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.     

Statistical moments of backscattered ultrasound in porous fiber reinforced composites

Statistical moments are important tools for understanding wave propagation in random media and are applied here to backscatter measurements in porous, fiber-reinforced polyimide composites. High temperature resins, such as PMR-15, used in load critical components are much more prone to void production than traditional epoxies. Porosity was induced in thin laminates by cure cycle perturbations and quantified destructively as to void content (0.3 to 8.1%) and radius distribution (10 to 250 mum). Measurements of the statistical moments of the scattered field were made using a single, large aperture transducer. Backscatter signals were acquired and used to calculate the rectified ensemble average, and the point signal-to-noise ratio. The lateral and axial full-width half-maximum (FWHM) were extracted from a two-dimensional auto-covariance of the field and compared to the limiting values set by the diffraction pattern and bandwidth of the measurement transducer. Results confirm that the statistical behavior of the echo amplitudes from porous fabric laminates follow predictions based on circular Gaussian statistics for void contents in the range 2 to 5%. Deviations from these predictions occur for void contents outside this range primarily due to changes in the void distribution. These results are important to the development of better void content measurement techniques and are particularly crucial when imaging specularly reflecting defects embedded in a porous volume because spatial and contrast resolution are limited by speckle and the image signal-to-noise.     

Ultrasonic scanner for in vivo measurement of cancellous bone properties from backscattered data

A dedicated ultrasonic scanner for acquiring RF echoes backscattered from the trabecular bone was developed. The design of device is based on the goal of minimizing of custom electronics and computations executed solely on the main computer processor and the graphics card. The electronic encoder-digitizer module executing all of the transmission and reception functions is based on a single low-cost field programmable gate array (FPGA). The scanner is equipped with a mechanical sector-scan probe with a concave transducer with 50 mm focal length, center frequency of 1.5 MHz and 60% bandwidth at -6 dB. The example of femoral neck bone examination shows that the scanner can provide ultrasonic data from deeply located bones with the ultrasound penetrating the trabecular bone up to a depth of 20 mm. It is also shown that the RF echo data acquired with the scanner allow for the estimation of attenuation coefficient and frequency dependence of backscattering coefficient of trabecular bone. The values of the calculated parameters are in the range of corresponding in vitro data from the literature but their variation is relatively high.     

Estimation of scatterer size from backscattered ultrasound: a simulation study

The reliability of the estimation of the size of scattering structures is assessed by realistic simulations and phantom experiments. The acoustic tissue model used in the simulation studies comprised a constant sound speed, homogeneous attenuation, and isotropic scattering. The scattering models considered were a discrete (spherical) model and two inhomogeneous-continuum models. The latter were characterized by an exponential and a Gaussian autocorrelation function, respectively. The backscattering spectra were, over the range from 5 to 10 MHz, fitted to linear, power, and autocorrelation functions of the three scattering models. The effects of the fitting function, the attenuation-either in an intervening layer or within the region of interest (ROI)-of the transmission pulse, and a spread in the scatterer sizes on the accuracy and the precision of the size estimates were assessed. The attenuation in the intervening tissue layer(s) as well as in the ROI itself has a significant effect on the accuracy of the size estimates and needs to be corrected. When performing the attenuation correction the inaccuracy of the attenuation estimate of the intervening layer leads to a large bias in the estimated scatterer size. Experimental results support the conclusion that scatterer size is a feasible tissue characterization parameter.     

Three-dimensional spatial and temporal temperature imaging in gel phantoms using backscattered ultrasound

