Showing papers on "Elastography published in 2004"

Measurement of viscoelastic properties of homogeneous soft solid using transient elastography: An inverse problem approach

Abstract: Two main questions are at the center of this paper. The first one concerns the choice of a rheological model in the frequency range of transient elastography, sonoelasticity or NMR elastography for soft solids (20-1000 Hz). Transient elastography experiments based on plane shear waves that propagate in an Agar-gelatin phantom or in bovine muscles enable one to quantify their viscoelastic properties. The comparison of these experimental results to the prediction of the two simplest rheological models indicate clearly that Voigt's model is the better. The second question studied in the paper deals with the feasibility of quantitative viscosity mapping using inverse problem algorithm. In the ideal situation where plane shear waves propagate in a sample, a simple inverse problem based on the Helmholtz equation correctly retrieves both elasticity and viscosity. In a more realistic situation with nonplane shear waves, this simple approach fails. Nevertheless, it is shown that quantitative viscosity mapping is still possible if one uses an appropriate inverse problem that fully takes into account diffraction in solids.

Noninvasive vascular elastography: theoretical framework

Abstract: Changes in vessel wall elasticity may be indicative of vessel pathologies. It is known, for example, that the presence of plaque stiffens the vascular wall, and that the heterogeneity of its composition may lead to plaque rupture and thrombosis. Another domain of application where ultrasound elastography may be of interest is the study of vascular wall elasticity to predict the risk of aneurysmal tissue rupture. In this paper, this technology is introduced as an approach to noninvasively characterize superficial arteries. In such a case, a linear array ultrasound transducer is applied on the skin over the region of interest, and the arterial tissue is dilated by the normal cardiac pulsation. The elastograms, the equivalent elasticity images, are computed from the assessment of the vascular tissue motion. Investigating the forward problem, it is shown that motion parameters might be difficult to interpret; that is because tissue motion occurs radially within the vessel wall while the ultrasound beam propagates axially. As a consequence of that, the elastograms are subjected to hardening and softening artefacts, which are to be counteracted. In this paper, the Von Mises (VM) coefficient is proposed as a new parameter to circumvent such mechanical artefacts and to appropriately characterize the vessel wall. Regarding the motion assessment, the Lagrangian estimator was used; that is because it provides the full two-dimensional strain tensor necessary to compute the VM coefficient. The theoretical model was validated with biomechanical simulations of the vascular wall properties. The results allow believing in the potential of the method to differentiate hard plaques and lipid pools from normal vascular tissue. Potential in vivo implementation of noninvasive vascular elastography to characterize abdominal aneurysms and superficial arteries such as the femoral and the carotid is discussed.

Single-cell elastography: probing for disease with the atomic force microscope.

Abstract: The atomic force microscope (AFM) is emerging as a powerful tool in cell biology. Originally developed for high-resolution imaging purposes, the AFM also has unique capabilities as a nano-indenter to probe the dynamic viscoelastic material properties of living cells in culture. In particular, AFM elastography combines imaging and indentation modalities to map the spatial distribution of cell mechanical properties, which in turn reflect the structure and function of the underlying cytoskeleton. Such measurements have contributed to our understanding of cell mechanics and cell biology and appear to be sensitive to the presence of disease in individual cells. This chapter provides a background on the principles and practice of AFM elastography and reviews the literature comparing cell mechanics in normal and diseased states, making a case for the use of such measurements as disease markers. Emphasis is placed on the need for more comprehensive and detailed quantification of cell biomechanical properties beyond the current standard methods of analysis. A number of technical and practical hurdles have yet to be overcome before the method can be of clinical use. However, the future holds great promise for AFM elastography of living cells to provide novel biomechanical markers that will enhance the detection, diagnosis, and treatment of disease.

