Extensions of nonlinear B/A parameter imaging methods for echo mode

Deep learning for assessing liver fibrosis based on acoustic nonlinearity maps: an in vivo study of rabbits

This study aimed to assess liver fibrosis in rabbits by deep learning models based on acoustic nonlinearity maps. Injection of carbon tetrachloride was used to induce liver fibrosis. Acoustic nonlinearity maps, which were built by data of echo signals, were used as input data for deep learning model. Convolutional neural network (CNN), CNN combined with support vector machine (SVM), CNN combined with random forest and CNN combined with logistic regression were used as deep learning model. Nested 10-fold cross-validation was used to search hyperparameters and evaluate performance of models. Histologic examination of liver specimens of the rabbits was performed to evaluate the fibrosis stage. Receiver operator characteristic curve and area under curve (AUC) were used for estimating the probability of the correct prediction of liver fibrosis stages. A total of 600 acoustic nonlinearity maps were used. Model of CNN combined with SVM demonstrated the best diagnostic performance compared with all other methods for diagnosis of significant fibrosis (≥F2, AUC = 0.82), advanced fibrosis (≥F3, AUC = 0.88) and cirrhosis (F4, AUC = 0.90). Model of CNN showed the second highest AUCs. The deep learning model based on acoustic nonlinearity maps demonstrated potential for evaluation of liver fibrosis.

The Impact of Vendor-Specific Ultrasound Beam-Forming and Processing Techniques on the Visualization of In Vitro Experimental "Scar": Implications for Myocardial Scar Imaging Using Two-Dimensional and Three-Dimensional Echocardiography

BACKGROUND

Myocardial scar appears brighter compared with normal myocardium on echocardiography because of differences in tissue characteristics. The aim of this study was to test how different ultrasound pulse characteristics affect the brightness contrast (i.e., contrast ratio [CR]) between tissues of different acoustic properties, as well as the accuracy of assessing tissue volume.

METHODS

An experimental in vitro "scar" model was created using overheated and raw pieces of commercially available bovine muscle. Two-dimensional and three-dimensional ultrasound scanning of the model was performed using combinations of ultrasound pulse characteristics: ultrasound frequency, harmonics, pulse amplitude, steady pulse (SP) emission, power modulation (PM), and pulse inversion modalities.

RESULTS

On both two-dimensional and three-dimensional imaging, the CR between the "scar" and its adjacent tissue was higher when PM was used. PM, as well as SP ultrasound imaging, provided good "scar" volume quantification. When tested on 10 "scars" of different size and shape, PM resulted in lower bias (-9.7 vs 54.2 mm³) and narrower limits of agreement (-168.6 to 149.2 mm³ vs -296.0 to 404.4 mm³, P = .03). The interobserver variability for "scar" volume was better with PM (intraclass correlation coefficient = 0.901 vs 0.815). Two-dimensional and three-dimensional echocardiography with PM and SP was performed on 15 individuals with myocardial scar secondary to infarction. The CR was higher on PM imaging. Using cardiac magnetic resonance as a reference, quantification of myocardial scar volume showed better agreement when PM was used (bias, -645 mm³; limits of agreement, -3,158 to 1,868 mm³) as opposed to SP (bias, -1,138 mm³; limits of agreement, -5,510 to 3,233 mm³).

CONCLUSIONS

The PM modality increased the CR between tissues with different acoustic properties in an experimental in vitro "scar" model while allowing accurate quantification of "scar" volume. By applying the in vitro findings to humans, PM resulted in higher CR between scarred and healthy myocardium, providing better scar volume quantification than SP compared with cardiac magnetic resonance.

A review on B/A measurement methods with a clinical perspective

The nonlinear parameter of ultrasound B/A has shown to be a useful diagnostic parameter, reflecting medium content, structure, and temperature. Despite its recognized values, B/A is not yet used as a diagnostic tool in the clinic due to the limitations of current measurement and imaging techniques. This review presents an extensive and comprehensive overview of the techniques developed for B/A measurement of liquid and liquid-like media (e.g., tissue), identifying the methods that are most promising from a clinical perspective. This work summarizes the progress made in the field and the typical challenges on the way to B/A estimation. Limitations and problems with the current techniques are identified, suggesting directions that may lead to further improvement. Since the basic theory of the physics behind the measurement strategies is presented, it is also suited for a reader who is new to nonlinear ultrasound.

