Ultrasonic Backscatter Research Articles

Monitoring radiofrequency ablation with ultrasound Nakagami imaging

Radiofrequency ablation (RFA) is a widely used alternative modality in the treatment of liver tumors. Ultrasound B-mode imaging is an important tool to guide the insertion of the RFA electrode into the tissue. However, it is difficult to visualize the ablation zone because RFA induces the shadow effect in a B-scan. Based on the randomness of ultrasonic backscattering, this study proposes ultrasound Nakagami imaging, which is a well-established method for backscattered statistics analysis, as an approach to complement the conventional B-scan for evaluating the ablation region. Porcine liver samples (n = 6) were ablated using a RFA system and monitored by employing an ultrasound scanner equipped with a 7.5 MHz linear array transducer. During the stages of ablation (0-12 min) and postablation (12-24 min), the raw backscattered data were acquired at a sampling rate of 30 MHz for B-mode, Nakagami imaging, and polynomial approximation of Nakagami imaging. The contrast-to-noise ratio (CNR) was also calculated to compare the image contrasts of the B-mode and Nakagami images. The results demonstrated that the Nakagami image has the ability to visualize changes in the backscattered statistics in the ablation zone, including the shadow region during RFA. The average Nakagami parameter increased from 0.2 to 0.6 in the ablation stage, and then decreased to approximately 0.3 at the end of the postablation stage. Moreover, the CNR of the Nakagami image was threefold that of the B-mode image, showing that the Nakagami image has a better image contrast for monitoring RFA. Specifically, the use of the polynomial approximation equips the Nakagami image with an enhanced ability to estimate the range of the ablation region. This study demonstrated that ultrasound Nakagami imaging based on the analysis of backscattered statistics has the ability to visualize the RFA-induced ablation zone, even if the shadow effect exists in the B-scan.

WE-E-134-07: Ultrasound Image Based Measurements of Myocardial Fiber Structure Within the Left and Right Ventricular Walls of the Heart

Purpose: Previous studies from our Laboratory demonstrated that quantitative measurements of myocardial fiber structure for individual hearts at specific transverse planes can be obtained from analyses of echocardiographic images. The objective of this study was to extend these echocardiographic-based measurements to depict the myocardial fiber structure within the ventricular walls of the entire heart. Methods: A series of 2D apical echocardiographic images were acquired from 7 excised, intact, formalin-fixed sheep hearts using a commercially available (GE Vivid) ultrasonic imaging system. Myocardial fiber orientations were determined from analyses of the measured transmural (radial) ultrasonic backscatter profiles within the ventricular walls in conjunction with a previously determined relationship between the observed backscatter level and the angle of insonification relative to myocardial fiber orientation. These data were assembled to produce a fiber orientation image map of the entire left and right ventricular walls of each heart. In addition, 3D volumetric, apical echocardiographic images were acquired from a subset of the excised hearts for comparison. Results: Image maps depicting myocardial fiber structure throughout the ventricular walls appear consistent with the known fiber structure of the heart. Images demonstrate left-ventricular mid-myocardial fibers oriented within the short axis plane and gradually becoming more longitudinally oriented toward the epicardial and endocardial surfaces. Preliminary data from 3D volumetric, apical echocardiographic images suggest similar results. Conclusion: These results demonstrate that measurements of myocardial fiber structure throughout the entire heart can be successfully derived from analyses of echocardiographic images. Further development of this method may provide a method for mapping the myocardial fiber orientation in individual patients over the heart cycle and provide a means for assessing potentially altered fiber structure associated with congenital and acquired heart diseases. [NIH R01 HL040302]; Funded, in part, by NIH R01 HL040302; Dr. Kirk D. Wallace is an employee of GE Global Research

Effect of selected signals of interest on ultrasonic backscattering measurement in cancellous bones

This study examined how the signals of interest (SOI) effect on the backscattering measurement numerically based on 3-D finite-difference time-domain (FDTD) method. High resolution microstructure mappings of bovine cancellous bones provided by micro-CT were used as the input geometry for simulations. Backscatter coefficient (BSC), integrated backscatter coefficient (IBC) and apparent integrated backscatter (AIB) were calculated with changing the start (L1) and duration (L2) of the SOI. The results demonstrated that BSC and IBC decrease as L1 increases, and AIB decreases more rapidly as L1 increases. The backscattering parameters increase with fluctuations as a function of L2 when L2 is less than 6 mm. However, BSC and IBC change little as L2 continues to increase, while AIB slowly decreases as L2 continues to increase. The results showed how the selections of the SOI effect on the backscattering measurement. An explicit standard for SOI selection was proposed in this study and short L1 (about 1.5 mm) and appropriate L2 (6 mm–12 mm) were recommended for the calculations of backscattering parameters.

