Goutam Ghoshal

Wigner distribution of a transducer beam pattern within a multiple scattering formalism for heterogeneous solids

Diffuse ultrasonic backscatter measurements have been especially useful for extracting microstructural information and for detecting flaws in materials. Accurate interpretation of experimental data requires robust scattering models. Quantitative ultrasonic scattering models include components of transducer beam patterns as well as microstructural scattering information. Here, the Wigner distribution is used in conjunction with the stochastic wave equation to model this scattering problem. The Wigner distribution represents a distribution in space and time of spectral energy density as a function of wave vector and frequency. The scattered response is derived within the context of the Wigner distribution of the beam pattern of a Gaussian transducer. The source and receiver distributions are included in the analysis in a rigorous fashion. The resulting scattered response is then simplified in the single-scattering limit typical of many diffuse backscatter experiments. Such experiments, usually done using a modified pulse-echo technique, utilize the variance of the signals in space as the primary measure of microstructure. The derivation presented forms a rigorous foundation for the multiple scattering process associated with ultrasonic experiments in heterogeneous media. These results are anticipated to be relevant to ultrasonic nondestructive evaluation of polycrystalline and other heterogeneous solids.

Numerical model of longitudinal wave scattering in polycrystals

The scattering of elastic waves in polycrystalline materials is relevant for ultrasonic materials characterization and nondestructive evaluation (NDE). Ultrasonic attenuation is used widely to extract microstructural parameters such as grain size. Accurate interpretation of experimental data requires robust scattering models. Such models typically assume constant density, uniform grain size, and ergodicity hypotheses. The accuracy and limits of applicability of these models cannot be fully tested with experiments due to practical limits of real materials processing. Here, this problem is examined in terms of numerical simulations using Voronoi polycrystals that are discretized using finite elements. Wave propagation is studied by integrating the system directly in time using a planestrain formulation. Voronoi polycrystals with cubic symmetry and random orientations are used making the bulk material statistically isotropic. Example numerical results for materials with various degrees of scattering that are of common interest are presented. The numerical results are presented and compared with scattering theory for a wide range of frequencies. The numerical results show good agreement with the theory for the examples examined with evidence that the correlation function is frequency dependent. These results are anticipated to impact ultrasonic NDE of polycrystalline media.

Diffuse ultrasonic backscatter at normal incidence through a curved interface.

Diffuse ultrasonic backscatter techniques are useful for probing heterogeneous materials to extract microstructural parameters and detect flaws which cannot be detected by conventional ultrasonic techniques. Such experiments, usually done using a modified pulse-echo technique, utilize the spatial variance of the signals as a primary measure of microstructure. Quantitative ultrasonic scattering models include components of both transducer beams as well as microstructural scattering information. Of particular interest for interpretation of many experiments is the propagation through a liquid-solid interface. Here, a recent single-scattering model is expanded to include components needed for comparison with experiments. In particular, the Wigner distribution of the displacement profile is derived to model the beam pattern of an ultrasonic transducer through a curved liquid-solid interface. A simple Gaussian beam is used to model the transducer beam pattern. This expression is then used in conjunction with an appropriate scattering operator to complete the derivation. The theory developed is then compared with experimental results for a fine-grained steel using both a planar and a cylindrical interface. These results are anticipated to impact ultrasonic nondestructive evaluation and characterization of heterogeneous media with arbitrary curvatures.

Inter‐comparison of ultrasonic backscatter coefficient measurements of live rat fibroadenomas across multiple imaging platforms.

The backscatter coefficient (BSC) as a function of frequency is a system and operator independent parameter. It is the basis for some quantitative ultrasound (QUS) based on spectral analysis being translated into clinical use. This study aims to extend previous work in well‐characterized physical phantoms to live animals where tissue properties are unknown. Six Sprague Dawley rats with spontaneous mammary tumors (five fibroadenomas and one carcinoma) were imaged with three clinical systems (Zonare Z.one, Ultrasonix RP, and Siemens S2000) and one single element laboratory system. Data were acquired from approximately the same region of the tumor with each scanner using independent setups. Scans of a reference phantom and Plexiglas plate were acquired for the clinical and laboratory systems, respectively. The data were analyzed using methods developed by the respective research group. BSC versus frequency plots show agreement in magnitude and trend among the different systems. The BSC estimates overlap each...

Monitoring temperature elevations in tissues using quantitative ultrasound.

High intensity focused ultrasound (HIFU) is a noninvasive technique that has great potential for improving thermal therapies but requires accurate monitoring of lesion formation. Quantitative ultrasound (QUS) is a novel imaging technique that can improve monitoring of HIFU treatment by quantifying tissue changes. Ultrasonic backscatter experiments were performed on tissue‐mimicking phantoms, fresh rabbit liver, and beef liver samples versus increases in temperature from 37 to 50 °C in 1 °C increments. Two parameters were estimated from the backscatter coefficient [effective scatterer diameter (ESD) and effective acoustic concentration (EAC)] and two parameters were estimated from envelope statistics (k parameter and μ parameter) of the backscatter. No significant changes in ESD were observed for the phantoms but the ESD increased by more than 10% with increasing temperature in the liver samples. Significant decreases in EAC of 20%–40% were observed in all the samples. No specific trends were observed in e...