Optical coherence tomographic elastography technique for measuring deformation and strain of atherosclerotic tissues

Abstract: Objectives: To evaluate optical coherence tomographic elastography as a method for assessing the elastic properties of atherosclerotic plaque and the parameters that influence interpretation. Methods: Phantoms and aorta were examined in vitro to quantify speckle modulation and measure the displacement and strain maps. A correlation method was used as a speckle tracking technique for measuring axial and lateral displacement vectors and calculation of strain maps. The influence of correlation kernel size on accuracy of the method was evaluated. Results: In terms of a percentage error between calculated and measured displacements, the best results for phantoms were obtained with a 41 × 41 kernel (1.88% error). For both phantom and aorta images, it was found that, with the increasing size of cross correlation kernel, the axial and lateral displacement maps are less noisy and the displacement vectors are more clearly defined. However, the large kernels tend to average out the differences in displacements of small particles in phantoms and decrease the ability of speckle tracking to make microstructural assessments. Therefore, it is important to select kernel size carefully, based on the image features. Conclusions: Optical tomographic elastography can be used to assess the microstructural properties of atherosclerotic tissue at micrometre scale resolution, but preselected analysis criteria must be understood in a critical interpretation of the results.

On the thermal effects associated with radiation force imaging of soft tissue

Abstract: Several laboratories are investigating the use of acoustic radiation force to image the mechanical properties of tissue. Acoustic Radiation Force Impulse (ARFI) imaging is one approach that rises brief, high-intensity, focused ultrasound pulses to generate radiation force in tissue. This radiation force generates tissue displacements that are tracked using conventional correlation-based ultrasound methods. The tissue response provides a mechanism to discern mechanical properties of the tissue. The acoustic energy that is absorbed by tissue generates radiation force and tissue heating. A finite element methods model of acoustic heating has been developed that models the thermal response of different tissues during short duration radiation force application. The beam sequences and focal configurations used during ARFI imaging are modeled herein; the results of these thermal models can be extended to the heating due to absorption associated with other radiation force-based imaging modalities. ARFI-induced thermal diffusivity patterns are functions of the transducer f-number, the tissue absorption, and the temporal and spatial spacing of adjacent ARFI interrogations. Cooling time constants are on the order of several seconds. Tissue displacement due to thermal expansion is negligible for ARFI imaging. Changes in sound speed due to temperature changes call be appreciable. These thermal models demonstrate that ARFI imaging of soft tissue is safe, although thermal response must be monitored when ARFI beam sequences are being developed.

Assessment of elastic parameters of human skin using dynamic elastography

Abstract: Sonoelastography and transient elastography are two ultrasound-based techniques that facilitate noninvasive characterization of the viscoelastic properties of soft tissues by investigating their response to shear mechanical excitation. Young's modulus is the principle assessment parameter. Because it defines local tissue stiffness, it is of major interest for the medical imaging and cosmetic industries as it could replace subjective palpation by yielding local, quantitative information. In this paper, we describe a new high-resolution device capable of measuring local Young's modulus in very thin layers (1-5 mm) and devoted to the in vivo evaluation of the elastic properties of human skin. It uses an ultrasonic probe (50 MHz) for tracking the displacements induced by a 300 Hz shear wave generated by a ring surrounding the transducer. The displacements are measured using a conventional cross-correlation technique between successive ultrasonic back-scattered echoes. First, this noninvasive technique has been experimentally proven to be accurate for investigating elasticity in different skin-mimicking phantoms. Second, data were acquired in vivo on human forearms. As expected, Young's modulus was found to be higher in the dermis than in the hypodermis and other soft tissues.

The feasibility of using elastography for imaging the Poisson's ratio in porous media.

Abstract: The feasibility of using elastography for experimentally estimating and imaging the Poisson's ratio of porous media under drained and undrained conditions was investigated. Using standard elastographic procedures, static and time-sequenced poroelastograms (strain ratio images) of homogeneous cylindrical gelatin and commercially available tofu samples were generated under sustained applied axial strain. The experimental data show similar trends to those that were observed in finite-elements simulations, and to those that were calculated from classical theoretical models proposed for biphasic materials with similar mechanical properties. To demonstrate the applicability of elastography to monitor time-dependent changes in nonhomogeneous porous structures as well, preliminary time-sequenced poroelastograms were obtained from two-layer porous phantoms and porcine muscle samples in vitro. The results suggest that elastography may have significant potential for quantitatively mapping the time-dependent mechanical behavior of poroelastic media, which is related to the dynamics of fluid flow and to the elasticity and permeability parameters of the media.

In vivo elasticity measurements of extremity skeletal muscle with MR elastography.

