bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultrasonic Error Compensation Method to Correct Instrumental and Systematic Errors in Volumetric Flow Measurements: A Theoretical Study
Authors: Fei Ding-Yu , Fu Cai-Ting , Chen Xunchang
Published: March 2013

A computational model has been developed to observe the effects of several instrumental parameters and systematic error sources on ultrasonic volumetric flow measurements in vessels using an error compensation method proposed previously by the authors. The effects of two essential instrumental parameters, the gate length and pulse duration, on two intermediate flow results and an optimal correction factor as a function of the beam width-to-vessel diameter ratio ( BWR) have been studied under steady flow conditions. The corrected flow results under typical combinations of those parameters show that by proper selection of a correction factor, the proposed method can compensate for those systematic errors. If the estimated BWR is within ±0.1 of the actual BWR value, the systematic error can be limited to within 6.5%. The study also shows that the compensation method can effectively reduce the errors caused by other systematic error sources such as the system sensitivity and high-pass filter function, as well as misalignment of the ultrasound beam from the center of the vessel.
Doppler Angle Estimation of Pulsatile Flows Using AR Modeling
Authors: Yeh Chih-Kuang , Li Pai-Chi
Published: March 2013

In quantitative ultrasonic flow measurements, the beam-to-flow angle (i.e., Doppler angle) is an important parameter. An autoregressive (AR) spectral analysis technique in combination with the Doppler spectrum broadening effect was previously proposed to estimate the Doppler angle. Since only a limited number of flow samples are used, real-time two-dimensional Doppler angle estimation is possible. The method was validated for laminar flows with constant velocities. In clinical applications, the flow pulsation needs to be considered. For pulsatile flows, the flow velocity is time-varying and the accuracy of Doppler angle estimation may be affected. In this paper, the AR method using only a limited number of flow samples was applied to Doppler angle estimation of pulsatile flows. The flow samples were properly selected to derive the AR coefficients and then more samples were extrapolated based on the AR model. The proposed method was verified by both simulations and in vitro experiments. A wide range of Doppler angles (from 30° to 78°) and different flow rates were considered. The experimental data for the Doppler angle showed that the AR method using eight flow samples had an average estimation error of 3.50° compared to an average error of 7.08° for the Fast Fourier Transform (FFT) method using 64 flow samples. Results indicated that the AR method not only provided accurate Doppler angle estimates, but also outperformed the conventional FFT method in pulsatile flows. This is because the short data acquisition time is less affected by the temporal velocity changes. It is concluded that real-time two-dimensional estimation of the Doppler angle is possible using the AR method in the presence of pulsatile flows. In addition, Doppler angle estimation with turbulent flows is also discussed. Results show that both the AR and FFT methods are not adequate due to the spectral broadening effects from the turbulence.
An Efficient Motion Estimation and Compensation Method for Ultrasound Synthetic Aperture Imaging
Authors: Kim K.S. , Hwang J.S. , Jeong J.S. , Song T.K.
Published: March 2013

This paper describes a method for overcoming motion artifacts in synthetic aperture imaging. The method is based on a computer simulation study on the influence of target motion on synthetic aperture techniques. A region-based motion compensation approach is used in which only the axial motion is estimated and compensated for a given region of interest under the assumption that the whole ROI moves uniformly. The estimated axial motion is calculated with a crosscorrelation method at the point where the focused signal has the maximum energy within the ROI. We also present a method for estimating axial motion using the autocorrelation method that is widely used to estimate average Doppler frequency. Both computer simulations and in vivo experiments show that the proposed crosscorrelation-based method can greatly improve the spatial resolution and SNR of ultrasound imaging by implementing SA techniques for two-way dynamic focusing without motion artifacts. In addition, the autocorrelation-based motion compensation method provides almost the same results as the crosscorrelation-based method, but with a dramatically reduced computational complexity.
Acoustic Radiation Force Impulse Imaging of In Vivo Vastus Medialis Muscle Under Varying Isometric Load
Authors: Nightingale Kathryn , Nightingale Roger , Stutz Deborah , Trahey Gregg
Published: March 2013

Acoustic Radiation Force Impulse (ARFI) imaging is a method for characterizing local variations in tissue mechanical properties. In this method, a single ultrasonic transducer array is used to both apply temporally short localized radiation forces within tissue and to track the resulting displacements through time. Images of tissue displacement immediately after force cessation, maximum tissue displacement, the time it takes for the tissue to reach its maximum displacement, and the recovery time constant of the tissue are generated from the ARFI data sets. The information in each of these images demonstrates good agreement with matched B-mode images. The study presented here was designed to evaluate the relationship between changes in these ARFI parameters with known tissue mechanical properties in vivo.