Nonlinear ultrasonic imaging in pulse-echo mode using Westervelt equation: a preliminary research

Acoustic nonlinear parameter β, was of great interest in tissue characterization in recent years. Nonlinear imaging methods have been reported to provide improved spatial and contrast resolution. We introduce a nonlinear imaging method derived from nonlinear wave equation based on Gaussian-form solution assumption, which can be applied in pulse-echo mode on diagnostic ultrasound. Through making the use of two pulse transmission, only nonlinear effects are reserved and other effects like scattering, diffraction and linear attenuation can be eliminated. For validation of this method a set of simulation results are generated with a nonlinear simulator. Simulated images also indicate that our method clearly describes the spatial distribution of B/A in the medium.

Acoustic Distortion Ratio Enhancement Using Multiple Pulse-Echo Method (MPEM) for Evaluation of B/A Nonlinear Parameter

In this article, we report on the analysis of the extended acoustic signature obtained from the pulse-echo method to evaluate the B/A nonlinear parameter in fluids. In the known form of the method, the first acoustic tone burst from the reflector is used for the parameter measurement. The multiple pulse-echo method (MPEM) makes use of several tone bursts coming from the reflector back wall. The distortion ratio can be increased when the source frequency is tuned to a reflector resonance. The repercussion of this increase in the measurement of the nonlinear parameter B/A is investigated. As a practical result, this work suggests that the fluid volume required for the measurement can be reduced.

Dynamically Characterizing Skeletal Muscles via Acoustic Non-linearity Parameter: In Vivo Assessment for Upper Arms

It is crucial to model skeletal muscles for muscle-centered health care, such as prosthetics. Here we hypothesize that the acoustic non-linearity parameter (B/A) can be utilized to partially represent the contraction state of skeletal muscles. Although previous work commonly measured the B/A value of tissues in vitro, the present study targets the biceps brachii muscle to investigate the relationship between the B/A value and the dynamics of the elbow. Furthermore, it is proposed that a correction method based on the angular spectrum theory be applied in vivo, and the dynamic metrics of the B/A value and its feasibility be verified through an underwater experiment. Seven participants were invited for the in vivo experiment, in which elbow torque and B/A values were measured simultaneously. The non-plane reflection was approximately treated through an integral method, leading to a modified B/A value. Then, linear regression was applied to characterize the B/A-torque relationship, with the calculated coefficient of determination (R²) ranging from 0.85-0.93. Experimental results indicate that the modified B/A value of the biceps brachii correlates well with elbow torque. This study not only paves the way to dynamic measurement of the B/A value of skeletal muscles in vivo, but also confirms that B/A can be used as a more comprehensive assessment criterion for muscle functions.

Nonlinear bulk elasticity imaging using dual frequency ultrasound

The nonlinear acoustic bulk properties of tissue, e.g., the coefficient of nonlinearity, β_n, or the nonlinear bulk elasticity, β_p=β_nκ₀, have been shown to be promising parameters for tissue characterization due to their sensitivity to tissue structure. Previously developed methods for imaging these parameters using single frequency ultrasound have shown success in a laboratory setting using the transmission mode. In the pulse-echo mode, however, unknown absorption, diffraction, and speckle produce unreliable estimates and instability, causing these methods to have achieved no clinical relevance. In this paper, a pulse-echo method for measurement of the nonlinear bulk elasticity is presented using a dual frequency approach. The method is less sensitive to diffraction and absorption due to a separate low frequency manipulation wave. The technique is tested in both simulations and in vitro in a heterogeneous phantom with two regions of different nonlinear properties. Both in simulations and in vitro, a spatial β_p map is produced where the two regions are clearly distinguished. In addition, the quantitative estimates of β_p obtained are close to the expected values, making the method a promising first step toward in vivo imaging of nonlinear bulk properties.

