Propagation In Soft Tissue Research Articles

Editorial: Pushing the physical limits of wave propagation in soft tissues: an add-on to shear wave elastography

Editorial: Pushing the physical limits of wave propagation in soft tissues: an add-on to shear wave elastography

Viscoelastic Characterization of Soft Tissue-Mimicking Gelatin Phantoms using Indentation and Magnetic Resonance Elastography.

Characterization of biomechanical properties of soft biological tissues is important to understand the tissue mechanics and explore the biomechanics-related mechanisms of disease, injury, and development. The mechanical testing method is the most straightforward way for tissue characterization and is considered as verification for in vivo measurement. Among the many ex vivo mechanical testing techniques, the indentation test provides a reliable way, especially for samples that are small, hard to fix, and viscoelastic such as brain tissue. Magnetic resonance elastography (MRE) is a clinically used method to measure the biomechanical properties of soft tissues. Based on shear wave propagation in soft tissues recorded using MRE, viscoelastic properties of soft tissues can be estimated in vivo based on wave equation. Here, the viscoelastic properties of gelatin phantoms with two different concentrations were measured by MRE and indentation. The protocols of phantom fabrication, testing, and modulus estimation have been presented.

Experimental evidence of shear waves in fractional viscoelastic rheological models.

Fractional viscoelastic rheological models, such as the Kelvin Voigt Fractional Derivative model, have been proposed in the literature for modelling shear wave propagation in soft tissue. In this article, our previously developed wave propagation model for transluminal propagation based on a Kelvin Voigt Fractional Derivative wave equation is experimentally validated. The transluminal procedure uses the transmission and detection of shear waves through the luminal wall. The model was compared against high-speed camera observations in translucent elastography phantoms with similar viscoelastic properties to prostate tissue. An ad hoc cross-correlation procedure was used to reconstruct the angular displacement from the high-speed camera observations. Rheometry and shear wave elastography were used for characterising the shear wave velocity dispersion curve for the phantoms. Fractional viscoelastic properties were derived after fitting the dispersion curve to its analytical expression. Propagation features and amplitude spectra from simulations and high-speed camera observations were compared. The obtained results indicate that the model replicates the experimental observations with acceptable accuracy. The model presented here provides a useful tool to model transluminal procedures based on wave propagation and its interaction with the mechanical properties of the tissue outside the lumen.

Ultrasonic shear wave propagation in soft tissues for microelastography

Dispersion curve shows evidence of a cutoff frequency above which shear waves no longer propagate, which may assist microelastography researchers in choosing the best frequency for experiments.

Analysis of the transient surface wave propagation in soft-solid elastic plates.

In dynamic elastography, the goal is to estimate the Young's modulus from audio-frequency wave propagation in soft-tissues. Within this frequency range, the shear wavelength is centimeter-sized while the compressional wavelength is meter-sized. Thus, the experimental data are usually collected in the near-field of the source. Near-field effects have been widely studied for bulk wave propagation. However, the near- and transient-fields of surface and guided waves have received less attention. In this work, the transient surface displacement field in soft-solid elastic plates in vacuum is analyzed. Due to the high Poisson's ratio, mode conversion has special characteristics in soft-solids. They are analyzed through this work where it is shown that the transient-field over the surface can be interpreted by tracing a few reflections. The authors show the existence of a critical distance needed for the formation of Rayleigh-Lamb modes. Below this distance, only direct surface waves propagate without contribution from reflected waves. Thus, the dispersion curve differs from that predicted by Rayleigh-Lamb modes. Instead, the authors propose a model based on the interference of surface waves, which agree with the experimental data. In addition, the conditions needed in order to retrieve the shear wave phase velocity from the surface field are given.

Color Doppler shear wave imaging and its application to breast imaging

Imaging of continuous shear wave (CSW) induced by a small mechanical vibrator attached on tissue surface is low-cost and easy-to-use elastography. A problem of CSW is shear wave's reflection and refraction at tissue boundary, which degrade the accuracy of shear wave velocity estimation. We propose a novel shear wave elastography of CSW (Color Doppler Shear Wave Imaging: CD-SWI). Shear wave’s propagation in soft tissue is reconstructed directly without adding any function to conventional color flow imaging instrument. Two conditions for shear wave frequency and displacement amplitude are required to obtain maps. However, they are not severe restrictions because frequency is chosen among several frequencies suited for elastography and the required displacement amplitude is a few tens of microns. Both Fourier analysis along time axis and a directional filter on wave-number vector space improve image quality. Accuracy and image quality of CD-SWI are evaluated by comparing with ARFI method. CD-SWI is applied to breast in vivo and shear wave propagation in breast (f = 297 Hz) is observed. Inherent shear wave propagation in soft tissue reconstructed by CD-SWI, such as diffraction and refraction around tissue with different stiffness as well as shear wave velocity, may give novel information in tissue characterization by shear wave.