Utilizing a modified Siemens Elegra scanner with a 75L40 transducer array, ARFI images of vastus medialis muscle were generated in three of the authors under four levels of activation (0, 5.7, 14.5, and 23.3 N-m). Four ARFI datasets were acquired for each loading condition. The observed trends were that displacement magnitude, the time it took for the tissue to reach its maximum displacement, and recovery time constant decreased with increasing load (i.e., increasing muscle stiffness). Significant differences were observed between load levels and subjects for all parameters (p<0.01). The results indicate that ARFI imaging may be capable of quantifying tissue stiffness in real-time measurements, although further investigation is required.
Analysis of an Adaptive Strain Estimation Technique in Elastography
Authors: Srinivasan S. , Kallel F. , Souchon R. , Ophir J.
Published: March 2013

Elastography is based on the estimation of strain due to tissue compression or expansion. Conventional elastography involves computing strain as the gradient of the displacement (time-delay) estimates between gated pre- and postcompression signals. Uniform temporal stretching of the postcompression signals has been used to reduce the echo-signal decorrelation noise. However, a uniform stretch of the entire postcompression signal is not optimal in the presence of strain contrast in the tissue and could result in loss of contrast in the elastogram. This has prompted the use of local adaptive stretching techniques. Several adaptive strain estimation techniques using wavelets, local stretching and iterative strain estimation have been proposed. Yet, a quantitative analysis of the improvement in quality of the strain estimates over conventional strain estimation techniques has not been reported. We propose a two-stage adaptive strain estimation technique and perform a quantitative comparison with the conventional strain estimation techniques in elastography. In this technique, initial displacement and strain estimates using global stretching are computed, filtered and then used to locally shift and stretch the postcompression signal. This is followed by a correlation of the shifted and stretched postcompression signal with the precompression signal to estimate the local displacements and hence the local strains. As proof of principle, this adaptive stretching technique was tested using simulated and experimental data.
Contrast and Velocity Ultrasonic Tomography of Long Bones
Authors: Ouedraogo E. , Lasaygues P. , Lefebvre J.P. , Gindre M. , Talmant M. , Laugier P.
Published: March 2013

Our objective is to derive quantitative sound speed images of cortical bone using ultrasonic transmission tomography. Cortical bone is a highly refracting medium, i.e., the sound velocity changes abruptly across the interface between soft tissue and bone. It results in a loss of data compared to classical tomography in soft tissues. In order to correct for degradation by refraction effects, the classical acquisition procedure of projection data is modified; the transducers are oriented according to Snell's law of refraction with the aim of optimizing the sound propagation as parallel longitudinal rays inside the bone. This strategy allows the subsequent application of straight-ray reconstruction by the backprojection technique, which is a classical procedure in x-ray tomography. The method is validated with Plexiglas® solid cylinders and tubes immersed in water. Improved sound velocity images are then derived using conventional Radon transform of the experimental time-of-flight data. The method is then extended to in vitro human femur immersed in water. The geometry of the bone cross-section is reconstructed from measurements using ultrasonic reflection tomography. The result is then introduced in the calculation of the position and orientation of the transducers, which are associated with the parallel acoustical paths in bone in the transmission measurements. The procedure leads to significant restoration enhancement over the non corrected image. The mean value of the velocity of 3,200 ms−1 in the cortical shell is consistent with the values known from literature. These preliminary quantitative images using combined reflected and transmission ultrasound show promise for bone imaging.
A Modified Block Matching Method for Real-Time Freehand Strain Imaging
Authors: Zhu Yanning , Hall Timothy J.
Published: March 2013

This manuscript reports a technical innovation that has been developed for real-time, freehand strain imaging. This work is based on a well-known block-matching algorithm with two significant modifications. First, since displacements are estimated row-by-row, displacement estimates from the previous row are used to predict the displacement estimates in the current row, thereby drastically reducing the search-region size and increasing computational efficiency. Second, a displacement error detection and correction method is developed to overcome the local tracking errors that may be more severe with freehand scanning and thereby improve the robustness of the algorithm. This algorithm has been implemented on a clinical ultrasound imaging system, and with real-time imaging feedback, long sequences of high quality strain images are observed using freehand compression. Displacement estimation errors with this method are experimentally measured and compared with results from simulation. We report only a specific implementation, with no comparison to other displacement estimators in the literature and no optimization of this specific technique. Images of tissue-mimicking phantoms with small spherical targets are used to test the ability to detect small lesions using the strain imaging technique. In vivo strain images of breast and thyroid are also shown.
Adaptive Clutter Filtering via Blind Source Separation for Two-Dimensional Ultrasonic Blood Velocity Measurement
Authors: Gallippi Caterina M. , Trahey Gregg E.
Published: March 2013