Thermal therapies such as radio frequency, heated saline, and high-intensity focused ultrasound ablations are often performed suboptimally due to the inability to monitor the spatial and temporal distribution of delivered heat and the extent of tissue necrosis. Ultrasound-based temperature imaging recently was proposed as a means to measure noninvasively the deposition of heat by tracking the echo arrival time shifts in the ultrasound backscatter caused by changes in speed of sound and tissue thermal expansion. However, the clinical applicability of these techniques has been hampered by the two-dimensional (2-D) nature of traditional ultrasound imaging, and the complexity of the temperature dependence of sound speed for biological tissues. In this paper, we present methodology, results, and validation of a 3-D spatial and temporal ultrasound temperature estimation technique in an alginate-based gel phantom to track the evolution of heat deposition over a treatment volume. The technique was experimentally validated for temperature rises up to approximately 10 degrees C by comparison with measurements from thermocouples that were embedded in the gel. Good agreement (rms difference = 0.12 degrees C, maximum difference = 0.24 degrees C) was observed between the noninvasive ultrasound temperature estimates and thermocouple measurements. Based on the results obtained for the temperature range studied in this paper, the technique demonstrates potential for applicability in image guidance of thermal therapy for determining the location of the therapeutic focal spot and assessing the extent of the heated region at subablative intensities.     

IGBT-based kilovoltage pulsers for ultrasound measurement applications

Two high-voltage pulser designs are presented that offer advantages in some ultrasound measurement applications, such as driving thick ultrasonic source transducers used for broadband measurements of attenuation or hydrophone frequency response and directivity. The pulsers use integrated gate bipolar transistors (IGBTs) as the switching devices, and in one design an output voltage pulse is produced that has a peak amplitude nearly twice that of the supply voltage. The pulsers are inexpensive and relatively easy to construct. The power supply need only provide the average current to charge the capacitors, as opposed to the much higher peak pulse current. With a 1200 V supply and a pulse repetition frequency of 200 Hz, the nondoubling and doubling pulsers provided peak voltages of greater than 1100 V and 2200 V, respectively, into loads ranging from 50 /spl Omega/ to 500 /spl Omega/. For a 50 /spl Omega/ load, slewing rates of 38 V/ns and 23 V/ns were measured for the nondoubling and doubling pulsers, respectively. For a 500 /spl Omega/ load these values were 56 V/ns and 36 V/ns.     

Efficiency of integrated waveguide probes for the detection of light backscattered from weakly scattering media

A semianalytical model for light collection by integrated waveguide probes is developed by extending previous models used to describe fiber probes. The efficiency of waveguide probes is compared to that of different types of fiber probes for different thicknesses of a weakly scattering sample. The simulation results show that integrated probes have a collection efficiency that is higher than that of small-core fiber probes, and, in the particular case of thin samples, also exceeds the collection efficiency of large-core highly multimode fiber probes. An integrated waveguide probe with one excitation and eight collector waveguides is fabricated and applied to excite and collect luminescence from a ruby rod. The experimental results are in good agreement with the simulation and validate the semianalytical model.     

Self-mixing detection of backscattered radiation in single-mode pulse-periodic CO2 lasers

The self-mixing (autodyne) effect in single-mode CO(2) lasers with pulse-periodic (PP) pumping of the active medium is theoretically analyzed and experimentally investigated. A semiempirical model of the autodyne effect in CO(2) lasers of this type is developed that allows the laser beat signal to be described from the known shape of the generated pulses. The self-mixing effect in PP CO(2) lasers is shown to be identical with that in continuous-wave CO(2) lasers, except that the autodyne amplification during the laser pulse proves time-dependent. It is demonstrated that the amplitude-frequency characteristic of the autodyne amplification for PP CO(2) lasers is also of resonance nature, but its bandwidth is broadened, as compared with that in the case of continuous laser pumping. As in the case of continuous pumping, the self-mixing effect in PP CO(2) lasers can be used to detect and analyze backscattered signals, specifically for measuring the rates of destruction of materials by the 10 μm radiation and for monitoring this process.     