Rescaled Local Interaction Simulation Approach for Shear Wave Propagation Modelling in Magnetic Resonance Elastography

Properties of soft biological tissues are increasingly used in medical diagnosis to detect various abnormalities, for example, in liver fibrosis or breast tumors. It is well known that mechanical stiffness of human organs can be obtained from organ responses to shear stress waves through Magnetic Resonance Elastography. The Local Interaction Simulation Approach is proposed for effective modelling of shear wave propagation in soft tissues. The results are validated using experimental data from Magnetic Resonance Elastography. These results show the potential of the method for shear wave propagation modelling in soft tissues. The major advantage of the proposed approach is a significant reduction of computational effort.

A Temporal View of Soft Tissue Quantitative Ultrasound

The objective of soft tissue quantitative ultrasound (QUS) is to improve diagnostic ultrasound imaging capabilities via quantitative outcomes. Over the past three or so decades, there have been an increasing number of QUS successes. A temporal view moves us back in history almost six decades when techniques and theoretical developments were in their earliest stages that impacted modern QUS successes. The earliest theoretical developments and techniques some six decades ago can be attributed to Lev Chernov, Philip Morse, Herman Feshbach, Uno Ingard, John Wild and Jack Reid. Later, Floyd Dunn developed important views as to how connective tissue affected the interaction between ultrasound and soft tissue. Then, as the theory of wave propagation in soft tissues with random inhomogeneities was extended and applied by Fred Lizzi, Jim Zagzebski and Mike Insana (and their colleagues), contemporary QUS successes started to emerge.

Acoustic localisation of coronary artery stenosis: Wave propagation in soft tissue mimicking gels

WAVE PROPAGATION IN SOFT TISSUE MIMICKING GEL H. Thomas Banks, Malcolm J. Birch, Mark P. Brewin, Steve E. Greenwald, Shuhua Hu, Zack R. Kenz, Carola Kruse, Dwij Mehta, Simon Shaw, John R. Whiteman Blizard Institute, Barts and The London School of Medicine and Dentistry, Queen Mary, University of London, UK; Clinical Physics, Barts Health NHS Trust, London, UK; Institute of Computational Mathematics, Brunel University, UK; Department of Mathematics, North Carolina State University, Raleigh NC, USA

Viscoelasticity determined by measured wave absorption coefficient for modeling waves in soft tissues

This paper reports the construction of viscoelasticity models for simulation of wave propagation in soft tissues. Aided by the Carson transform, Fung’s model and Iatridis’s model are transformed into the frequency domain with the imaginary part of the transformed relaxation function shown to be related to wave absorption effect. Based on measured wave absorption coefficients, the viscoelasticity models in the time domain are determined. Results show that Fung’s model does not support the frequency-dependent damping effect, but Iatridis’ model does. To validate the approach, numerical simulation of propagating wavelets in a skeletal pig muscle is performed. The constructed relaxation function is discretized by parallelly connected Standard Linear Solid (SLS) models. The hereditary integration is then transformed into a set of partial differential equations by introducing the internal variables. Together, the governing equations include the equation of motion, the constitutive equation, and the equations of internal variables. Velocity, stress, and internal variables are the primary unknowns. The model equations are solved by using the space–time CESE method. The calculated wave absorption effect compares well with the measured data.

In vivoestimation of elastic wave parameters using phase-stabilized swept source optical coherence elastography

We report a highly sensitive method based on phase-stabilized swept source optical coherence elastography (PhS-SSOCE) to measure elastic wave propagation in soft tissues in vivo. The waves were introduced using a mechanical stimulus and were assessed using the phase response of the swept source optical coherence tomography signal. The technique was utilized to measure age-related changes in elastic flexural wave velocity and attenuation in mice cornea in vivo. Results demonstrate that the wave velocity increases with animal age, supporting previous observations that stiffness of mice cornea gradually increases with age. Our studies suggest that the PhS-SSOCE technique could potentially be used to obtain biomechanical properties of ocular tissues in vivo.