Abstract: MR elastography (MRE) has been shown to be capable of non-invasively measuring tissue elasticity even in deep-lying regions. Although limited studies have already been published examining in vivo muscle elasticity, it is still not clear over what range the in vivo elasticity values vary. The present study intends to produce further information by examining four different skeletal muscles in a group of 12 healthy volunteers in the age range of 27-38 years. The examinations were performed in the biceps brachii, the flexor digitorum profundus, the soleus and the gastrocnemius. The average shear modulus was determined to be 17.9 (+/- 5.5), 8.7 (+/- 2.8), 12.5 (+/- 7.3) and 9.9 (+/- 6.8) kPa for each muscle, respectively. To ascertain the reproducibility of the examination, the stiffness measurements in two volunteers were repeated seven times for the biceps brachii. These examinations yielded a mean shear modulus of 11.3 +/-.7 and 13.3 +/- 4.7 kPa for the two subjects. For elasticity reconstruction, an automated reconstruction algorithm is introduced which eliminates variation due to subjective manual image analysis. This study yields new information regarding the expected variation in muscle elasticity in a healthy population, and also reveals the expected variability of the MRE technique in skeletal muscle.

MR elastography of the prostate: Initial in-vivo application

Abstract: Purpose To analyze the initial assessment of the technical feasibility of in-vivo MR elastography (MRE) of the prostate gland in healthy volunteers. Materials and methods Dynamic sinusoidal MR elastography was performed in 7 healthy volunteers in prone position. The mechanical wave was induced via an external oscillator attached to the pubic bone. A 1.5 Tesla MR system (Philips Medical Systems, Netherland) was used with 4 combined surface coils for signal reception. MRE data acquisition was performed with a motion-sensitive spin-echo MR sequence that was phase-locked to the mechanical oscillation. Subsequently, these images were used to reconstruct the local distribution of elasticity inside the prostate gland. The applied reconstruction algorithm was tested by means of phantom measurements. Results Sufficient penetration of the mechanical wave into the prostate gland was achieved in all volunteers, allowing the acquisition of utilizable image data sets. The reconstructed distribution of elasticity (shear-modulus) inside the healthy prostate gland correlated with the zonal anatomy of the gland. The elasticity of the central portion (2.2 +/- 0.3 kPa) appeared to be lower than the peripheral prostatic portion (3.3 +/- 0.5 kPa). Conclusion In-vivo MRE of the prostate gland is technically feasible. The proposed experimental set-up allows the efficient insertion of the mechanical wave into the prostate gland and provides a successful MR data acquisition.

Axial strain calculation using a low-pass digital differentiator in ultrasound elastography

Abstract: In ultrasound elastography, tissue axial strains are calculated from the gradient of the estimated axial displacements. However, the common differentiation operation amplifies the noises in the displacement estimation, especially at high frequencies. In this paper, a low-pass digital differentiator (LPDD) is proposed to calculate the axial strain from the estimated tissue displacement. Several LPDDs that have been well developed in the field of digital signal processing are presented. The corresponding performances are compared qualitatively and quantitatively in computer simulations and in preliminary phantom and in vitro experiments. The results are consistent with the theoretical analysis of the LPDDs.

OCT-based arterial elastography: robust estimation exploiting tissue biomechanics.

Abstract: We present a novel multi-resolution variational framework for vascular optical coherence elastography (OCE). This method exploits prior information about arterial wall biomechanics to produce robust estimates of tissue velocity and strain, reducing the sensitivity of conventional tracking methods to both noise- and strain-induced signal decorrelation. The velocity and strain estimation performance of this new estimator is demonstrated in simulated OCT image sequences and in benchtop OCT scanning of a vascular tissue sample.

Elastography imaging modalities for characterizing properties of tissue

Abstract: An image reconstruction algorithm begins with an initial acquisition of a preoperative imaging volume followed by a second imaging sequence subsequent to an applied deformation. A computational domain (model) is generated from the preoperative image series and boundary conditions are derived from a pre-post deformation comparison, as well as from information gathered from deformation source application (i.e., displacement and/or force). Using boundary conditions, a series of model-based image deformations is accomplished while varying model material properties. A calculation of a Jacobian matrix relating the change in regional mutual information is performed with respect to the change in material properties. Upon completion of this process, matrix regularization techniques are used to condition the system of equations and allow for inversion and subsequent delivery of model-property adjustments.