Analysis of acoustic nonlinearity parameter B/A in liquids containing ultrasound contrast agents

The acoustic nonlinearity parameter B/A plays a significant role in the characterization of acoustic properties of various biomaterials and biological tissues. It has the potential to be a favorable imaging modality in contrast ultrasound imaging with coated microbubbles. However, the development of effective means for evaluating the nonlinearity parameter of suspensions of ultrasound contrast agents (UCAs, also known as bubbly liquids) remains open. The present paper formulates a new equation based on the thermodynamic method that correlates both attenuation and phase velocity of linear ultrasound. The simplicity of the present method makes the B/A estimation possible with a relatively rigorous mathematical derivation. The calculated nonlinearity parameter contains the contribution of dynamic effects of bubbles, and its low-frequency limit agrees with B/A estimated by the method of mixture law when the volume fraction is below 10^-4. Furthermore, the maximum B/A in bubbly liquids can reach up to10⁵, while the minimum can be as low as -10⁵. The negative nonlinearity parameter indicates significantly different thermodynamic properties of bubbly liquids.

Exploiting Ballou's rule for better tissue classification

Ultrasound tissue characterization based on the coefficient of nonlinearity, β_n = 1 + B/2A, has been demonstrated to produce added diagnostic value due to its large variation and sensitivity to tissue structure. However, the parameter has been observed to be significantly correlated to the speed of sound and density. These relationships are analyzed empirically as well as theoretically by developing a pressure-density relation based on a thermodynamic model and the Mie intermolecular potential. The results indicate that for many soft tissues, the coefficient of nonlinearity is largely determined by the isentropic compressibility, κ_s. Consequently, for tissue characterization, estimating the nonlinear response of the medium, given by β_p = β_nκ_s, appears to be beneficial due to correlated quantities.

Nonlinear ultrasound parameter to monitor cell death in cancer cell samples

A scaling subtraction method was proposed to analyze the radio frequency data from cancer cell samples exposed to an anti-cancer drug and to estimate a nonlinear parameter. The nonlinear parameter was found to be well correlated (R²= 0.62) to the percentage of dead cells in apoptosis and necrosis. The origin of the nonlinearity may be related to a change in contacts between cells, since the nonlinear parameter was well correlated to the average total coordination number of binary packings (R²≥ 0.77). These results suggest that the scaling subtraction method may be used to early quantify chemotherapeutic treatment efficiency.

Calibration of focused ultrasonic transducers and absolute measurements of fluid nonlinearity with diffraction and attenuation corrections

This paper presents analytical and experimental techniques for absolute determination of the acoustic nonlinearity parameter (β) in fluids using focused transducers. When focused transducers are used for β measurements, the geometrical and mechanical calibrations are generally required for accurate determination of the receiver transfer function from which the absolute pressure can be calculated. The fundamental and second harmonic wave amplitudes in harmonic generation measurements should be modified to account for beam diffraction and material absorption. All these issues are resolved in this study and the proposed technique is validated through the β measurement in water. An experimental method is developed to determine the effective radius and focal length of focused transducers. A simplified self-reciprocity calibration procedure for a broadband focused receiver is described. The diffraction and attenuation corrections for the fundamental and second harmonic waves are explicitly derived using the multi-Gaussian beam model, and the effects on the β determination are discussed. When the diffraction and attenuation corrections are all properly made, the measurement of β over a large range of propagation distances is possible with errors less than 8%.

Ultrasound coefficient of nonlinearity imaging

Imaging the acoustical coefficient of nonlinearity, β, is of interest in several healthcare interventional applications. It is an important feature that can be used for discriminating tissues. In this paper, we propose a nonlinearity characterization method with the goal of locally estimating the coefficient of nonlinearity. The proposed method is based on a 1-D solution of the nonlinear lossy Westerfelt equation, thereby deriving a local relation between β and the pressure wave field. Based on several assumptions, a β imaging method is then presented that is based on the ratio between the harmonic and fundamental fields, thereby reducing the effect of spatial amplitude variations of the speckle pattern. By testing the method on simulated ultrasound pressure fields and an in vitro B-mode ultrasound acquisition, we show that the designed algorithm is able to estimate the coefficient of nonlinearity, and that the tissue types of interest are well discriminable. The proposed imaging method provides a new approach to β estimation, not requiring a special measurement setup or transducer, that seems particularly promising for in vivo imaging.