Estimation of porosity content of composite materials by applying discrete wavelet transform to ultrasonic backscattered signal

As the use of composite materials in the aerospace industry increases, the development of advanced nondestructive evaluation (NDE) techniques for composite materials is in demand. Ultrasonic quantitative NDE technique for composite materials may provide good information on manufacturing quality, material strength and perhaps useful lifetime. It is well known that the effects of porosity in composite laminates on ultrasonic attenuation and velocity can be used in gauging the porosity content in composites, but back surface echoes may be absent or unusable due to complex geometry and bonding effects. In such cases the backscattered signals may be processed to extract porosity information. Measuring the porosity content in composite material by ultrasonic backscattering signal is a significant challenging problem in NDE of composite material. Backscattering signals are random and sensitive to volume fraction of pore and thickness of ply in composite material. Therefore the backscattering signal has various frequency bands and hence a signal decomposition method is required to analyze the ultrasonic backscattering signals. In this study, the discrete wavelet transform (DWT) using a MATLAB decomposition algorithm was applied to ultrasonic backscattered signals acquired in various porous composite laminates containing a porosity content that ranges from 0.01 to 11.90%. The ultrasonic backscattered signals were decomposed into two parts: the high frequency components called “Details” and the low frequency components called “Approximation”. And then, the correlation analysis was performed between the porosity content and the peak amplitude and magnitude of peak frequency of the decomposed signal. Overall, the correlation was reasonably good. As a conclusion, the DWT technique showed good benefits for analyzing the porosity content in composites using ultrasonic backscattered signal from composite materials.

An Approach for the Visualization of Temperature Distribution in Tissues According to Changes in Ultrasonic Backscattered Energy

Previous studies developed ultrasound temperature-imaging methods based on changes in backscattered energy (CBE) to monitor variations in temperature during hyperthermia. In conventional CBE imaging, tracking and compensation of the echo shift due to temperature increase need to be done. Moreover, the CBE image does not enable visualization of the temperature distribution in tissues during nonuniform heating, which limits its clinical application in guidance of tissue ablation treatment. In this study, we investigated a CBE imaging method based on the sliding window technique and the polynomial approximation of the integrated CBE (ICBEpa image) to overcome the difficulties of conventional CBE imaging. We conducted experiments with tissue samples of pork tenderloin ablated by microwave irradiation to validate the feasibility of the proposed method. During ablation, the raw backscattered signals were acquired using an ultrasound scanner for B-mode and ICBEpa imaging. The experimental results showed that the proposed ICBEpa image can visualize the temperature distribution in a tissue with a very good contrast. Moreover, tracking and compensation of the echo shift were not necessary when using the ICBEpa image to visualize the temperature profile. The experimental findings suggested that the ICBEpa image, a new CBE imaging method, has a great potential in CBE-based imaging of hyperthermia and other thermal therapies.

3P5-6 Statistical Analysis of the Envelope of Three-dimensional High-frequency Ultrasonic Backscatter from Dissected Human Lymph Nodes(Poster Session)

3P5-6 Statistical Analysis of the Envelope of Three-dimensional High-frequency Ultrasonic Backscatter from Dissected Human Lymph Nodes(Poster Session)

Ultrasonic Backscattering in Cubic Polycrystals with Ellipsoidal Grains and Texture

An ultrasonic model for backscattering from polycrystalline microstructure is developed for polycrystals with uniaxial texture and elongated cubic crystallites. The uniaxial texture or crystallographic orientation of the grains is described by a modified Gaussian orientation distribution function (ODF) with a texture parameter. Macroscopically such a textured polycrystalline medium exhibits hexagonal symmetry. The preferred texture direction and elongation are independently defined in a global system. The dependence of backscattering coefficients and their directional ratios on both texture and grain anisotropy are discussed. Attenuation coefficients in the high frequency range for arbitrary wave propagation direction are obtained and then the ratios in the three axis directions are studied. The model is compared with experimental data available in the literature for Al rolled alloys and shows good agreement when accounting for both texture and grain anisotropy effects.