Modality independent elastography (MIE): a new approach to elasticity imaging

Abstract: The correlation between tissue stiffness and health is an accepted form of organ disease assessment. As a result, there has been a significant amount of interest in developing methods to image elasticity parameters (i.e., elastography). The modality independent elastography (MIE) method combines a nonlinear optimization framework, computer models of soft-tissue deformation, and standard measures of image similarity to reconstruct elastic property distributions within soft tissue. In this paper, simulation results demonstrate successful elasticity image reconstructions in breast cross-sectional images acquired from magnetic resonance (MR) imaging. Results from phantom experiments illustrate its modality independence by reconstructing elasticity images of the same phantom in both MR and computed tomographic imaging units. Additional results regarding the performance of a new multigrid strategy to MIE and the implementation of a parallel architecture are also presented.

Adapting the Lagrangian speckle model estimator for endovascular elastography: Theory and validation with simulated radio-frequency data

Abstract: Intravascular ultrasound (IVUS) is known to be the reference tool for preoperative vessel lesion assessments and for endovascular therapy planning. Nevertheless, IVUS echograms only provide subjective information about vessel wall lesions. Since changes in the vascular tissue stiffness are characteristic of vessel pathologies, catheter-based endovascular ultrasound elastography (EVE) has been proposed in the literature as a method for outlining the elastic properties of vessel walls. In this paper, the Lagrangian Speckle Model Estimator (LSME) is formulated for investigations in EVE, i.e., using a polar coordinate system. The method was implemented through an adapted version of the Levenberg-Marquardt minimization algorithm, using the optical flow equations to compute the Jacobbian matrix. The theoretical framework was validated with simulated ultrasound rf data of mechanically complex vessel wall pathologies. The results, corroborated with Ansys finite element software, demonstrated the potential of EVE to provide useful information about the heterogeneous nature of atherosclerotic plaques.

The role of the coupling term in transient elastography.

Abstract: The transient radiation of low-frequency elastic waves through isotropic and homogeneous soft media is investigated using the Green's function approach. A careful analysis of the coupling term is performed and yields the introduction of a very near field region in which its amplitude behaves as 1/r. To address the calculation of impulse responses, a simplified Green's function is proposed for semi-infinite media and compared to exact solutions. Impulse response calculations are successfully compared with experimental measurements obtained for circular radiators of different diameters using transient elastography. Results presented in this paper provide a better understanding of the role of the coupling term in elastography and should be used to compensate diffraction and coupling effects observed in transient elastography.

3D Elastography Using Freehand Ultrasound

Abstract: We present a novel technique for 3D elastography using freehand ultrasound. The scan is straightforward to perform, requiring just a single sweep over the area of interest with an unmodified 2D probe. The 3D elastogram is constructed in real time and can be visualised immediately following the sweep. Results are presented for a jelly phantom containing a hard inclusion.

Combined ultrasound, optoacoustic, and elasticity imaging

Abstract: Combination of three complementary imaging technologies - ultrasound imaging, elastography, and optoacoustic imaging - is suggested for detection and diagnostics of tissue pathology including cancer. The fusion of these ultrasound-based techniques results in a novel imaging system capable of simultaneous imaging of the anatomy (ultrasound imaging), cancer-induced angiogenesis (optoacoustic imaging) and changes in mechanical properties (elasticity imaging) of tissue to uniquely identify and differentiate pathology at various stages. To evaluate our approach, analytical and numerical studies were performed using heterogeneous phantoms where ultrasonic, optical and viscoelastic properties of the materials were chosen to closely mimic soft tissue. The results of this study suggest that combined ultrasound-based imaging is possible and can provide more accurate, reliable and earlier detection and diagnosis of tissue pathology. In addition, monitoring of cancer treatment and guidance of tissue biopsy are possible with a combined imaging system.

Noise reduction using spatial-angular compounding for elastography

Abstract: Ultrasound elastography has developed into an imaging modality suitable for detection and diagnosis of cancers in the breast, prostate, and thyroid and for monitoring ablative therapies in the liver, kidneys, and other sites. In this article, a new approach is described that enables the reduction of noise artifacts in elastography without a significant reduction in either the contrast or spatial resolution. The technique uses angular-weighted compounding of local angular strains estimated from echo signals scanned at different insonification angles. Strain estimated along angular insonification directions can be separated into strain tensor components along the axial (direction of compression) and lateral directions. The mechanical stimulus is applied only along one direction. Angular-weighting factors are derived from the relationship between the axial and lateral strains under the assumption of tissue incompressibility. Experimental results using a uniformly elastic, tissue-mimicking phantom demonstrate the improvement in the signal-to-noise ratio obtained with angular-weighted compounding. Variation in the signal-to-noise ratio obtained using different angular increments also is investigated. Elastograms obtained from an inclusion phantom also demonstrate the improvement in contrast detail resolution obtained using spatial-angular compounding.