A method for adaptive clutter rejection via blind source separation (BSS) using principal and independent component analyses is presented in application to blood velocity measurement in the carotid artery. In particular, the filtering method's efficacy for eliminating clutter and preserving lateral blood flow signal components is presented. The performance of IIR filters is compromised by shorth data ensembles (10 to 20 temporal samples) as implemented for color-flow and high frame-rate imaging due to initialization requirements. Further, the ultrasonic imaging system's transfer function maps axial wall and lateral blood motion to overlapping spectra. As such, frequency domain-based approaches to wall filtering are ineffective for distinguishing wall from blood motion signals. Rather than operating in the frequency domain, BSS performs clutter rejection by decomposing the input data ensemble into N constitutive source signals in time, where N is the ensemble length. Source signal energy coupled with respective signal depth and time course profiles reveal which source signals correspond to blood, noise and clutter components. Clutter components may then be removed without disruption of lateral blood flow information needed for two-dimensional blood velocity measurement. A simplistic data simulation is employed to offer an intuitive understanding of BSS methods for signal separation. The adaptive BSS filter is further demonstrated using a Field II simulation of blood flow through the carotid artery including tissue motion. BSS clutter filter performance is compared to the performance of FIR, IIR and polynomial regression clutter filters. Finally, the filter is employed for clinical application using a Siemens Elegra scanner, carotid artery data with lateral blood flow collected from healthy volunteers, and Speckle Tracking; velocity magnitude and angle profiles are shown. Once again, the BSS clutter filter is contrasted to FIR, IIR and polynomial regression clutter filters using clinical examples. Velocities computed with Speckle Tracking after BSS wall filtering are highest in the center of the artery and diminish to low velocities near the vessel walls, with velocity magnitudes consistent with physiological expectations. These results demonstrate that the BSS adaptive filter sufficiently suppresses wall motion signal for clinical lateral blood velocity measurement using data ensembles suitable for color-flow and high frame-rate imaging.
Temperature Dependence of Ultrasonic Propagation Speed and Attenuation in Canine Tissue
Authors: Techavipoo U. , Varghese T. , Zagzebski J.A. , Stiles T. , Frank G.
Published: March 2013

Previously reported data on the temperature dependence of propagation speed in tissues generally span only temperature ranges up to 60°C. However, with the emerging use of thermal ablative therapies, information on variation in this parameter over higher temperature ranges is needed. Measurements of the ultrasonic propagation speed and attenuation in tissue in vitro at discrete temperatures ranging from 25 to 95°C was performed for canine liver, muscle, kidney and prostate using 3 and 5 MHz center frequencies. The objective was to produce information for calibrating temperature-monitoring algorithms during ablative therapy. Resulting curves of the propagation speed vs. temperature for these tissues can be divided into three regions. In the 25–40°C range, the speed of sound increases rapidly with temperature. It increases moderately with temperature in the 40–70°C range, and it then decreases with increasing temperature from 70–95°C. Attenuation coefficient behavior with temperature is different for the various tissues. For liver, the attenuation coefficient is nearly constant with temperature. For kidney, attenuation increases approximately linearly with temperature, while for muscle and prostate tissue, curves of attenuation vs. temperature are flat in the 25–50°C range, slowly rise at medium temperatures (50–70°C), and level off at higher temperatures (70–90°C). Measurements were also conducted on a distilled degassed water sample and the results closely follow values from the literature.
Improved Fourier-Transform-Based Parallel Receive Beam Formation
Authors: Li Meng-Lin , Li Pai-Chi
Published: March 2013

A Fourier transform (FT)-based technique for forming parallel receive beams has been previously employed to increase the imaging frame rate in ultrasonic imaging. However, the image quality in FT-based parallel reconstruction is degraded because differences in range focusing delays are ignored and a wide transmit beam needs to be used. In this paper, an adaptive weighting technique based on a focusing-quality index is used to reduce the sidelobes of the FT-derived parallel receive beams. The focusing-quality index is derived from the spatial spectrum of the received aperture data after the receive delays have been applied. Since the spatial spectrum of the baseband aperture data is also used to approximate receive beams in FT-based parallel reconstruction, the adaptive weighting technique can be directly combined with the FT-based technique for forming parallel receive beams with only a slight increase in system complexity. Real ultrasound data are used to demonstrate the efficacy of the proposed technique on both wire targets and speckle-generating objects. The results clearly demonstrate the effectiveness in reducing the sidelobes. In addition, the image background noise is suppressed. The principles, experimental results, and the extension of the proposed technique to 3D ultrasound imaging are described in this paper.
An ASIC Design for Versatile Receive Front-End Electronics of an Ultrasonic Medical Imaging System — 16 Channel Analog Inputs and 4 Dynamically Focused Beam Outputs
Authors: Park Song B. , Kwak Jaeyoung , Lee Kwyro
Published: March 2013

An ultra large-scale ASIC is designed for the receive front-end electronics of an ultrasonic medical imaging system. The chip receives 16 channel analog rf signals and outputs 4 sets of sample-point-wise dynamically focused partial beam data. Four complete beam data sets are obtained in parallel by simply cascading as many chips as needed in an array system. High resolution of the focusing delay is obtained by nonuniformly selecting each channel data from a quadruply-interpolated rf data stream. The proposed ASIC can be applied to most practical array transducers in the frequency range of 2 to 10 MHz. The digital part of the designed ASIC can be implemented on a chip area of 17.9 μm2 with 0.18 mm CMOS technology, leaving sufficient room for 16 ADCs of 8 bits, 50 MHz on the 5.7 mm × 5.7 mm chip with a 208 pin package.
Vascular Elasticity from Regional Displacement Estimates
Authors: Mai Jerome J. , Lupotti Fermin A. , Insana Michael F.
Published: March 2013