A backscatter difference technique for ultrasonic bone assessment

Ultrasonic backscatter techniques may offer a useful approach for detecting changes in cancellous bone caused by osteoporosis and other diseases. The goal of this study was to investigate the utility of a backscatter difference technique for ultrasonic bone assessment. Measurements were performed on 22 cube-shaped specimens of human cancellous bone using four broadband transducers with center frequencies 2.25, 5, 7.5, and 10 MHz. The backscatter difference spectrum D(f) was obtained by subtracting power spectra (in dB) from two different portions of the same backscatter signal. D(f) was found to be a monotonically increasing, quasi-linear function of frequency when averaged over multiple measurement sites on multiple specimens. The frequency slope of D(f) demonstrated weak to moderate correlations with specimen density (R = 0.21-0.80). The frequency averaged mean of D(f) demonstrated moderate to good correlations with density (R = 0.70-0.95). These results suggest that parameters based on the frequency averaged mean of the backscatter difference spectrum may be useful for bone assessment purposes.

Particle characterisation in highly concentrated dispersions using ultrasonic backscattering method

Determining particle size and concentration in highly concentrated suspensions and emulsions is challenging, especially under process conditions. In general, ultrasound therefore can be used for particle characterisation due to the ability of sound waves to pass opaque dispersions, whereas optical detection principles mostly are limited to low particulate contents. An established acoustic method, the ultrasonic attenuation spectroscopy, uses a transmission setup for measuring the attenuation of a dispersion. A major drawback of this measurement method is caused by the fact, that the measuring gap tends to plug, which again limits the inline capability. To overcome this limitation, an ultrasonic reflection setup is used for gathering the sound waves, which are reflected, respectively backscattered by the dispersion. Statistically analysing the corresponding backscattering signal yields the sound attenuation as well as a scattering intensity equivalent. Both measurement parameters can be shown to be sensitive against particle size and concentration. Based on a single scattering theory, a semi-empirical approach is presented for interpretation of measurement results with respect to particle size and concentration. Measurements, performed on a glass beads in water dispersion, show good agreement with theory for dimensionless wave number 0.1<ka⩽1, even for concentrations up to 30vol.%.

In vivovalidation of a bimodal technique combining time-resolved fluorescence spectroscopy and ultrasonic backscatter microscopy for diagnosis of oral carcinoma

Tissue diagnostic features generated by a bimodal technique integrating scanning time-resolved fluorescence spectroscopy (TRFS) and ultrasonic backscatter microscopy (UBM) are investigated in an in vivo hamster oral carcinoma model. Tissue fluorescence is excited by a pulsed nitrogen laser and spectrally and temporally resolved using a set of filters/dichroic mirrors and a fast digitizer, respectively. A 41-MHz focused transducer (37-μm axial, 65-μm lateral resolution) is used for UBM scanning. Representative lesions of the different stages of carcinogenesis show that fluorescence characteristics complement ultrasonic features, and both correlate with histological findings. These results demonstrate that TRFS-UBM provide a wealth of co-registered, complementary data concerning tissue composition and structure as it relates to disease status. The direct co-registration of the TRFS data (sensitive to surface molecular changes) with the UBM data (sensitive to cross-sectional structural changes and depth of tumor invasion) is expected to play an important role in pre-operative diagnosis and intra-operative determination of tumor margins.

Characterization of polycrystals by ultrasonic attenuation-to-backscattering ratio measurements

Scattering of ultrasonic waves in polycrystalline materials depends on the relative misorientation of the crystallites, their elastic properties and the crystallite size and morphology. Those parameters affect both ultrasonic attenuation and backscattering. In structural material alloys the elastic properties of crystallites and microtextures are often unknown, thus complicating model-based microstructure size determination from ultrasonic measurements. It has been shown by analysis and simulation that the measured attenuation-to-backscattering ratio is independent of the unknown elastic material characteristics and thus may be used advantageously for inverse determination of mictostructural characterization of polycrystalline materials. The method is based on recently developed models for ultrasonic attenuation and backscattering in polycrystals with nonequiaxed grains of general ellipsoidal shape. Using the model approximation an effective size parameter is defined and obtained from the backscattering-t...

Ultrasonic scattering for inverse characterization of complex microstructures

This paper overviews resent theoretical and experimental results for inverse characterization of duplex polycrystalline microstructures from measurements of ultrasonic attenuation and backscattering. The duplex microstructures are formed by elongated regions of clustering crystallites with preferred orientations and are typical for titanium alloys. New insights into the dependences of the backscattering and attenuation on frequency and averaged ellipsoidal grain radii are obtained. In particular the dominant effect of the averaged ellipsoidal radius in the direction of wave propagation, instead of the ellipsoidal cross-section, is shown. Also the opposite effect on attenuation and backscattering of grain size in the direction of wave propagation is demonstrated by theory and experiment. Both attenuation and backscattering depend on effective elastic properties of clusters, their size and orientation and the measurement system parameters. It is shown that for material property inversion it is advantageous to decouple elastic and geometrical properties by a special series of measurements and data inversion: first determine grain geometry and size from relative directional and/or attenuation-to-backscattering ratio measurements; this is followed by absolute attenuation and backscattering measurements to obtain the crystallite orientation distribution function in the clusters.