Model-based reconstructive elasticity imaging of deep venous thrombosis

Abstract: Deep venous thrombosis (DVT) and its sequela, pulmonary embolism, is a significant clinical problem. Once detected, DVT treatment is based on the age of the clot. There are no good noninvasive methods, however, to determine clot age. Previously, we demonstrated that imaging internal mechanical strains call identify and possibly age thrombus in a deep vein. In this study the deformation geometry for DVT elasticity imaging and its effect on Young's modulus estimates is addressed. A model-based reconstruction method is presented to estimate elasticity in which the clot-containing vessel is modeled as a layered cylinder. Compared to all unconstrained approach in reconstructive elasticity imaging, the proposed model-based approach has several advantages: only one component of the strain tensor is used; the minimization procedure is very fast; the method is highly efficient because an analytic solution of the forward elastic problem is used; and the method is not very sensitive to the details of the external load pattern-a characteristic that is important for free-hand, external, surface-applied deformation. The approach was tested theoretically using a numerical model, and experimentally on both tissue-like phantoms and all animal model of DVT. Results Suggest that elasticity reconstruction may prove to be a practical adjunct to triplex scanning to detect, diagnose, and stage DVT.

Observation of nonlinear shear wave propagation using magnetic resonance elastography.

Abstract: MR elastography (MRE) is an MRI modality that is increasingly being used to image tissue elasticity throughout the body. One MRE technique that has received a great deal of attention is based on visualizing shear waves, which reveal stiffness by virtue of their local wavelength. However, the shape of propagating shear waves can also provide valuable information about the nonlinear stress-strain behavior of tissue. Here an experiment is proposed that allows the observation of nonlinear wave propagation based on spatial-temporal phase contrast images. A theoretical description of the wave propagation was developed that reflects typical MRE excitation, which involves excitation modes both parallel and perpendicular to B0. Based on this model, it is shown that both odd and even higher harmonics are produced with their amplitudes dependent on the details of the actuator, imaging geometry, and the nonlinear tissue properties. With appropriate motion encoding, harmonic vibrations arising from nonlinear tissue response can be detected. The effect is demonstrated on an agarose gel phantom using a sinusoidal shear vibration of 150 Hz, and clearly shows the presence of harmonics at 600 and 750 Hz. Using an estimate of the strain energy of the phantom, we were able to determine the nonlinear tissue properties.

Palpation imaging using a haptic system for virtual reality applications in medicine.

Abstract: In the field of medical diagnosis, there is a strong need to determine mechanical properties of biological tissue, which are of histological and pathological relevance. Malignant tumors are significantly stiffer than surrounding healthy tissue. One of the established diagnosis procedures is the palpation of body organs and tissue. Palpation is used to measure swelling, detect bone fracture, find and measure pulse, or to locate changes in the pathological state of tissue and organs. Current medical practice routinely uses sophisticated diagnostic tests through magnetic resonance imaging (MRI), computed tomography (CT) and ultrasound (US) imaging. However, they cannot provide direct measure of tissue elasticity. Last year we presented the concept of the first haptic sensor actuator system to visualize and reconstruct mechanical properties of tissue using ultrasonic elastography and a haptic display with electrorheological fluids. We developed a real time strain imaging system for tumor diagnosis. It allows biopsies simultaneously to conventional ultrasound B-Mode and strain imaging investigations. We deduce the relative mechanical properties by using finite element simulations and numerical solution models solving the inverse problem. Various modifications on the haptic sensor actuator system have been investigated. This haptic system has the potential of inducing real time substantial forces, using a compact lightweight mechanism which can be applied to numerous areas including intraoperative navigation, telemedicine, teaching and telecommunication.

A quantitative comparison of modulus images obtained using nanoindentation with strain elastograms.