A recently-developed ultrasonic technique for measuring elastic properties of vascular tissue is evaluated using computer simulations, phantom and in vivo human measurements. A time sequence of displacement images is measured over the cardiac cycle to describe the spatial and temporal patterns of deformation surrounding arteries. This information is combined with a mathematical model to estimate an elastic modulus. Computer simulations of ultrasonic echo data from deformed tissues are analyzed to define a signal processing approach. Measurements in flow phantoms, with and without vessel-simulating channel walls, provide an assessment of the accuracy and precision of this technique for vascular elasticity measurements. Finally, preliminary results for the stiffness index (β) in a study group of healthy human volunteers are compared with previously reported data. We find that careful measurement technique is required to control measurement variability.
Ultrasound Attenuation Imaging Using Compound Acquisition and Processing
Authors: Tu Haifeng , Varghese Tomy , Madsen Ernest L. , Chen Quan , Zagzebski James A.
Published: March 2013

A method that combines both spatial and frequency compounding is described for measuring attenuation in tissue. The technique applies a reference phantom to account for imaging system dependencies of echo signals. Emphasis is given to local attenuation estimates, to reduce the variance of the attenuation measurements over small regions of interest (ROI) and to enable coarse attenuation imaging. Experiments using a uniform phantom show that the standard deviation of local attenuation estimates within a ROI drops when greater degrees of compounding are applied. Attenuation images of a specially designed phantom containing inclusions with attenuation contrast illustrate the accuracy and precision of the technique.
Correction for Simultaneous Catheter Eccentricity and Tilt in Intravascular Elastography
Authors: Shi H. , Varghese T. , Chen Q. , Gimelli G.
Published: March 2013

Intravascular elastography can provide significant new information about the elastic properties of vascular tissue and plaque, useful for the diagnosis of disease and appropriate selection of interventional methods. Knowledge of the plaque composition, vulnerability and its elastic properties can assist the clinician in selecting appropriate interventional techniques. However, several noise sources have to be addressed to obtain quality intravascular elastograms. Misalignment of the vessel lumen and the ultrasound beam can produce erroneous strain estimates in elastography. Errors in the strain estimate are introduced due to the eccentricity and tilt of the intravascular transducer within the vessel lumen. Previous work in this area has provided theoretical expressions for the correction of eccentricity and tilt errors when they occur independent of each other. However, under most imaging conditions, both eccentricity and tilt errors are simultaneously present. In this paper, we extend the theoretical correction factor by accounting for the influence of both of these errors occurring simultaneously in the positioning of the catheter within the vessel lumen.
A Novel and Robust Method for Rapid Strain Estimation in Elastography
Authors: Alam S. Kaisar
Published: March 2013

In elastography, change in signal shape from tissue deformation and nonaxial tissue motion reduce correlation between the pre- and postcompression echo signals. Appropriate global temporal stretching of postcompression signals can reduce the decorrelation. Adaptive stretching performs a search for the stretch factor that maximizes the correlation between the pre- and postcompression echo signal segments at each data window location. Adaptive stretching is robust but computation intensive. In contrast, global stretching is fast but performs well only in areas where local strains are close to the applied strain. We developed a method that strikes a balance between the speed of global stretching and the performance of adaptive stretching. In this method, several strain maps are computed by performing global stretching with a range of different stretch factors. The area in each computed strain image with strain values closely corresponding to the uniform stretch factor will contain ‘good quality’ strain estimates. To produce a single elastogram at the end, we identify the strain map with the maximum correlation at each location and the strain value in that strain map at that location is chosen for the combined map. Results from data generated by finite-element simulation and phantom experiments demonstrate that the described strain estimator is significantly less susceptible to signal degradation than conventional strain estimators.
Monitoring Thermally-Induced Lesions with Supersonic Shear Imaging
Authors: Bercoff J. , Pernot M. , Tanter M. , Fink M.
Published: March 2013

Thermally-induced lesions are generally stiffer than surrounding tissues. We propose here to use the supersonic shear imaging technique (SSI) for monitoring high-intensity focused ultrasound (HIFU) therapy. This new elasticity imaging technique is based on remotely creating shear sources using an acoustic radiation force at different locations in the medium. In these experiments, an HIFU probe is used to generate lesions in fresh tissue samples. A diagnostic transducer, controlled by our ultrafast scanner, is located in the therapeutic probe focal plane. It is used for both generating the shear waves and imaging the resulting propagation at frame rates reaching 5,000 images/s. Movies of the shear wave propagation can be computed off-line. The therapeutic and imaging sequences are interleaved and a set of wave propagation movies is performed during the heating process. From each movie, elasticity estimations have been performed using an inversion algorithm. It demonstrates the feasibility of detecting and quantifying the hardness of HIFU-induced lesions using SSI.
Experimental Studies of the Thermal Effects Associated with Radiation Force Imaging of Soft Tissue
Authors: Palmeri Mark L. , Frinkley Kristin D. , Nightingale Kathryn R.
Published: March 2013

Many groups are studying acoustic radiation force-based imaging modalities to determine the mechanical properties of tissue. Acoustic Radiation Force Impulse (ARFI) imaging is one of these modalities that uses standard diagnostic ultrasound scanners to generate localized, impulsive, acoustic radiation force in tissue. This radiation force generates tissue displacements that are tracked using conventional correlation-based ultrasound methods. The dynamic response of tissue to this impulsive radiation force provides information about the mechanical properties of the tissue.