Diffuse ultrasonic backscatter measurements for monitoring stress in rail

Recent research in polycrystalline materials, both theoretically and experimentally, has demonstrated a dependence of diffuse ultrasonic backscatter (DUB) on applied stress. This dependence has been used to develop a measurement device for monitoring longitudinal stress in continuous welded rail (CWR). However when moving this research from the laboratory setting to field applications many additional challenges are encountered with respect to the experimental method. In this presentation, results from initial field experiments are presented. This work is performed on a switch track from a railroad mainline to a short line railroad siding. Comparison between DUB measurements and actual stress in the rail is accomplished through the use of four stress modules (based on standard strain gauge technology) that were installed at four different locations along test site. In these field tests, DUB experiments using two orthogonal shear waves are investigated. Procedures to mitigate errors in our experiments and techniques to refine the measurement for more accurate results are discussed. These field experiments highlight the utility of this approach with respect to practical and financial considerations for determining induced stresses in CWR. [Research supported by FRA.]

Evaluating duplex microstructures in polycrystalline steel with diffuse ultrasonic backscatter

The performance of metallic components is governed in large part by the microstructure of the base material from which the component is manufactured. In this presentation, diffuse ultrasonic backscatter techniques are discussed with respect to their use for monitoring the microstructure of polycrystalline steel as a result of the manufacturing process. To improve the mechanical properties, the surface of polycrystalline steel is quenched, a process which transforms the initial phase to a pearlite phase within grains. A diffuse ultrasonic backscatter model is developed that includes the duplex microstructure through the addition of an additional length scale in the two-point spatial correlation function. This function defines the probability that two randomly chosen points will fall into the same grain and/or same crystallite. The model clearly shows the dependence of the diffuse ultrasonic backscatter signal with respect to frequency, average grain size and lamellar spacing of the crystallites. Experimental results are used to show how the two length scales can be extracted from the measurements. The spatial variation of the microstructure with respect to depth from the quench surface is also examined. These diffuse ultrasonic techniques are shown to have the sensitivity to deduce the duplex microstructure throughout the sample.

Cross-imaging system comparison of backscatter coefficient estimates from a tissue-mimicking material

A key step toward implementing quantitative ultrasound techniques in a clinical setting is demonstrating that parameters such as the ultrasonic backscatter coefficient (BSC) can be accurately estimated independent of the clinical imaging system used. In previous studies, agreement in BSC estimates for well characterized phantoms was demonstrated across different laboratory systems. The goal of this study was to compare the BSC estimates of a tissue mimicking sample measured using four clinical scanners, each providing RF echo data in the 1-15 MHz frequency range. The sample was previously described and characterized with single-element transducer systems. Using a reference phantom for analysis, excellent quantitative agreement was observed across the four array-based imaging systems for BSC estimates. Additionally, the estimates from data acquired with the clinical systems agreed with theoretical predictions and with estimates from laboratory measurements using single-element transducers.

Estimation the mean correlation length of metals by using mode-converted diffuse ultrasonic backscatter model

Diffuse ultrasonic backscatter measurements can explain the phenomenon of the scattering at interfaces in heterogeneous materials. The theoretical models of diffuse ultrasonic backscatter for longitudinal (L-L) and shear (T-T) wave scattering within polycrystalline materials have recently been developed. A mean correlation length of metals is successfully calculated from the L-L model at normal incidence in a pulse-echo inspection. Above the first critical angle, mode-conversion scattering occurs when the longitudinal waves are converted to shear waves (L-T) at material grain boundaries. With a similar formalism, a mode-conversion scattering (L-T) model is presented to describe this process. The model is then to fit the experimental response for a pitch-catch transducer configuration and the correlation length is extracted by modifying the beam function. The mean correlation length from the L-T model is in agreement with both the L-L model and the results obtained from optical micrographs. This presentation outlines the theoretical framework and the method to extract the mean correlation length. Mode-converted backscatter removes the influence of the front-wall reflection and may lead to improvements in microstructural characterization and material property evaluation. [Research supported by FRA.]