Abstract: Tissue stiffness is generally known to be associated with pathologic changes. Ultrasound (US) elastography, on the other hand, is capable of imaging tissue strain, which may or may not be well-correlated with tissue stiffness. Hence, a quantitative comparison between the elastographic tissue strain images and the corresponding tissue modulus images needed to be performed to evaluate the usefulness of elastography in imaging tissue stiffnesss properties. Simulations were performed to demonstrate and quantify the similarities between modulus images and strain elastograms. This was followed by comparing nanoindenter-based modulus images with strain elastograms of thin slices of tissue-mimicking phantoms. Finally, some beef slices, canine prostates, ovine kidneys and breast cancers grown in mice were used to demonstrate the qualitative correspondence between modulus images and strain elastograms. The simulations and the experiments indicated that it is feasible to perform quantitative comparisons between strain images (using elastography) and modulus images on certain tissue structures and geometries. A good quantitative correspondence (correlation values of greater than 0.8) between structures in the modulus and strain images could be obtained at scales equal to or larger than 20 Qlambda (where Q is the quality factor defined as the ratio of the center frequency over the band width and lambda is the wavelength of the US system) modulus contrasts larger than 5, applied strains between 0.5% and 3% and window lengths for computing strain elastograms between 3 Qlambda and 5 Qlambda. The gelatin-phantom experiments showed lower values of correlation (values around 0.5) than with theory and simulations. The decrease in correlation was attributed to the presence of measurement noise in both strain elastography and modulus imaging, an increase of dimensionality of the problem (from 2-D to 3-D), local anisotropy, heterogeneity and nonstationarity. Experiments on real tissue slices showed further decrease in the correlation to around 0.3, possibly due to additional confounding factors such as time-dependent mechanical properties and geometrical distortions in the tissue during imaging. The work presented in this paper demonstrates that there is an intrinsic relationship between strain elastograms and the actual distribution of soft tissue elastic moduli, and bodes well for continued work in the area of elastography.

Characterization of cardiovascular liver motion for the eventual application of elasticity imaging to the liver in vivo.

Abstract: Elastography, which uses ultrasound to image the tissue strain that results from an applied displacement, can display tumours and heat-ablated tissue with high contrast. However, its application to liver in vivo may be problematic due to the presence of respiratory and cardiovascular sources of displacement. The aim of this study was to measure the cardiovascular-induced component of natural liver motion for the purpose of planning future work that will either use the motion to produce elasticity images or will compensate for it when employing an external source of displacement. A total of 36 sequences of 7 s real-time radio frequency (RF) echo images of the liver were acquired from six healthy volunteers during breath-hold using a stationary 3.5 MHz transducer. For each image sequence, the axial and lateral components of displacement were measured for each pair of consecutive RF images using 2D-echo tracking. The spatio-temporal character of these displacements was then analysed using a novel approach, employing proper orthogonal decomposition, whereby the dominant motion patterns are described by eigenvectors with the highest eigenvalues. The motion patterns of different liver segments were complex, but they were also found to be cyclic, highly repeatable and capable of producing measurable displacements in the liver. These observations provide good evidence to suggest that it may be possible to correct for natural liver motion when using an externally applied displacement for elasticity imaging. It was also found that about 65%-70% of all liver motion could be described using the first eigenvector. Use of only this component of the motion will greatly simplify the design of a mechanical system to be used in an objective study of elasticity imaging of phantoms and excised tissues in the presence of simulated cardiovascular-induced liver motion.

Elastographic versus x-ray CT imaging of radio frequency ablation coagulations: an in vitro study.

Abstract: Techniques to imageelasticity parameters (i.e., elastography) have recently become of great interest to researchers. In this paper we use conventional ultrasoundelastography and x-rayCT to imageradio frequency(RF)ablation sites of excised canine liver enclosed in gelatin. Thermal coagulations of different sizes were produced by applying the RF procedure for various times and end point temperatures. Dimensions, areas and volumes computed from CT and elastography were compared with those on whole mount pathology specimens. Ultrasoundelastography exhibited high contrast for the thermal coagulations and performed better than CT. The correlation between pathology and elastography for this sample set of 40 thermal coagulations (r=0.94 for volume estimation, r=0.87 for area estimation) is better than the correlation between pathology and CT (r=0.89 for volume estimation, r=0.82 for area estimation).