TU‐C‐220‐03: Visualization of Breast Carcinoma Using Photoacoustic Imaging: The Ongoing Twente Experience

Purpose: Worldwide, every year more than 1,300,000 women are diagnosed with breast cancer and annually, about 450,000 women die from the disease [1]. However, the current practice of imaging the breast for the detection and diagnosis of cancer suffers from shortcomings [2]. The interactions of the probe energy—whether X‐rays, magnetic field, or ultrasound (US)—with a tumor are not sufficiently specific or sensitive compared with non‐ pathological tissue. This results in occurrences of false positives and false negatives, which lead to psychological and physical morbidity and mortality. Furthermore, though X‐ray mammography is the gold standard imaging modality, X‐rays are carcinogenic. Ultrasound and MRI are restricted in their use to being secondary procedures owing to low sensitivity (US), poor specificity (MRI), and high‐expense (MRI). There is thus a requirement for a technique that is relatively harmless and has the potential to detect the presence of carcinoma in a manner that is more specific and reliable than inspection of morphologic and anatomic changes. Methods Photoacoustic imaging has much potential in detecting the presence of a carcinoma based on the optical absorption contrast manifested by the tumor compared to healthy tissue. This higher absorption is due to the presence of blood contents in the enhanced vascularization associated with the growth and spread of cancer. In PA, short pulses of light are used to illuminate the tissue: absorption induces heating followed by rapid thermal expansion. This generates US waves that propagate through tissue, to be detected by acoustic transducers positioned at the surface. The time‐of‐flight (TOF), amplitude and duration of acoustic pulses recorded on the tissue surface possess information regarding the location, absorption, and dimensions of the source. By recording signals in a 2‐D grid of detector positions, a 3‐D reconstruction of the absorber is possible. The advantage of this approach compared with purely optical imaging is that the spatial resolution is superior because of the low scattering experienced by the US during propagation in soft tissue. In the Biomedical Photonic Imaging Group (BMPI) of the University of Twente (UT) we have worked to develop photoacoustic imaging for breast imaging [3]. In 2007, the BMPI group reported their first results of NIR photoacoustic imaging of breast cancer in human subjects: in 4 of 5 cases of suspect breasts higher photoacoustic contrast associated was observed [4]. Results: In December 2010, a new clinical study has been started at the Centre for Breast Care of the Medisch Spectrum Twente in Oldenzaal. In this study, a number of patients will be measured within a period of 1.5 years. Patients are measured within the normal diagnostic path at the Centre for Mammacare, in between the ultrasound investigation and the ultrasound guided biopsy. The study can be divided into three phases and for each phase patients are included based on the suspiciousness of their lesion. In the Phase 1 of the study, the focus is on a small number of patients with lesions that are highly suspicious for malignancy. The goal of this part of the study is to optimize the imaging methods for the visualization of malignancies. At the time of going to press Phase 1 has been successfully completed. Ten technically acceptable measurements have been performed all of breasts highly suspect for malignancy [5]. The photoacoustic images have been compared with conventional imaging methods such as X‐ray imaging, ultrasound imaging and in some cases Magnetic Resonance Imaging. Conclusions These results confirm the conclusions of the 2007 study: that NIR photoacoustic imaging has potential in the diagnosis of breast cancer. On‐going work involves imaging patients in the subsequent phases. In phase 2 of the study, a large number of patients with lesions that are suspicious for malignancy are measured in order to define the photoacoustic markers that are indicative for the presence of a malignancy. In the last phase of the study, the absence of those photoacoustic malignancy markers will be verified in subjects with either healthy breast tissue or benign lesions.

Kramers–Kronig relationships applied to shear wave propagation in soft tissues.

Multiple methods have been proposed to characterize the viscoelastic material properties of soft tissues to differentiate normal from pathological tissue. Shearwave dispersion ultrasound vibrometry is a method that uses the dispersion of the shear wave velocity to characterize tissue shear elasticity and viscosity. It has recently been shown [M. W. Urban and J. F. Greenleaf, Phys. Med. Biol. 54, 5919–5933 (2009)] that the Kramers–Kronig relationships can be applied to shear wave measurements in ex vivo skeletal muscle over a frequency bandwidth of 100–600 Hz. We will describe this process where shear wave attenuation is modeled using a power law, and the shear wave velocity dispersion is predicted by applying the Kramers–Kronig relationships. In this study, we apply this method to shear wave propagation measurements made in different soft tissues such as ex vivo porcine skeletal muscle and kidney and in vivo porcine liver. The shear wave velocity predictions using the Kramers–Kronig relationships agree with the velocity measurements. The mean square error for these tissues ranged from 0.0024–0.0625 m2/s2. The Kramers–Kronig relationships can be used in shear wave propagation measurements in different types of soft tissues. [This work was supported in part by NIH Grants Nos. EB002167 and DK082408.]

A Kramers–Kronig-based quality factor for shear wave propagation in soft tissue

Shear wave propagation techniques have been introduced for measuring the viscoelastic material properties of tissue, but assessing the accuracy of these measurements is difficult for in vivo measurements in tissue. We propose using the Kramers–Kronig relationships to assess the consistency and quality of the measurements of shear wave attenuation and phase velocity. In ex vivo skeletal muscle we measured the wave attenuation at different frequencies, and then applied finite bandwidth Kramers–Kronig equations to predict the phase velocities. We compared these predictions with the measured phase velocities and assessed the mean square error (MSE) as a quality factor. An algorithm was derived for computing a quality factor using the Kramers–Kronig relationships.