Particle size and density of a slurry from ultrasonic backscattering measurements at a solid interface

The pivotal experiment was performed with a setup in which a plastic cylinder was mounted on the top of a horizontal Rexolite plate and a transducer mounted directly below the cylinder; a single layer of stationary 1588-μm acrylic spheres was placed in the cylinder filled with water. Two well-separated signals were received by the transducer operating in the pulse-echo mode: (1) a signal due to the reflection from water at the interface and (2) a time-delayed signal resulting from the backscattering from the spheres of diameter D. The important observation was that the time delay was equal to 2 D/c using standard notation. A method was developed to use the FFT phase difference between the incident and scattered signals at the interface to determine the time-delay as a function of frequency, the backscattering coefficient M versus frequency, a particle size distribution, and an average value of the diameter. Experimental average diameter results are shown in the square brackets for nominal particle sizes: (1) 1588-μm acrylic spheres [1564 μm], (2) polystyrene spheres for diameters from 200 μm to 500 μm [260 μm-536 μm], (3) suspended slurry of 250-300 μm polystyrene spheres at 2.25 MHz [253 μm], (4) 794 μm [759 μm] and 1588-μm [1623 μm] Teflon spheres, (5) 1588-μm stainless steel spheres [1674 μm], and (6) suspended slurry of 250-300 μm polystyrene spheres [275 μm] at 3.5 MHz for seven volume fractions. Density and particle size measurements were obtained for the latter. For the density measurement, the FFT amplitude of the scattered signal was summed from 2 to 4 MHz for each slurry. A plot of the square root of the FFT-amplitude-sum versus the volume fraction yields a straight line, passing through the origin. A calibration of the experimental setup is obtained by fitting a straight line through the data with error bars. Thus, the volume fraction for a slurry of unknown concentration can be determined by measuring the FFT-amplitude-sum. The density of the slurry is obtained from the volume fraction. These results make it feasible to develop an online and real-time pipeline sensor to measure particle size and slurry density.

Influence of stresses on grain size and attenuation parameters in ultrasonic scattering models

Recent findings have shown the dependence of material stresses on the strength of ultrasonic backscatter. The stress dependence arrives when considering the covariance of effective elastic moduli of a medium with a randomly oriented grain structure. In some instances, the change in magnitude of the scattered response due to stress is significant and can be utilized as a technique for NDT stress monitoring. Conversely, ignoring the stress sate could result in significant error when attempting to extract micrsotructural parameters using ultrasonic NDE techniques. This presentation touches on the error generated in estimating the average grain size when using ultrasonic scattering models which include stress dependent backscatter coefficients. Results are given for three scattering modes; longitudinal to longitudinal, shear to shear, and (mode converted) longitudinal to shear under a variety of loading configurations. Furthermore, the stress dependence in the covariance influences common multiple echo attenuation measurements when grain scattering is present. Theoretical attenuation values of the different modes are given for various stress-states. Understanding how stress influences these parameters can potentially improve existing ultrasonic NDT techniques. [Research supported by the Federal Railroad Administration.]

Detection of inclusions embedded in a polycrystalline medium: A diagrammatic approach

Diffuse ultrasonic backscatter techniques are used to evaluate and to quantify polycrystalline microstructure in structural materials including steel, aluminum, and concrete. Modeling of the backscatter received from grain scattering is important for both microstructural characterization and flaw detection. Here, the problem of an inclusion embedded in a polycrystalline background is examined using a diagrammatic approach. Expressions are derived that for the attenuation and diffuse backscatter using a Green’s function approach that incorporates Feynman diagrams for simplicity in the analysis. The resulting equations are solved in the single-scattering limit that couples geometric probabilities and correlation functions such that the impact of the inclusion can be quantified. The effects of frequency, average grain size, polycrystalline properties, inclusion properties and inclusion size are considered. Further work related to inspection with focused beams or immersion type ultrasonic inspections is also presented. This work is anticipated to impact the studies of model-assisted probability and inclusion content quantification in ultrasonic non-destructive evaluation.

Statistical Analysis of High Frequency Ultrasonic Backscattered Signals from Basal Cell Carcinomas

A statistical approach was implemented in the study of histologic characteristics from ex vivo basal cell carcinomas, based on the properties of backscattered acoustic waves, for the purpose of evaluating the method as a diagnostic tool. The study was developed using an ultrasound biomicroscope working at a frequency of 45 MHz. The parameters examined were signal-to-noise ratio (SNR), and shape parameters from the Weibull (bW) and generalized gamma (cGG and υGG) probability density functions. Twenty-seven carcinomatous skin samples were obtained from volunteer patients and classified into two groups (BCC1 and BCC2) based on the distribution patterns of their tumor nests; also, seven non-tumoral samples were used for comparative purposes. Significant differences between groups were obtained for all studied parameters. The successful differentiation between some tissue groups suggests its potential use for carcinoma characterization.