-dipoles in the inner voids is approximately 2 kV; once the macro-dipoles are built up, the PP films will exhibit piezoelectric effect. With increasing polarization voltage, the space charge density gradually increases, resulting in significant enhancement of piezoelectric effect. For the PP films polarized at a peak voltage of 8 kV, the space charge density, d33 coefficient, and the figure of merit FOMv(d33 g33) are 0.56 mC/m2, 379 pC/N and 8.6 GPa-1, respectively. Since not only the FOMv of the PP films is almost two orders of magnitude larger than that of PVDF, but also the acoustic impedance in such a material is very low (～ 0.025 MRayl), the PP films have an obvious advantage as applied in airborne ultrasonic transmit-receive or pulse-echo systems.

Figure of merit of piezoelectret transducers for pulse-echo or transmit-receive systems for airborne ultrasound

Piezoelectret transducers may be characterized relative to other kinds of piezoelectric transducers with respect to their combined transmit-receive performance by a figure of merit (FOM). Reasonable FOMs for the specific case of broadband (non-resonant) airborne ultrasonics are discussed in this paper. These FOMs are specifically suitable for a measuring system where low input voltage to the transmitter is desirable and where the receiver is assessed by its voltage- or charge-related signal-to-noise ratio. It is found that these FOMs depend chiefly on the piezoelectric d33- and g33-coefficients.

Superresolution imaging in ultrasound B-scan imaging

A number of imaging systems exhibit speckle, which is caused by the interaction of a coherent pulse reflecting off of random reflectors. The limitations of these systems are quite serious since the speckle phenomenon creates a pattern of nulls and peaks from subresolvable scatterers or targets that are difficult to interpret. Another limitation of these pulse-echo imaging systems is that their resolution is dependent on the full spatial extent of the propagating pulse, usually several wavelengths in the axial or propagating dimension and typically longer in the transverse direction. This limits the spatial resolution to many multiples of the wavelength. This paper focuses on the particular case of ultrasound B-scan imaging and develop an inverse filter solution that eliminates both the speckle phenomenon, and the poor resolution dependency on the pulse length and width, to produce SURUS (super-resolution ultrasound) images. The key to the inverse filter is the creation of pulse shapes that have stable inverses. This is derived by use of the standard Z-transform and related properties. Although the focus of this paper is on examples from ultrasound imaging systems, the results are applicable to other pulse-echo imaging systems that also can exhibit speckle statistics.

20 years translation of ultrasonic elastography: technology innovation and clinical applications

Stiffness change of tissue is often seen with the progression of pathology. In the past two decades, ultrasonic elastography has emerged as a powerful complementary technique to B-mode ultrasonic imaging, in which strain of target tissues can be imaged and their stiffness can be assessed. The physical principle of elastography is, when deformation is generated in the tissue by an external or internal mechanical stimulus, coherent acoustic echoes can be tracked by a pulse-echo system, then induced strain inside the tissue can be estimated and the relative mapping of strain can be imaged. In addition, the viscoelastic properties of the tissue can be assessed if the mechanical wave velocity propagating in the tissue was measured. Now this technique has been developed to be more flexible and accurate. The qualitative results presented by quasi-static elastogram have changed to numerical quantification of viscoelastic modulus achieved by other innovative methods, such as sonoelastography, transient elastography, acoustic radiation force impulse imaging (ARFI), shear-wave dispersion ultrasound vibrometry (SDUV) and supersonic shear imaging (SSI). Aiming for different tissue or organs, these methods can achieve viscoelasticity assessments on various target objects with sizes from tens of millimeters to several microns. Short or trains of mechanical or acoustic radiation force impacts have been used to generate displacements with different patterns. The induced deformation in the tissue has also been tracked by different algorithms based on both cross-correlation and texture matching approaches. Some of these techniques have been studied by our group, and the combination of ultrasonic elastography and particle imaging velocimetry (PIV) using microbubble contrast agents has been also explored. Ultrasonic elastography has been verified on many normal/abnormal tissues and now widely used in clinical practice, such as on breast disease (fibroadenomas, cysts and cancers), liver fibrosis and cirrhosis, dermatology (melanomas and scars), cardiovascular disease (cardiac muscle disease and arteriosclerosis), musculoskeletal studies, minimally invasive surgery and hyperthermia therapy (temperature monitoring and lesion detection in HIFU) . During these 20 years, ultrasonic elastography has been developed to be a powerful adjunctive technique to traditional medical imaging methods and expected to become an excellent tool for clinical diagnosis and treatment.