Visualization and GPU-accelerated simulation of medical ultrasound from CT images

We present a fast GPU-based method for simulation of ultrasound images from volumetric CT scans and their visualization. The method uses a ray-based model of the ultrasound to generate view-dependent ultrasonic effects such as occlusions, large-scale reflections and attenuation combined with speckle patterns derived from pre-processing the CT image using a wave-based model of ultrasound propagation in soft tissue. The main applications of the method are ultrasound training and registration of ultrasound and CT images.

Basic nonlinear acoustics: An introduction for radiological physicists

Presented is a brief introduction to nonlinear acoustics, a topic of increasing importance in modern diagnostic ultrasound. Specifically treated is shock wave and harmonic production in lossless media. We also present a description of linear attenuation mechanisms in soft tissue and finally nonlinear propagation in soft tissue.

Computer simulation of forward wave propagation in soft tissue

A method for simulating forward wavefront propagation in heterogeneous tissue is discussed. The intended application of this method is for the study of aberration produced when performing ultrasound imaging through a layer of soft tissue. A one-way wave equation that permits smooth variation in all acoustically important variables is derived. This equation also describes tissue exhibiting nonlinear elasticity and arbitrary frequency-dependent relaxation. A numerical solution to this equation is found by means of operator splitting and propagation along the spatial depth coordinate. The numerical solution is accurate when compared to analytical solutions for special cases, and when compared to numerical solutions of the full wave equation by other methods. The presented implementation provides a fast numerical method for studying the impact of aberration in medical ultrasound imaging through soft tissue--both on the transmitted beam and the nonlinearly generated harmonic beam.

Local inversion of transient shear-wave propagation for elasticity and viscosity mapping in soft tissues

Observation of transient shear-wave propagation in soft tissue is of great interest for the study of tissue viscoelastic properties. In previous work, we introduced a technique, called Supersonic Shear Imaging (SSI), able to generate transient shear waves using the acoustic radiation force and image their propagation in real time in soft tissues. In this work, a local inverse problem of the shear wave propagation permitting the recovery of shear elasticity and viscosity is presented. Compressional and shear waves are decoupled by applying the curl operator to the experimental 2-D displacement field. The role of viscosity on the accuracy of the elasticity estimation is studied. The influence of out of plane shear propagation on the inversion algorithm is discussed. Finally, in media presenting shear viscoelasticity heterogeneities, finite difference simulations are used to study the spatial resolution of the algorithm and its sensitivity to the signal-to-noise ratio. Experiments on calibrated tissue-mimicking phantoms presenting different viscoelastic properties are presented validating the simulation results. First in vivo results on female breasts are presented.

Diffraction field of a low frequency vibrator in soft tissues using transient elastography

For the last 10 years, interest has grown in low frequency shear waves that propagate in the human body. However, the generation of shear waves by acoustic vibrators is a relatively complex problem, and the directivity patterns of shear waves produced by the usual vibrators are more complicated than those obtained for longitudinal ultrasonic transducers. To extract shear modulus parameters from the shear wave propagation in soft tissues, it is important to understand and to optimize the directivity pattern of shear wave vibrators. This paper is devoted to a careful study of the theoretical and the experimental directivity pattern produced by a point source in soft tissues. Both theoretical and experimental measurements show that the directivity pattern of a point source vibrator presents two very strong lobes for an angle around 35 degrees . This paper also points out the impact of the near field in the problem of shear wave generation.

Correlation Between the Light Scattering and the Mitochondrial Content of Normal Tissues and Transplantable Rodent Tumors

The development of noninvasive optical studies necessitates an understanding of the biological parameters which affect light propagation in soft tissues. In the present report, we have measured the optical properties of various normal (i.e., perfused liver, brain, skeletal muscle, white adipose tissue) and neoplastic rodent tissues (i.e., glioma, hepatoma, mammary adenocarcinoma) by using time-resolved spectroscopy. The contribution of the hemoglobin (+ myoglobin in the case of muscle) to the total light absorption at 780 nm has been determined. This contribution varies from about 25% (brain, skeletal muscle) to about 100% (white adipose tissue, 13762A mammary adenocarcinoma, 9L glioma). These results are explained by different blood volume fractions in the tissues and by the existence at 780 nm of other chromophores, such as the mitochondrial cytochrome oxidase. Secondly, the dependence of the light scattering of the tissue on both the cell and the mitochondrial content has been analyzed. The results indicate that there is no correlation between the light scattering and the DNA content, measured as an indicator of the cell number in the tissue. The scattering coefficient is proportional to both the succinate dehydrogenase activity and the mitochondrial protein content of the tissue, which are indicators of the mitochondria content of the tissue when based upon estimates of tissue wet weight.