双频超声波测距

本文介绍一种在空气中实时，高精度，大范围双频超声波测距的原理及方法，该方法利用双频超声波，并结合一些简单的信号自理方法，实现在１０几米范围内高精度实时地探测目标距离，该方法可以广泛用于类似于智能机器人定位，建筑工程验收，物资管理等一些需要测距的地方。     

Optical resonant ultrasound spectroscopy for fluid properties measurement

The properties of fluids are studied using unusually small containment spherical resonators. Proper identification of resonant fluid signatures allows determination of pressure and density of the internal gas with great accuracy using an appropriate equation of state (EOS). Low noise and high sensitivity detection of vibration are critical parameters to characterizing the contained gas when its pressure approaches 1 atm. or less. The benefits of using spherical resonators to determine fluid properties are discussed, and some example calculations of sound speed are presented. In addition to measuring fluids, a comparative experimental approach is taken to explore and, eventually, to optimize vibration detection. In the experiments, two detection methods, a contact piezoelectric transducer (PZT) device and a non-contact optical device, are compared simultaneously and quantitatively. This is done in a unique manner without change in vibration coupling to the sample between tests. A commercially available resonant ultrasound spectroscopy system is used as the contact system, while another commercial device (used as the non-contact vibration detector) combined with the same excitation source (used in the contact system) comprises the other system. The non-contact detector is an optical interferometric receiver that provides adaptation to optically rough surfaces and high sensitivity to acoustic displacements through optical interference in photorefractive GaAs. Both vibration detection systems are compared with particular emphasis on displacement sensitivity, frequency response, and noise level. Furthermore, the results from comparing detection modalities are presented, and their effects on fluid properties measurement are discussed     

Singular spectrum analysis applied to backscattered ultrasound signals from in vitro human cancellous bone specimens

Mean scatterer spacing (MSS) holds particular promise for the detection of changes in quasiperiodic tissue microstructures such as may occur during development of disease in the liver, spleen, or bones. Many techniques that may be applied for MSS estimation (temporal and spectral autocorrelation, power spectrum and cepstrum, higher order statistics, and quadratic transformation) characterize signals that contain a mixture of periodic and nonperiodic contributions. In contrast, singular spectrum analysis (SSA), a method usually applied in nonlinear dynamics, first identifies components of signals corresponding to periodic structures and, second, identifies dominant periodicity. Thus, SSA may better separate periodic structures from nonperiodic structures and noise. Using an ultrasound echo simulation model, we previously demonstrated SSA's potential to identify MSS of structures in quasiperiodic scattering media. The current work aims to observe the behavior of MSS estimation by SSA using ultrasound measurements in phantom materials (two parallel, nylon-line phantoms and four foam phantoms of different densities). The SSA was able to estimate not only the nylon-line distances but also nylon-line thickness. The method also was sensitive to the average pore-size differences of the four sponges. The algorithms then were applied to characterize human cancellous bone microarchitectures. Using 1-MHz center-frequency, radio-frequency ultrasound signals, MSS was measured in 24 in vitro bone samples and ranged from 1.0 to 1.7 mm. The SSA MSS estimates correlate significantly to MSS measured independently from synchrotron microtomography, r2 = 0.68. Thus, application of SSA to backscattered ultrasound signals seems to be useful for providing information linked to tissue microarchitecture that is not evident from clinical images.     

Determination of the force constant of a single-beam gradient trap by measurement of backscattered light

A single-beam gradient trap could potentially be used to hold a stylus for scanning force microscopy. With a view to development of this technique, we modeled the optical trap as a harmonic oscillator and therefore characterized it by its force constant. We measured force constants and resonant frequencies for 1-4-μm-diameter polystyrene spheres in a single-beam gradient trap using measurements of backscattered light. Force constants were determined with both Gaussian and doughnut laser modes, with powers of 3 and 1 mW, respectively. Typical values for spring constants were measured to be between 10(-6) and 4 × 10(-6) N/m. The resonant frequencies of trapped particles were measured to be between 1 and 10 kHz, and the rms amplitudes of oscillations were estimated to be around 40 nm. Our results confirm that the use of the doughnut mode for single-beam trapping is more efficient in the axial direction.     

Quasi-balanced two-wave mixing interferometer for remote ultrasound detection

We present an improved detection scheme for a two-wave mixing interferometer with a Bi₁₂SiO₂₀ crystal. The proposed detection scheme allows quasi-balanced detection of ultrasonic signals whereby electrical disturbances are suppressed. Quasi-balancing is achieved by changing the polarity of the high voltage at the photorefractive crystal, leading to an inversion of the optical interference signal, in combination with inversion of the detector signal using a signal inverter before the data acquisition device. The polarity of the high voltage is changed by utilizing an H-bridge consisting of five high-voltage relays. Microcontrollers are used to synchronize the reversion of the high voltage at the photorefractive crystal and the inversion of the measured signals. We demonstrate remote measurement of ultrasonic waves and shown that electrical disturbances are suppressed using the quasi-balanced mode.     