Superresolution imaging of scatterers in ultrasound B-scan imaging

A number of imaging systems exhibit speckle, which is caused by the interaction of a coherent pulse reflecting off of random reflectors. The limitations of these systems are quite serious because the speckle phenomenon creates a pattern of nulls and peaks from subresolvable scatterers or targets that are difficult to interpret. Another limitation of these pulse-echo imaging systems is that their resolution is dependent on the full spatial extent of the propagating pulse, usually several wavelengths in the axial or propagating dimension and typically longer in the transverse direction. This limits the spatial resolution to many multiples of the wavelength. This paper focuses on the particular case of ultrasound B-scan imaging and develops an inverse filter solution that eliminates both the speckle phenomenon and the poor resolution dependency on the pulse length and width to produce super-resolution ultrasound (SURUS) images. The key to the inverse filter is the creation of pulse shapes that have stable inverses. This is derived by use of the standard Z-transform and related properties. Although the focus of this paper is on examples from ultrasound imaging systems, the results are applicable to other pulse-echo imaging systems that also can exhibit speckle statistics.

Time-domain calculation of spectral centroid from backscattered ultrasound signals

Spectral centroid estimation from backscattered ultrasound RF signals is the preliminary step for quantitative ultrasound analysis in many medical applications. The traditional approach of estimating the spectral centroid in the frequency domain takes a long time because discrete Fourier transform (DFT) processing for each RF segment is required. To avoid this, we propose time-domain methods to estimate the spectral centroid in this paper. First, we derive the continuous-time-domain equations for the spectral centroid estimation using Parseval's theorem and Hilbert transform theory. Then, we extend the method to the discrete-time domain to ease the implementation while maintaining the same accuracy as the calculation in the frequency domain. From the result, we observe that it is not practical to apply the discrete-time equations directly, because a high sampling rate is needed to approximate the time derivative in the discrete-time domain. Therefore, we also derive the feasible version of the discrete-time equations using a circular autocorrelation function, which has no constraints on the sampling rate for real RF signals acquired from pulse-echo ultrasound systems. Simulation results using numerical phantoms show that the time-domain calculation is approximately 4.4 times faster on average than the frequency-domain method when the software's built-in functions were used. The average estimation error compared with that of the frequency-domain method using DFT is less than 0.2% for the entire propagation depths. The proposed time-domain approach to estimate the spectral centroid can be easily implemented in real-time ultrasound systems.

Elasticity Imaging and Sensing Using Targeted Motion: From Macro to Nano

The methods used to estimate tissue elasticity based on the motion of local inhomogeneities such as laser-induced gas microbubbles, magnetic nanoparticles and other targets under an externally applied force are reviewed. The theoretical bases of the motion of a target in a viscoelastic medium are described. Given various targets differing in size, these methods allow for elasticity measurements at different levels of spatial resolution. Short acoustic radiation force and magnetic field pulses were used to initiate the motion of various test objects (solid spheres, laser-induced microbubbles and magnetic nanoparticles) with sizes ranging from several millimeters to tens of nanometers. The induced motion was monitored by conventional pulse-echo ultrasound systems adapted for these measurements. The elasticity of the medium was evaluated based on a theoretical model describing the motion of a particle in a viscoelastic medium. Shear elasticity, evaluated using the developed approach, agrees well with independent measurements of mechanical properties of the medium. In this paper we review our recent theoretical and experimental results and discuss the applications of elasticity imaging and sensing using targeted motion, i.e., motion induced via the external forces acting on local inhomogeneities.

High frequency ultrasonic imaging using thermal mechanical noise recorded on capacitive micromachined transducer arrays

The cross-correlation of diffuse thermal-mechanical noise recorded by two sensors yields an estimate of the ultrasonic waves propagating between them. We used this approach at high frequencies (1-30 MHz) on a capacitive micromachined ultrasonic transducer (CMUT) ring array (d = 725 μm), monolithically integrated with low noise complementary metal oxide semiconductor electronics. The thermal-mechanical noise cross-correlations between the CMUT array elements in immersion reveal both evanescent surface waves (below 10 MHz) and waves propagating primarily in the fluid (above 10 MHz). These propagating waves may allow passive imaging of scatterers closer to the array as compared to conventional pulse-echo systems, providing potentially higher resolution.

