Quantitative characterization of viscoelastic behavior in tissue-mimicking phantoms and ex vivo animal tissues.

Viscoelasticity of soft tissue is often related to pathology, and therefore, has become an important diagnostic indicator in the clinical assessment of suspect tissue. Surgeons, particularly within head and neck subsites, typically use palpation techniques for intra-operative tumor detection. This detection method, however, is highly subjective and often fails to detect small or deep abnormalities. Vibroacoustography (VA) and similar methods have previously been used to distinguish tissue with high-contrast, but a firm understanding of the main contrast mechanism has yet to be verified. The contributions of tissue mechanical properties in VA images have been difficult to verify given the limited literature on viscoelastic properties of various normal and diseased tissue. This paper aims to investigate viscoelasticity theory and present a detailed description of viscoelastic experimental results obtained in tissue-mimicking phantoms (TMPs) and ex vivo tissues to verify the main contrast mechanism in VA and similar imaging modalities. A spherical-tip micro-indentation technique was employed with the Hertzian model to acquire absolute, quantitative, point measurements of the elastic modulus (E), long term shear modulus (η), and time constant (τ) in homogeneous TMPs and ex vivo tissue in rat liver and porcine liver and gallbladder. Viscoelastic differences observed between porcine liver and gallbladder tissue suggest that imaging modalities which utilize the mechanical properties of tissue as a primary contrast mechanism can potentially be used to quantitatively differentiate between proximate organs in a clinical setting. These results may facilitate more accurate tissue modeling and add information not currently available to the field of systems characterization and biomedical research

Forward model for quantitative pulse-echo speed-of-sound imaging

Computed ultrasound tomography in echo mode (CUTE) allows determining the spatial distribution of speed-of-sound (SoS) inside tissue using handheld pulse-echo ultrasound (US). This technique is based on measuring the changing phase of beamformed echoes obtained under varying transmit (Tx) and/or receive (Rx) steering angles. The SoS is reconstructed by inverting a forward model describing how the spatial distribution of SoS is related to the spatial distribution of the echo phase shift. CUTE holds promise as a novel diagnostic modality that complements conventional US in a single, real-time handheld system. Here we demonstrate that, in order to obtain robust quantitative results, the forward model must contain two features that were not taken into account so far: a) the phase shift must be detected between pairs of Tx and Rx angles that are centred around a set of common mid-angles, and b) it must account for an additional phase shift induced by the error of the reconstructed position of echoes. In a phantom study mimicking liver imaging, this new model leads to a substantially improved quantitative SoS reconstruction compared to the model that has been used so far. The importance of the new model as a prerequisite for an accurate diagnosis is corroborated in preliminary volunteer results

Independent tuning of acoustic and mechanical properties of phantoms for biomedical applications of ultrasound

In this work the preparation of tissue mimicking materials (TMMs) with independently tunable acoustic and elastic properties is reported. Although a large number of hydrogel, synthetic polymer, polysaccharides or other natural based materials have been proposed and used for the realization of TMMs, both for diagnostic and therapeutic applications of ultrasounds, up to today, simulation of acoustic properties was often performed using solid particles, reducing dramatically the transparency and inevitably affecting the homogeneity and the elastic properties of the TMM. By means of concentrated salts solutions and different polysaccharides, an easy method to prepare these TMMs have been developed. This approach would lead to obtain homogenous TMMs with Young modulus ranging over 3 orders of magnitude, i.e. from 2 to 1500 kPa, with independently tunable attenuation properties. An accurate mechanical and acoustic characterization of these TMMs have been performed. Finally, by means of a preliminary trials on protein denaturation induced by a high focused ultrasound transducer in a transparent TMMs with different attenuation values, the mechanism underlying on the formation and propagation of lesion has been investigated. Obtained results suggest that this 'chemical' approach would strongly support in vitro investigations on the open issues related to diagnostic and therapeutic application of ultrasounds

Broadband acoustic measurement of an agar-based tissue mimicking material - a longitudinal study