A digital-signal-processor-based measurement system for on-linefault detection

This paper deals with the design, construction, and setting up of a measurement apparatus, based on an architecture using two parallel digital signal processors (DSP's), for on-line fault detection in electric and electronic devices. In the proposed architecture, the first DSP monitors a device output on-line in order to detect faults, whereas the second DSP estimates and updates the system-model parameters in real-time in order to track their eventual drifts. The problems which arose when the proposed apparatus was applied to a single-phase inverter are discussed, and some of the experimental results obtained in fault and nonfault conditions are reported     

Amplitude-dependent losses in ultrasound exposure measurement

Energy losses resulting from the nonlinear propagation of ultrasonic pulses in water have been measured using a polyvinylidene difluoride membrane hydrophone and a radiation-force balance. The focused ultrasonic transducers used were of low focal gain operating at source intensities and frequencies typical of those used in medical diagnostic applications. Energy transfer into harmonic components has been quantified by hydrophone measurements at the focus. At values of shock parameter sigma>pi/2, total loss of intensity was observed, with the greatest loss reaching 2.75 dB of the intensity predicted by linear extrapolation from low-amplitude measurements. A similar but smaller-magnitude reduction in the radiation force measured by a force balance was observed. These results are related to ranges of acoustic parameters obtained from surveys on clinical equipment. It is concluded that a significant majority of contemporary clinical scanners can generate ultrasonic pulses which will lose energy during transmission through water due to amplitude-dependent nonlinear losses, and that it is necessary to consider these, and other nonlinear phenomena, when predicting exposure conditions in vivo.     

现代医疗超声测量系统的设计

1引言 超声在医疗中的应用正变得越来越重要.从其应用看,我们大致可以将医疗超声分为诊断超声和治疗超声两个应用方向.     

Noncontact measurement of vibration using airborne ultrasound

A noncontact ultrasonic method for measuring the surface normal vibration of objects was studied. The instrument consists of a pair of 420 kHz ultrasonic air transducers. One is used to emit ultrasounds toward the moving surface, and the other receives the ultrasound reflected from the object under test. Two effects induce a phase modulation on the received signal. The first effect results from the variation of the round trip time interval tau required for the wavefront to go from the emitter to the moving surface and back to the receiver. This is the Doppler effect directly proportional to the surface displacement. The second effect results from the nonlinear parametric interactions of the ultrasonic beams (forward and backward) with the low frequency sound field emitted in the air by the vibrating surface. This latter phenomenon, which is a volume effect, is proportional to the velocity of the vibrating surface and increases with the distance between the transducers and the surface under test. The relative contribution of the Doppler and parametric effects are evaluated, and both have to be taken into account for ultrasonic interferometry in air.     

Air-coupled piezoelectric detection of laser-generated ultrasound

A pulsed laser has been used to generate ultrasonic transients in samples of metal and fiber-reinforced polymer composite material. These have been detected using an air-coupled piezoelectric transducer. It is demonstrated that such a transduction system can be used for longitudinal waves in bulk material, Rayleigh waves at solid surfaces and Lamb waves in thin plates.     

物体振动幅度的远距离非接触测量技术

根据物体振动幅度检测的工作特点,采用线阵CCD实现对物体的远距离、非接触性、动态自动化幅度检测技术。本系统的硬件电路设计完成了CCD模块与USB端口的数据传输,通过确定固定条码标尺中心位置的上下偏移量实现对物体振动幅度的检测。采用Visual C++作为开发环境,完成了用户应用程序的开发。该系统具有测量精度高、操作简便、安全性好,便于野外作业等优点,在桥梁、铁轨等目标的振动幅度检测项目中得到很好的应用。