Quantitative Assessment of Thin-Layer Tissue Viscoelastic Properties Using Ultrasonic Micro-Elastography With Lamb Wave Model
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Abstract:
Characterizing the viscoelastic properties of thin-layer tissues with micro-level thickness has long remained challenging. Recently, several micro-elastography techniques have been developed to improve the spatial resolution. However, most of these techniques have not considered the medium boundary conditions when evaluating the viscoelastic properties of thin-layer tissues such as arteries and corneas; this might lead to estimation bias or errors. This paper aims to integrate the Lamb wave model with our previously developed ultrasonic micro-elastography imaging system for obtaining accurate viscoelastic properties in thin-layer tissues. A 4.5-MHz ring transducer was used to generate an acoustic radiation force for inducing tissue displacements to produce guided wave, and the wave propagation was detected using a confocally aligned 40-MHz needle transducer. The phase velocity and attenuation were obtained from k-space by both the impulse and the harmonic methods. The measured phase velocity was fit using the Lamb wave model with the Kelvin-Voigt model. Phantom experiments were conducted using 7% and 12% gelatin and 1.5% agar phantoms with different thicknesses (2, 3, and 4 mm). Biological experiments were performed on porcine cornea and rabbit carotid artery ex vivo. Thin-layer phantoms with different thicknesses were confirmed to have the same elasticity; this was consistent with the estimates of bulk phantoms from mechanical tests and the shear wave rheological model. The trend of the measured attenuations was also confirmed with the viscosity results obtained using the Lamb wave model. Through the impulse and harmonic methods, the shear viscoelasticity values were estimated to be 8.2 kPa for $0.9~\text {Pa}{\cdot} \text {s}$ and 9.6 kPa for $0.8~\text {Pa}{\cdot} \text {s}$ in the cornea and 27.9 kPa for $0.1~\text {Pa}\cdot \text {s}$ and 26.5 kPa for $0.1~\text {Pa}\cdot \text {s}$ in the artery.Keywords:
Acoustic Radiation Force
Elasticity
Phase velocity
Lamb waves
Purpose: The early recognition of changes in the pancreatic parenchyma plays an important role in the diagnosis of pancreatic disorders. The newly developed ultrasound technique of shearwave elastography or Acoustic Radiation Force Impulse Virtual Touch Tissue Quantification (ARFI-VTTQ) visualizes and quantifies tissue elasticity and superimposes findings on conventional B-scan images. Objective of the present study was to establish normal values for the ARFI-VTTQ method in healthy subjects.
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Ultrasound-based elastography including strain elastography, acoustic radiation force impulse (ARFI) imaging, point shear wave elastography and supersonic shear imaging (SSI) have been used to differentiate breast tumors among other clinical applications. The objective of this study is to extend a previously published virtual simulation platform built for ultrasound quasi-static breast elastography toward acoustic radiation force-based breast elastography. Consequently, the extended virtual breast elastography simulation platform can be used to validate image pixels with known underlying soft tissue properties (i.e. 'ground truth') in complex, heterogeneous media, enhancing confidence in elastographic image interpretations. The proposed virtual breast elastography system inherited four key components from the previously published virtual simulation platform: an ultrasound simulator (Field II), a mesh generator (Tetgen), a finite element solver (FEBio) and a visualization and data processing package (VTK). Using a simple message passing mechanism, functionalities have now been extended to acoustic radiation force-based elastography simulations. Examples involving three different numerical breast models with increasing complexity-one uniform model, one simple inclusion model and one virtual complex breast model derived from magnetic resonance imaging data, were used to demonstrate capabilities of this extended virtual platform. Overall, simulation results were compared with the published results. In the uniform model, the estimated shear wave speed (SWS) values were within 4% compared to the predetermined SWS values. In the simple inclusion and the complex breast models, SWS values of all hard inclusions in soft backgrounds were slightly underestimated, similar to what has been reported. The elastic contrast values and visual observation show that ARFI images have higher spatial resolution, while SSI images can provide higher inclusion-to-background contrast. In summary, our initial results were consistent with our expectations and what have been reported in the literature. The proposed (open-source) simulation platform can serve as a single gateway to perform many elastographic simulations in a transparent manner, thereby promoting collaborative developments.
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This chapter reviews the current research and applications for acoustic radiation force (ARF)-based methods, namely ARF impulse (ARFI) imaging and shear wave elasticity imaging (SWEI) methods, in the diagnosis and treatment of cardiac diseases. ARFI imaging is an ultrasound-based elastography method that provides information about the local mechanical properties of tissue. SWEI imaging is another ARF-based elastography technique that uses acoustic impulses to displace tissue. SWEI and ARFI imaging are currently implemented on software-modified diagnostic ultrasound scanners and use conventional and prototype ultrasound transducers for the generation of radiation force and the tracking of the tissue response. Ultrasound elastography can provide direct tissue stiffness measurements throughout the cardiac cycle, and cardiac functional indices can be extracted from these mechanical measurements. Noninvasive measurement of cardiac stiffness through transthoracic imaging has great prognostic potential to diagnose a myriad of cardiomyopathies.
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We aimed to evaluate the effectiveness of acoustic radiation force impulse (ARFI) elastography in differentiating between hepatic lesions. The prospective study included 117 patients with liver masses. Shear wave velocity (SWV) values for lesions were determined by ARFI imaging and compared statistically. The difference between SWV values for benign and malignant hepatic masses was significant ( p < 0.01). The threshold SWV value for malignant hepatic lesions was established at 2.52 m/s, and the sensitivity and specificity of this cut-off value were 97% and 66%, respectively. We concluded that ARFI elastography provides supplementary data that aid in the differential diagnosis of liver masses.
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