Commercially available ultrasound quality assurance test phantoms rely upon the long-term acoustic stability of tissue-mimicking-materials (TMMs). The measurement of the acoustic properties can be technically challenging and it is important to ensure its stability. The standard technique is to film-wrap samples of TMM and to measure the acoustic properties in a water bath. In this study, a modified technique is proposed whereby the samples of TMM are measured in a preserving fluid that is intended to maintain their characteristics. The acoustic properties were evaluated using a broadband pulse-echo substitution technique over the frequency range of 4.5 – 50 MHz at 0, 6 and 12 months using both techniques. For both techniques, the measured mean values for the speed of sound and the attenuation were very similar and within the IEC recommended value. However, the results obtained using the proposed modified technique demonstrated greater stability over the 1-year period when compared with the results acquired using the standard technique

Broadband Acoustic Measurement of the Agar-based Tissue Mimicking Material: a Longitudinal Study

Commercially available ultrasound quality assurance test phantoms rely on the long-term acoustic stability of the tissue-mimicking-material (TMM). Measurement of the acoustic properties of the TMM can be technically challenging, and it is important to ensure its stability. The standard technique is to film-wrap samples of TMM and to measure the acoustic properties in a water bath. In this study, a modified technique was proposed whereby the samples of TMM are measured in a preserving fluid that is intended to maintain their characteristics. The acoustic properties were evaluated using a broadband pulse-echo substitution technique over the frequency range 4.5–50 MHz at 0, 6 and 12 months using both techniques. For both techniques, the measured mean values for the speed of sound and attenuation were very similar and within the International Electrotechnical Commission-recommended value. However, the results obtained using the proposed modified technique exhibited greater stability over the 1-y period compared with the results acquired using the standard technique

SCANNING ACOUSTIC MICROSCOPE FOR CHARACTERIZATION OF ARTERIAL PLAQUE

Unstable arterial plaque is likely the key component of atherosclerosis, a disease which is responsible for two-thirds of heart attacks and strokes, leading to approximately 1 million deaths in the United States. Ultrasound imaging is able to detect plaque but as of yet is not able to distinguish unstable plaque from stable plaque. In this work a scanning acoustic microscope (SAM) was implemented and validated as tool to measure the acoustic properties of a sample. The goal for the SAM is to be able to provide quantitative measurements of the acoustic properties of different plaque types, to understand the physical basis by which plaque may be identified acoustically. The SAM consists of a spherically focused transducer which operates in pulse-echo mode and is scanned in a 2D raster pattern over a sample. A plane wave analysis is presented which allows the impedance, attenuation and phase velocity of a sample to be de- termined from measurements of the echoes from the front and back of the sample. After the measurements, the attenuation and phase velocity were analysed to ensure that they were consistent with causality. The backscatter coefficient of the samples was obtained using the technique outlined by Chen et al [8]. The transducer used here was able to determine acoustic properties from 10-40 MHz. The results for the impedance, attenuation and phase velocity were validated for high and low-density polyethylene against published results. The plane wave approximation was validated by measuring the properties throughout the focal region and throughout a range of incidence angles from the transducer. The SAM was used to characterize a set of recipes for tissue-mimicking phantoms which demonstrate indepen- dent control over the impedance, attenuation, phase velocity and backscatter coefficient. An initial feasibility study on a human artery was performed.This work was supported in part by CenSSIS the Center for Subsurface Sensing and Imaging Systems under the Engineering Research Centers Program of the National Science Foundation (Award EEC- 9986821

An interferometric technique for B/A measurement

An isentropic phase method is described for measuringin vitro the acoustic nonlinearity parameterB/A of several aqueous buffers, protein solutions, lipid oils, and emulsions. The technique relies upon the use of an acoustic interferometer to measure the small changes in sound speed that accompany a rapid hydrostaticpressure change of between one and two atmospheres. Average accuracies of 0.85% are attainable with this method

Testing procedures and acquisition systems for contact sensor¿based vocal monitoring devices

L'abstract è presente nell'allegato / the abstract is in the attachmen