Tissue mimicking phantom for ultrasonic elastography with finely adjustable elastic and echographic properties

Abstract: We have developed a tissue-mimicking phantom for ultrasonic elastography The phantom has tunable elastic and echographic properties Polyacrylamide gel, a typical chemical gel in that it shows greater stability than physical gels such as gelatin or agar, provided the basis of the phantom Fine metal-oxide particles were added to modify the echogenicity of the gel Independent adjustment of the concentrations of acrylamide and metal-oxide particles enabled fine-tuning of the elastic and echographic properties of the phantom The tuning was fine enough that we were able to develop a "stealth phantom", which possesses a small hard region that is very difficult to detect in echography but clearly distinguishable in elastography

Atherosclerosis imaging: intravascular ultrasound.

Abstract: Most acute coronary syndromes result from the rupture or erosion of high-risk plaques. Clinical imaging studies have shown that atherosclerotic plaque formation and rupture are widespread processes that are often asymptomatic. The rationale for atherosclerosis imaging is the in-vivo identification of high-risk lesions, which may subsequently lead to prevention of future cardiovascular events. Although intravascular ultrasound (IVUS) imaging studies demonstrated that echolucent appearance of the plaque and expansive (positive) remodelling are associated with unstable clinical presentation, these characteristics were not adequate for accurate plaque characterisation. Recent technical developments in ultrasound equipment and analytical methods, utilising several characteristics of the digitised ultrasound signal with radiofrequency analysis and elastography, promise accurate tissue characterisation. Other imaging modalities, including optical coherence tomography, also contribute to a more precise characterisation of the composition of atherosclerotic plaques. A non-imaging approach is the focal assessment of temperature differences using sensitive intravascular thermography catheters, presumably reflecting focal inflammatory changes of vulnerable lesions. Although the histological characteristics of the atheroma are critically important in the sequence of events leading to acute coronary syndromes, the clinical relevance of identifying these characteristics is not yet clear. There is increasing evidence that identifying and treating individual culprit lesions may not be enough to prevent the ischaemic cardiac events in most patients, because the acute coronary syndrome is not a disease of a single site or a few discrete segments, but rather a systemic disease that involves the entire coronary tree. In addition to detection and quantitation of early coronary atherosclerosis and disease activity, accurate and reproducible methods could help to identify high-risk patients and allow serial monitoring during various therapeutic interventions. Serial IVUS imaging makes it possible to visualise the vessel wall that harbours the atheroma at different time points. Typically, serial IVUS allows the assessment of the percentage change in atheroma volume, with considerable statistical power to detect small changes. Using this methodology, aggressive lipid lowering by a high-dose statin agent has been shown to stop the progression of atherosclerosis, and a new mutant high-density lipoprotein complex was found to be effective in regressing atheroma burden. Although intravascular ultrasound is very accurate for quantification of atheroma burden, widespread application and accurate and reproducible non-invasive imaging modalities are needed for large-scale risk assessment algorithms. Cardiovascular computed tomography is at the forefront of the non-invasive imaging modalities. Future prospective imaging studies will be necessary to identify focal or systemic characteristics of high-risk lesions and to demonstrate the relationship between plaque burden, biochemical markers and clinical events.

3D ultrasound-based dynamic and transient elastography: first in vitro results

Abstract: 3D magnetic resonance (MR) elastography is a well-established technique based on monochromatic mechanical excitations to study soft tissue mechanical properties. MR imaging observations are used to calculate tissue viscoelastic properties and has been validated in vivo, but the technique is limited by the acquisition time (about ten minutes). We study the feasibility of performing 3D dynamic elastography using an ultrasound based imaging system. The ultrasound approach provides a low cost system and reduces the acquisition time by a factor of 300 (to about 2 seconds). Two ultrasonic arrays are moved perpendicularly using a stepper-motor-control positioning system to acquire ultrasonic images in a given volume of interest. Mechanical excitation is induced by a 20 mm square plate linked to an external vibrator. Displacements induced by the vibrator are calculated from ultrasound images using a 2-dimensional estimator (Tanter, M. et al., 2002). Each ultrasonic array is able to calculate in-plane (axial and lateral) displacements. Lateral displacements are the same from both arrays and are averaged. The configuration leads to the estimation of the three displacement components in a full 3D area. Imaging sequences for 3D displacements have been realized. Experiments were conducted in a phantom mimicking heterogeneous tissue with two harder inclusions. 3D shear elasticity, viscosity and anisotropy maps of the studied phantoms are presented and show the feasibility of building a full ultrasound based system for 3D dynamic elastography providing a complete description of tissue mechanical properties in a few seconds.