Characterisation of polymer-shelled microbubbles in wall-less flow phantom using high-frequency ultrasound and video microscopy

A high frequency ultrasound pulse echo system and a video microscope were combined to investigate the relationship between backscatter from polymer shelled ultrasound contrast agents (UCAs) and their diameter. Individual UCAs (manufactured by Point Biomedical or Philips Research) were imaged while being sonicated with 40 MHz tone bursts. The backscatter magnitude produced by the Philips UCAs was proportional to UCA size, which is consistent with theoretically predicted behaviour of encapsulated microbubbles driven at frequencies above resonance. Despite being smaller, the Point UCAs produced a backscatter magnitude twice that of Philips UCAs, indicating that Point UCAs might behave quasi-resonantly when excited at 40 MHz.

Capacitive micromachined ultrasonic transducers: A platform technology

Capacitive micromachined ultrasonic transducers (CMUTs) have been under development as alternative transducers for generating and detecting ultrasound. The advent of silicon micromachining enables the realization of the full potential of these transducers and provides performance that makes CMUTs competitive and superior to piezoelectric transducers. In immersion applications, CMUTs are possible with fractional bandwidth of over 100%, an electromechanical coupling coefficient close to unity; are made in the form of a single element or 1-D or 2-D arrays of tens of thousand of elements, as well as annular arrays. They have been operated in the frequency range of 100 kHz to 50 MHz, and included in systems with a dynamic range of the order of 150 dB/V/Hz. Custom electronics have been developed and integrated with arrays of transducers to form compact imaging catheters. In airborne applications, CMUTs have been used as mass sensors with atto-gram resolution, and in pulse echo systems operation at high temperature (600^C). This presentation will first review the operation of CMUTs, the technology used to make them, and some airborne and imaging applications.

A System for Nuclear Fuel Inspection Based on Ultrasonic Pulse-Echo Technique

Nuclear Pressurized Water Reactor (PWR) technology has been widely used for electric energy generation. The follow-up of the plant operation has pointed out the most important items to optimize the safety and operational conditions. The identification of nuclear fuel failures is in this context. The adoption of this operational policy is due to recognition of the detrimental impact that fuel failures have on operating cost, plant availability, and radiation exposure. In this scenario, the defect detection in rods, before fuel reloading, has become an important issue. This paper describes a prototype of an ultrasonic pulse-echo system designed to inspect failed rods (with water inside) from PWR. This system combines development of hardware (ultrasonic transducer, mechanical scanner and pulser-receiver instrumentation) as well as of software (data acquisition control, signal processing and data classification). The ultrasonic system operates at center frequency of 25 MHz and failed rod detection is based on the envelope amplitude decay of successive echoes reverberating inside the clad wall. The echoes are classified by three different methods. Two of them (Linear Fisher Discriminant and Neural Network) have presented 93% of probability to identify failed rods, which is above the current accepted level of 90%. These results suggest that a combination of a reliable data acquisition system with powerful classification methods can improve the overall performance of the ultrasonic method for failed rod detection.

Método ultrassônico de detecção de microvibração: Desenvolvimento da técnica

Many investigations have been devoted, during the last years, to implement an ultrasonic method able to generate shear waves and to detect the microvibrations caused by the wave propagation in biological tissues. Applications of such method include measuring rheological parameters for tissue characterization. Considering the potential to use shear wave propagation, this work presents an ultrasonic method able to detect microvibrations (UDmV), which is based on Kalman filtering to estimate the medium vibrations from a signal contaminated with noise. The UDmV was tested employing a speaker to generate microvibrations on a membrane and a pulse-echo ultrasonic system, operating at a center frequency of 4.96 MHz, to detect the membrane microvibrations. The UDmV demonstrated promising future potential, being able to detect the vibration frequency and amplitude with the same order of magnitude as presented in the literature (≈10 mm). Moreover, the method implemented in the present work confirmed previous results, from the literature, related with a reduction of the estimated vibration amplitude as the vibration frequency increases. The method developed in the present work can be employed to estimate the phase velocity and the attenuation coefficient of a shear wave propagating in a viscoelastic medium.