Two-dimensional shear wave speed and crawling wave speed recoveries from in vitro prostate data

Ultrasound elastography using shear wave interference patterns: a finite element study of affecting factors

Elastography as one of the non-invasive medical imaging techniques which can help determine the stiffness of organs and other structures is currently attracting more attention. An interesting imaging rate-independent technique which has been discussed in literature uses shear wave interference patterns (SWIP). In this method, two external continuous harmonic vibration sources were used to induced SWIP and the resulting tissue displacements are mapped using ultrasonic imaging called sonoelastography. In this paper, a finite element model (FEM) of viscoelastic soft tissue with circular stiffer lesion inside, is simulated for testing the effect of stimulation characteristics on the propagation of SWIPs and shear speed map reconstruction. Also, we proposed an elastography probe, including miniature vibration sources and ultrasound transducer, which can be appropriate for experimental tests. The elastographic average speed ratio (ASR) and some scores like Dice coefficient, related to the binary image of shear speed map, are calculated for quantitatively measuring the effect of different contributing harmonic vibration parameters. Results show that the potential of providing useful diagnostic information can be improved if the preferable parameters are considered for implementation. According to these results the ASR, Dice and Jaccard scores would diverge from the ground truth of FEA if the parameter level is not selected correctly. Particularly, the Dice and Jaccard coefficients are obtained about 0.9 and 0.8, respectively, for the best vibration parameters level choice.

A digital viscoelastic liver phantom for investigation of elastographic measurements

To develop elastography imaging technologies and implement image reconstruction algorithms, testing is done with phantoms. Although the validation step is usually taken using real data and physical phantoms, their geometry as well as composition, biomechanical parameters, and details of applying stress cannot be modified readily. Such considerations have gained increasing importance with the growth of elastography techniques as one of the non-invasive medical imaging modalities, which can map the elastic properties and stiffness of soft tissues. In this article, we develop a digital viscoelastic phantom using computed tomography (CT) imaging data and several application software tools based on illustrations of normal liver anatomy so as to investigate the biomechanics of elastography via finite element modeling (FEM). Here we discuss how to create this phantom step by step, demonstrate typical shear wave elastography (SWE) experiments of applying transient stress to the liver model, and calculate quantitative measurements. In particular, shear wave velocities are investigated through a parametric study designed based on tissue stiffness and distance from the applied stress. According to the results of FEM analysis, low errors were obtained for shear wave velocity estimation for both mechanical stress (~2-5%) and acoustic radiation force (~3-7%). Results show that our model is a powerful framework and benchmark for simulating and implementing different algorithms in shear wave elastography, which can serve as a guide for upcoming researches and assist scientists to optimize their subsequent experiments in terms of design.

Reconstructing 3-D maps of the local viscoelastic properties using a finite-amplitude modulated radiation force

A modulated acoustic radiation force, produced by two confocal tone-burst ultrasound beams of slightly different frequencies (i.e. 2.0 MHz ± Δf/2, where Δf is the difference frequency), can be used to remotely generate modulated low-frequency (Δf ≤ 500 Hz) shear waves in attenuating media. By appropriately selecting the duration of the two beams, the energy of the generated shear waves can be concentrated around the difference frequency (i.e., Δf ± Δf/2). In this manner, neither their amplitude nor their phase information is distorted by frequency-dependent effects, thereby, enabling a more accurate reconstruction of the viscoelastic properties. Assuming a Voigt viscoelastic model, this paper describes the use of a finite-element-method model to simulate three-dimensional (3-D) shear-wave propagation in viscoelastic media containing a spherical inclusion. Nonlinear propagation is assumed for the two ultrasound beams, so that higher harmonics are developed in the force and shear spectrum. Finally, an inverse reconstruction algorithm is used to extract 3-D maps of the local shear modulus and viscosity from the simulated shear-displacement fields based on the fundamental and second-harmonic component. The quality of the reconstructed maps is evaluated using the contrast between the inclusion and the background and the contrast-to-noise ratio (CNR). It is shown that the shear modulus can be accurately reconstructed based on the fundamental component, such that the observed contrast deviates from the true contrast by a root-mean-square-error (RMSE) of only 0.38 and the CNR is greater than 30 dB. If the second-harmonic component is used, the RMSE becomes 1.54 and the corresponding CNR decreases by approximately 10-15 dB. The reconstructed shear viscosity maps based on the second harmonic are shown to be of higher quality than those based on the fundamental. The effects of noise are also investigated and a fusion operation between the two spectral components is applied to enhance the reconstruction quality. Finally, a modified shear-wave spectroscopy technique, shown to be more robust to noise, is described for the estimation of the viscoelastic properties inside and outside the spherical inclusion under conditions of increased noise.

Analysis of the modulated acoustic radiation-force profile for a dual-beam confocal geometry

A localized modulated radiation force can be produced when two confocal ultrasound beams of nearly equal frequencies interfere in an attenuating medium such as tissue. It is well-established that this force generates both shear and longitudinal waves. By scanning the focal point over a plane and observing the propagation of these waves, the mechanical properties of the medium can be imaged. In this paper, the modulated radiation force is analytically derived in the case of attenuating media, by expanding on the theory of ultrasound-stimulated-vibro-acoustography (USVA) for lossless media. Furthermore, weak nonlinearities are considered in the formulation, since higher source pressures may prove to be necessary to improve the radiation-force profile - only the fundamental component is, however, studied in this paper. An analysis of the generated radiation force is performed and the effects of various parameters are investigated on its amplitude and spatial distribution. It will be shown that by carefully selecting the confocal geometry of the beams, as well as, the source pressure and center frequency, the spatial profile of the radiation force can be optimized. This, subsequently, could improve not only the resolution of the point-spread-function in USVA, but also, the profile of the shear waves in elastography applications.

Modeling shear waves through a viscoelastic medium induced by acoustic radiation force

In this study, a finite element model of a tissue-mimicking, viscoelastic phantom with a stiffer cylindrical inclusion subjected to an acoustic radiation force (ARF) is presented, and the resulting shear waves through the heterogeneous media are simulated, analyzed, and compared with experimental data. Six different models for the ARF were considered and compared. Each study used the same finite element model, but applied the following: (1) full radiation push; (2) focal region push; (3) single element focal point source; or (4) various thresholds of the full radiation push. For each case, displacements at discrete locations were determined and compared. The finite element simulation results for the full radiation push matched well with the experimental data with respect to replicating the shear wave speed and attenuation in the peak displacements through the background medium and inclusion, but did not illustrate comparable recovery after the peak displacements. As a result of this study, it has been shown that a focal region or point source push is not adequate to accurately model the effects of the full radiation push, but thresholding the full push can produce comparable results and reduce computation time.