The generation of micron-scale displacements using acoustic radiation force in tissue requires the use of high-intensity acoustic beams, and the soft tissue heating associated with these high-intensity beams must be evaluated to ensure safety when performing ARFI imaging in vivo. Experimental studies using thermocouples have validated Finite Element Method (FEM) models that simulate the heating of soft tissue during ARFI imaging. Spatial maps of heating measured with the thermocouples are in good agreement with FEM model predictions, with cooling time constants measured and modeled to be on the order of several seconds. Transducer heating during ARFI imaging has been measured to be less than 1 °C for current clinical implementations. These validated FEM models can now be used to simulate soft tissue heating associated with different transducers, beam spacing, focal configurations and thermal material properties. These experiments confirm that ARFI imaging of soft tissue is safe, although thermal response must be monitored when developing ARFI beam sequences for specific tissue types and organ systems.
Recent Developments in Tissue-Type Imaging (TTI) for Planning and Monitoring Treatment of Prostate Cancer
Authors: Feleppa Ernest J. , Porter Christopher R. , Ketterling Jeffrey , Lee Paul , Dasgupta Shreedevi , Urban Stella , Kalisz Andrew
Published: March 2013

Because current methods of imaging prostate cancer are inadequate, biopsies cannot be effectively guided and treatment cannot be effectively planned and targeted. Therefore, our research is aimed at ultrasonically characterizing cancerous prostate tissue so that we can image it more effectively and thereby provide improved means of detecting, treating and monitoring prostate cancer. We base our characterization methods on spectrum analysis of radiofrequency (rf) echo signals combined with clinical variables such as prostate-specific antigen (PSA). Tissue typing using these parameters is performed by artificial neural networks. We employed and evaluated different approaches to data partitioning into training, validation, and test sets and different neural network configuration options. In this manner, we sought to determine what neural network configuration is optimal for these data and also to assess possible bias that might exist due to correlations among different data entries among the data for a given patient. The classification efficacy of each neural network configuration and data-partitioning method was measured using relative-operating-characteristic (ROC) methods. Neural network classification based on spectral parameters combined with clinical data generally produced ROC-curve areas of 0.80 compared to curve areas of 0.64 for conventional transrectal ultrasound imaging combined with clinical data. We then used the optimal neural network configuration to generate lookup tables that translate local spectral parameter values and global clinical-variable values into pixel values in tissue-type images (TTIs). TTIs continue to show cancerous regions successfully, and may prove to be particularly useful clinically in combination with other ultrasonic and nonultrasonic methods, e.g., magnetic-resonance spectroscopy.
Real-Time 3D Transesophageal Echocardiography
Authors: Pua Eric C. , Idriss Salim F. , Wolf Patrick D. , Smith S. W.
Published: March 2013

Transesophageal echocardiography (TEE) is an essential diagnostic tool in patients with poor transthoracic echocardiographic windows or when detailed imaging of structures distant from the chest wall is necessary. A real-time 3D TEE probe has been fabricated in our laboratory in order to increase the amount of information available during a transesophageal procedure. The 1 cm diameter esophageal probe utilizes a 2-dimensional, 5 MHz array at its tip with a 6.3 mm diameter aperture, including 504 active channels. The array has a periodic vernier geometry with an element pitch of 0.18 mm, built onto a multilayer flexible (MLF) interconnect circuit. In order to accommodate 504 channels within the device, a 1 m long Gore MicroFlat™ cable was utilized for wiring the MLF to the corresponding system connectors. Pulse-echo tests in a water tank have yielded a −6 dB bandwidth of 25.3%. Fully connected to the system through 3 m of cable, the probe shows an average 50 Ω insertion loss of −85 dB with a standard deviation of 4 dB, as determined through pitch-catch measurements for a sampling of 10 elements. Using the completed 3D TEE probe with the Volumetrics Medical Imaging 3D scanner, real-time volumetric images of in vivo canine cardiac anatomy have been acquired, displaying atrial views, mitral valve function and interventional catheter guidance.
Effects of Swept Scanning on Velocity Estimation
Authors: Jeng Geng-Shi , Li Pai-Chi
Published: March 2013