Comparison of ultrasonic backscatter coefficient estimates from rat mammary tumors using single element and array transducers.

The ultrasonic backscatter coefficient (BSC) is the fundamental quantitative estimate from measurements that can be parametrized to yield the effective scatterer diameter and acoustic concentration. The ability to accurately estimate the BSC using different imaging systems (i.e., a system independent estimate) is significant for clinical application of QUS imaging. In this study, BSCs were estimated from spontaneous mammary tumors in rats using both single‐element transducers and linear arrays from a clinical imaging system. The BSC as a function of frequency was computed from the rf backscattered signals from Sprague Dawley rats that developed either fibroadenoma or carcinoma tumors. The tumors were scanned using three single‐element transducers with a collective −10‐dB bandwidth of 1.4–18 MHz and two linear arrays from the Ultrasonix RP system with a collective −10‐dB bandwidth of 2–8 MHz. For the single‐element transducers, a smooth Plexiglas plate was used to acquire a reference scan. For the linear a...

A novel acoustic cell processing platform for cell concentration and washing

FloDesign Sonics has developed a technology to enable a single use (gamma irradiated) continuous cell concentration and wash application for manufacturing of cell-based therapies. The device has been designed to be able to process several liters of a suspended cell culture, e.g., T-cells, at concentrations of 1 to 10 M cells/ml. The cell suspension flows through the device and the acoustic radiation force field is used to trap and hold the cells in the acoustic field. After concentrating the cells, one or multiple washing steps are accomplished by flowing the washing fluid through the device, using the acoustic field to trap the cells while displacing the original cell culture fluid. The holdup volume of the device is about 30 ml. Results are shown for prototypes with a 1x0.75 inch flow chamber driven by 2 MHz PZT-8 transducers operating at flow rates of 1-2L/h, measured cell recoveries of 90% have been achieved with concentration factors of 20 to 50 for Jurkat T-cell suspensions, depending on cell concen...

Three-dimensional model for acoustic field created by a piezoelectric plate in a resonator

A three-dimensional model is developed to describe an acoustic field excited by a piezoelectric plate of finite size in a fluid filled resonator. First, the eigenfunctions (modes) of a bare plate are derived using general piezoelectric equations considering the elastic and electric properties of the plate. Then, the piezoelectric plate is placed into a fluid media such that only one plate side is in fluid and an acoustic field generated by the plate in the fluid is estimated. Finally, a reflector is placed to be parallel to the piezoelectric plate and acoustic field in a resonator is evaluated. The solution for a piezoelectric plate of finite size is obtained using Singular Value Decomposition (SVD) method. Equations for acoustic and electric variables are presented. Radiation force on spherical particles in the standing wave field is derived and discussed. Numerical results are presented to show the three-dimensional modal displacement and electrical characteristics of the plate at various frequencies an...

Quantitative ultrasound imaging to monitor in vivo high-intensity ultrasound treatment

The success of any minimally invasive treatment procedure can be enhanced significantly if combined with a robust noninvasive quantitative imaging modality. Quantitative ultrasound (QUS) imaging has been widely investigated for monitoring various treatment responses such as chemotherapy and thermal therapy. Previously we have shown the feasibility of using spectral based quantitative ultrasound parameters to monitor high-intensity focused ultrasound (HIFU) treatment of in situ tumors [Ultrasonic Imaging, 2014]. In the present study we examined the use the various QUS parameters to monitor HIFU treatment of an in vivo mouse mammary adenocarcinoma model. Spectral parameters in terms of the backscatter coefficient, integrated backscattered energy, attenuation coefficient, and effective scatterer size and concentration were estimated from radiofrequency signals during the treatment. The characteristic of each parameter was compared to the temperature profile recorded by needle thermocouple inserted into the tumor a few millimeters away from the focal zone of the intersecting HIFU and the imaging transducer beams. The changes in the QUS parameters during the HIFU treatment followed similar trends observed in the temperature readings recorded from the thermocouple. These results suggest that QUS techniques have the potential to be used for non-invasive monitoring of HIFU exposure.

Quantitative ultrasound imaging for assessing and monitoring therapy

Conventional ultrasound, which is routinely used for diagnostic imaging applications, is mainly qualitative. However, novel quantitative ultrasound (QUS) imaging modes are being adapted to quantify tissue properties for diagnosing disease, classifying tissues and monitoring and assessing therapy. Ultrasound is a propagating wave that interacts with a medium as a function of the spatially-dependent mechanical properties of the medium. By analyzing the backscattered wave, various properties of the propagating media can be quantified. QUS techniques based on parameterizing spectral features and envelope statistics of the backscattered signal were used to monitor and assess therapy from high intensity focused ultrasound (HIFU) treatment. QUS parameters were obtained by fitting theoretical models to backscatter coefficients (BSCs) that are estimated from backscattered radiofrequency signals. Additional parameters were estimated by fitting the homodyned K distribution to the statistics of the envelope of the ba...