The swept-scan technique adopted in high-frequency ultrasound involves mechanically scanning a single-element transducer to acquire image data. Unlike conventional step scanning, where the image data are acquired at discrete positions, the swept-scan technique acquires the image data while the transducer is continuously moving. Such a scanning method is particularly advantageous for Doppler flow estimation because its frame rate is higher than that for the step-scan technique. However, the effects of the transducer motion on the accuracy of velocity estimation have not been studied comprehensively. This study employed a k-space approach to experimentally investigate the effects of swept scanning on both conventional Doppler axial velocity estimation and spectral-broadening-based lateral velocity estimation using a 45-MHz transducer. The results indicate that such effects must be corrected in order to obtain an accurate estimation of flow velocities.
Design and Fabrication of Ultrafine Piezoelectric Composites
Authors: Yin J. , Lukacs M. , Harasiewicz K.A. , Foster F.S.
Published: March 2013

Making fine scale (<20 μm) piezoelectric composites for high frequency (>50 MHz) ultrasound transducers remains challenging. Interdigital phase bonding (IPhB), described in this paper, presents a new technique developed to make piezoelectric composites at the ultrafine scale using a conventional dicing saw. Using the IPhB technique, a composite structure with a pitch that is less than the dicing saw blade thickness can be created. The approach is flexible enough to make composites of different combination of pitch and volume ratio. Using a conventional dicing saw with a 50 μm thick blade, composite with a 25 μm pitch and a volume ratio of 61 percent are fabricated. Such a composite is suitable for fabrication of ultrasonic transducers and arrays with central frequencies of up to 85 MHz. Single element transducers working at central frequencies of 50–60 MHz were made of these composites as a mean to characterize the acoustic performance. Measurement results of the transducers show that the longitudinal electromechanical coupling coefficient is greater than 0.6 and that there are no noticeable lateral resonances in the frequency range of 55-150 MHz. Design criteria for fine scale elements are also discussed based on theoretical results from finite element analysis (FEA).
Elastic Moduli of Thyroid Tissues under Compression
Authors: Lyshchik A. , Higashi T. , Asato R. , Tanaka S. , Ito J. , Hiraoka M. , Brill A.B. , Saga T. , Togashi K.
Published: March 2013

The aim of this study was to evaluate the elastic moduli of thyroid tissues under uniaxial compression and to establish the biomechanical fundamentals for accurate interpretation of thyroid elastograms. A total of 67 thyroid samples (24 samples of normal thyroid tissue, 2 samples of thyroid tissue with chronic thyroiditis, 12 samples of adenomatous goiter lesions and 7 samples of follicular adenoma, 19 samples of papillary adenocarcinoma (PAC) and 3 samples of follicular adenocarcinoma (FAC)) obtained from 36 patients who had received thyroid surgery were subjected to biomechanical testing within three hours after surgical resection at precompression strains of 5%, 10% and 20% and applied strains of 1%, 2%, 5% and 10% of sample height. As a result, the mean values of elastic moduli for benign thyroid lesions at all examined precompression levels were significantly higher than those for normal thyroid tissue measured at the same load (p<0.01). At low precompression (5%) and compression (1–2%) levels, benign thyroid nodule samples were 1.7 times harder than normal thyroid tissue. At high precompression (20%) and compression (10%) levels, this difference increased to 2.4 times. Stiffness of PAC samples was significantly higher than those for normal thyroid tissue and benign thyroid tumors measured at the same load (p<0.01). At low precompression (5%) and compression (1–2%) levels, PAC samples were 5.0 times harder than normal thyroid tissue. At high precompression (20%) and compression (10%) levels, this difference increased to 17.7 times. In contrast, samples of FAC were much softer than PAC (p<0.05) and were comparable in stiffness to normal thyroid tissues. The significant differences in the stiffness between normal thyroid tissue and thyroid tumors may provide useful information for accurate interpretation of thyroid elastograms.
Adaptive Imaging Using an Optimal Receive Aperture Size
Authors: Li Meng-Lin , Huang Sheng-Wen , Üstüner Kutay , Li Pai-Chi
Published: March 2013

Sidelobe contribution from off-axis targets degrades image quality in a coherent array imaging system. In ultrasound imaging, focusing errors resulting from sound-velocity inhomogeneities in human tissue — also known as phase aberrations — reduce the coherence of the received signals and elevate the sidelobe level. This paper proposes an adaptive receive-aperture technique based on thresholding of the coherence factor (CF). The CF describes the coherence of the received array signals and can be used as an index of focusing quality. This paper demonstrates that thresholding of the CF allows the mainlobe-dominated signals to be distinguished from the sidelobe-dominated signals, after which the receive-aperture size at each imaging position can be optimally determined so as to enhance the mainlobe-dominated signals and suppress the sidelobe-dominated signals. Thus, image quality degradation resulting from sound-velocity inhomogeneities can be reduced. Simulations and measured ultrasound data are used to evaluate the efficacy of the proposed technique. The characteristics of the proposed technique including the effects of the signal-to-noise ratio (SNR) and the transmit focal depth, and speckle reduction are discussed. The proposed technique is also compared with the parallel adaptive receive compensation algorithm and shown to produce a better improvement in image quality.
Noise Performance and Signal-to-Noise Ratio of Shear Strain Elastograms
Authors: Kumar Arun Thitai , Ophir Jonathan , Krouskop Thomas A.
Published: March 2013

In this paper, we develop a theoretical expression for the signal-to-noise ratio (SNR) of shear strain elastograms. The previously-developed ideas for the axial strain filter (ASF) and lateral strain filter (LSF) are extended to define the concept of the shear strain filter (SSF). Some of our theoretical results are verified using simulations and phantom experiments. The results indicate that the signal-to-noise ratio of shear-strain elastograms ( SNRsse) improves with increasing shear strain and with improvements in system parameters such as the sonographic signal-to-noise ratio ( SNRs) beamwidth, center frequency and fractional bandwidth. The results also indicate that the amount of axial strain present along with the shear strain is an important parameter that determines the upper bound on SNRsse. The SNRsse will be higher in the absence of additional deformation due to axial strain.
A Tutorial on the Use of ROC Analysis for Computer-Aided Diagnostic Systems
Authors: Scheipers Ulrich , Perrey Christian , Siebers Stefan , Hansen Christian , Ermert Helmut
Published: March 2013

The application of the receiver operating characteristic (ROC) curve for computer-aided diagnostic systems is reviewed. A statistical framework is presented and different methods of evaluating the classification performance of computer-aided diagnostic systems, and, in particular, systems for ultrasonic tissue characterization, are derived.

Most classifiers that are used today are dependent on a separation threshold, which can be chosen freely in many cases. The separation threshold separates the range of output values of the classification system into different target groups, thus conducting the actual classification process. In the first part of this paper, threshold specific performance measures, e.g., sensitivity and specificity; are presented. In the second part, a threshold-independent performance measure, the area under the ROC curve, is reviewed. Only the use of separation threshold-independent performance measures provides classification results that are overall representative for computer-aided diagnostic systems.

The following text was motivated by the lack of a complete and definite discussion of the underlying subject in available textbooks, references and publications. Most manuscripts published so far address the theme of performance evaluation using ROC analysis in a manner too general to be practical for everyday use in the development of computer-aided diagnostic systems. Nowadays, the user of computer-aided diagnostic systems typically handles huge amounts of numerical data, not always distributed normally. Many assumptions made in more or less theoretical works on ROC analysis are no longer valid for real-life data. The paper aims at closing the gap between theoretical works and real-life data. The review provides the interested scientist with information needed to conduct ROC analysis and to integrate algorithms performing ROC analysis into classification systems while understanding the basic principles of classification.
A New Method for Generating Poroelastograms in Noisy Environments
Authors: Righetti Raffaella , Ophir Jonathan , Garra Brian S. , Chandrasekhar Rajah M. , Krouskop Thomas A.
Published: March 2013

Poroelastography has been recently introduced as a new elastographic technique that may be used to describe the spatial and temporal behavior of poroelastic materials. The experimental methodology proposed thus far for phantoms and tissues in vitro requires the acquisition of a precompression rf frame, the application of a unit step strain compression to the sample and the acquisition of subsequent post-compression frames from the material. Elastograms and poroelastograms are generated by cross-correlating the sequentially-acquired postcompression frames with the reference precompression frame. The application of poroelastography to tissues in vivo must address the echo decorrelation problems that are encountered due to uncontrolled tissue motion, which may become significant shortly after the acquisition of the precompression frame. In this paper, we investigate the feasibility of performing poroelastography experiments using an alternative experimental scheme. In the proposed experimental methodology, the reference precompression frame is continuously moved while the time interval between the frames that are correlated is kept short. This allows long data acquisition times with simultaneous minimization of the decorrelation due to undesired tissue motion in vivo. We validated this new method using both a step and a ramp compression functions. We performed poroelastographic simulations and experiments in phantoms and in tissues in vivo. The results were compared to those obtained using the traditional acquisition methodology. This study shows that the two methods yield similar results in vitro and suggests that the new method may be more robust to decorrelation noise in applications in vivo.
Field Computation for Two-Dimensional Array Transducers with Limited Diffraction Array Beams
Authors: Lu Jian-Yu , Cheng Jiqi
Published: March 2013

A method is developed for calculating fields produced with a two-dimensional (2D) array transducer. This method decomposes an arbitrary 2D aperture weighting function into a set of limited diffraction array beams. Using the analytical expressions of limited diffraction beams, arbitrary continuous wave (cw) or pulse wave (pw) fields of 2D arrays can be obtained with a simple superposition of these beams. In addition, this method can be simplified and applied to a 1D array transducer of a finite or infinite elevation height. For beams produced with axially symmetric aperture weighting functions, this method can be reduced to the Fourier-Bessel method studied previously where an annular array transducer can be used. The advantage of the method is that it is accurate and computationally efficient, especially in regions that are not far from the surface of the transducer (near field), where it is important for medical imaging.

Both computer simulations and a synthetic array experiment are carried out to verify the method. Results (Bessel beam, focused Gaussian beam, X wave and asymmetric array beams) show that the method is accurate as compared to that using the Rayleigh-Sommerfeld diffraction formula and agrees well with the experiment.
Spatial Angular Compounding for Elastography without the Incompressibility Assumption
Authors: Rao Min , Varghese Tomy
Published: March 2013

Spatial-angular compounding is a new technique that enables the reduction of noise artifacts in ultrasound elastography. Previous results using spatial angular compounding, however, were based on the use of the tissue incompressibility assumption. Compounded elastograms were obtained from a spatially-weighted average of local strain estimated from radiofrequency echo signals acquired at different insonification angles. In this paper, we present a new method for reducing the noise artifacts in the axial strain elastogram utilizing a least-squares approach on the angular displacement estimates that does not use the incompressibility assumption. This method produces axial strain elastograms with higher image quality, compared to noncompounded axial strain elastograms, and is referred to as the least-squares angular-compounding approach for elastography. To distinguish between these two angular compounding methods, the spatial-angular compounding with angular weighting based on the tissue incompressibility assumption is referred to as weighted compounding. In this paper, we compare the performance of the two angular-compounding techniques for elastography using beam steering on a linear-array transducer. Quantitative experimental results demonstrate that least-squares compounding provides comparable but smaller improvements in both the elastographic signal-to-noise ratio and the contrast-to-noise ratio, as compared to the weighted-compounding method. Ultrasound simulation results suggest that the least-squares compounding method performs better and provide accurate and robust results when compared to the weighted compounding method, in the case where the incompressibility assumption does not hold.
Ultrasonic Backscatter and Attenuation (11–27 MHz) Variation with Collagen Fiber Distribution in Ex Vivo Human Dermis
Authors: Bridal S. Lori , Fournier Céline , Coron Alain , Leguerney Ingrid , Laugier Pascal
Published: March 2013

This ex vivo study explores the relationship of ultrasonic attenuation and backscatter to dermal microarchitecture by comparing ultrasonic measurements of these parameters (11–27 MHz) to a microscopic analysis of three parameters describing the collagen distribution (mean thickness and spacing of collagen bundles along the insonification direction and the percent area occupied by collagen). Skin samples ( N = 31) were obtained from patients undergoing breast or abdominal reduction surgery. Radio-frequency (rf) signals were acquired in a B-scan format using an ultrasound system developed for skin imaging (Ultrasons Technologies, Tours, France). Ultrasonic data were analyzed to calculate average integrated backscatter ( IBS in dB) and frequency dependence of backscatter ( n, dimensionless) of each specimen at depths centered approximately 370, 620 and 880 μm beneath the skin surface. Average integrated attenuation coefficient ( IA in dB.cm−1) and frequency dependence of attenuation coefficient (β in dB.cm−1.MHz−1) were estimated across the depth between 240 and 1,000 μm. The three collagen distribution parameters were estimated using digitized microcopic fields from matched regions of histological sections stained with hematoxylin-eosin-saffron. No significant correlation was identified between collagen distribution parameters and IA or β. For the most superficial depth studied in abdominal skin, n was inversely correlated to collagen bundle thickness ( r = −0.67, p = 0.002) and percent area (r = −0.65, p = 0.003). At the same depth, IBS was inversely correlated to percent area of collagen ( r = −0.51, p = 0.03). The rather high collagen packing (48 to 82% area) measured in histological sections and the inverse relationship observed between IBS and percent area of collagen suggest that a packing factor should be included in models relating skin collagen distribution to ultrasound spectral parameters. A better understanding of the relationship between ultrasound parameters and the microarchitecture of the dermis should help to interpret changes in ultrasonic parameters observed during in vivo ultrasonic skin examinations.
Characterizing Acoustic Attenuation of Homogeneous Media Using Focused Impulsive Acoustic Radiation Force
Authors: Palmeri Mark L. , Frinkley Kristin D. , Oldenburg Katherine G. , Nightingale Kathryn R.
Published: March 2013

A new method to characterize a material's attenuation using acoustic radiation force is proposed. Comparison of displacement magnitudes generated in a homogeneous material by acoustic radiation force excitations can be used to estimate the material's attenuation when the excitations are applied over a range of focal depths while maintaining a constant lateral focal configuration. Acoustic attenuations are related to the inverse of the excitation focal depth that yields the greatest focal zone displacement for this protocol. Experimental studies in calibrated tissue-mimicking phantoms are presented to demonstrate the feasibility of this method. Attenuations ranging from 0.3–1.5 dB/cm/MHz were characterized over excitation focal depths ranging from 5–30 mm, with an accuracy of 0.1 ± 0.15 dB/cm/MHz. As currently implemented, this method is limited to characterizing materials that have homogeneous material properties and acoustic attenuations. This method for characterizing acoustic attenuation can be performed using conventional diagnostic scanners without any additional hardware and could also be performed concurrently with acoustic radiation force-based imaging modalities to generate images of mechanical properties and attenuation that are spatially co-registered with B-mode images.