Elastography: a quantitative method for imaging the elasticity of biological tissues

Adaptive spectral strain estimators for elastography

In conventional elastography, internal tissue deformations, induced by external compression applied to the tissue surface, are estimated by cross-correlation analysis of echo signals obtained before and after compression. Conventionally, strains are estimated by computing the gradient of estimated displacement. However, gradient-based algorithms are highly susceptible to noise and decorrelation, which could limit their utility. We previously developed strain estimators based on a frequency-domain (spectral) formulation that were shown to be more robust but less precise compared to conventional strain estimators, In this paper, we introduce a novel spectral strain estimator that estimates local strain by maximizing the correlation between the spectra of pre- and postcompression echo signals using iterative frequency-scaling of the latter; we also discuss a variation of this algorithm that may be computationally more efficient but less precise. The adaptive spectral strain estimator combines the advantages of time- and frequency-domain methods and has outperformed conventional estimators in experiments and 2-D finite-element simulations.

On the feasibility of elastic wave visualization within polymeric solids using magnetic resonance elastography

In this paper, the feasibility of extending previously described magnetic resonance elastography (MRE) dynamic displacement (and associated elasticity) measurement techniques, currently used successfully in tissue, to solid materials which have much higher shear rigidity and much lower nuclear spin densities, is considered. Based on these considerations, the MRE technique is modified in a straightforward manner and used to directly visualize shear wave displacements within two polymeric materials, one of which is relatively stiff.

Quantifying elasticity and viscosity from measurement of shear wave speed dispersion

The propagation speed of shear waves is related to frequency and the complex stiffness (shear elasticity and viscosity) of the medium. A method is presented to solve for shear elasticity and viscosity of a homogeneous medium by measuring shear wave speed dispersion. Harmonic radiation force, introduced by modulating the energy density of incident ultrasound, is used to generate cylindrical shear waves of various frequencies in a homogeneous medium. The speed of shear waves is measured from phase shift detected over the distance propagated. Measurements of shear wave speed at multiple frequencies are fit with the theoretical model to solve for the complex stiffness of the medium. Experiments in gelatin phantoms show promising results validated by an independent method. Practical considerations and challenges in possible medical applications are discussed.

A continuous-wave ultrasound system for displacement amplitude and phase measurement

A noninvasive, continuous-wave ultrasonic technique was developed to measure the displacement amplitude and phase of mechanical structures. The measurement system was based on a method developed by Rogers and Hastings ["Noninvasive vibration measurement system and method for measuring amplitude of vibration of tissue in an object being investigated," U.S. Patent No. 4,819,643 (1989)] and expanded to include phase measurement. A low-frequency sound source was used to generate harmonic vibrations in a target of interest. The target was simultaneously insonified by a low-power, continuous-wave ultrasonic source. Reflected ultrasound was phase modulated by the target motion and detected with a separate ultrasonic transducer. The target displacement amplitude was obtained directly from the received ultrasound frequency spectrum by comparing the carrier and sideband amplitudes. Phase information was obtained by demodulating the received signal using a double-balanced mixer and low-pass filter. A theoretical model for the ultrasonic receiver field is also presented. This model coupled existing models for focused piston radiators and for pulse-echo ultrasonic fields. Experimental measurements of the resulting receiver fields compared favorably with theoretical predictions.

Plethysmographic arterial waveform strain discrimination by Fisher's method

Plethysmography has been used for over 50 years to measure gross change in tissue blood volume. Over the cardiac cycle, perfused tissue initially expands as the blood flow into the arterioles exceeds the flow through the capillary bed. Later in the cardiac cycle, the accumulated blood drains into the venous vasculature, allowing the tissue to return to its presystolic blood volume. Specific features in the plethysmographic waveform can be used to identify normal and abnormal perfusion. We are developing a Doppler strain-imaging technique to measure the local pulsatile expansion and relaxation of tissue analogous to the gross measurement of tissue volume change with conventional plethysmography. A phantom has been built to generate plethysmographic-style strains with amplitudes of less than 0.1% in a tissue-mimicking material. With Fisher's discriminant analysis, it is shown that normal and abnormal plethysmographic-style strains can be differentiated with high sensitivities using the Fourier components of the strain waveforms normalized to compensate for the variance in the strain amplitude estimate.

Elastographic versus x-ray CT imaging of radio frequency ablation coagulations: Anin vitrostudy

Techniques to image elasticity parameters (i.e., elastography) have recently become of great interest to researchers. In this paper we use conventional ultrasound elastography and x-ray CT to image radio 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. Ultrasound elastography 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).

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

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 x 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.

The influence of viscosity on the shear strain remotely induced by focused ultrasound in viscoelastic media

Shear wave elasticity imaging (SWEI), an emerging acoustic technology for medical diagnostics, is based on remote generation of shear waves in tissue by radiation force in the focal region of an ultrasonic beam. In this study, the feasibility of Doppler ultrasonic technique to visualize the remotely induced shear waves was demonstrated. The generation of shear displacement in the focal region of a pulsed 1-MHz ultrasound beam with pulse duration of approximately about 2 ms and intensity levels on the order of 145 W/cm2, and consequent propagation of shear wave in tissue-mimicking and muscle tissue in vitro, were measured. The analysis of temporal behavior of shear displacement within the focal plane allowed estimation of shear wave velocities. The velocities were 4 and 7 m/s in hard phantom and tissue containing phantom, respectively. The measured shear displacements on the order of micrometers in gel-based phantoms are in reasonable agreement with theoretical estimates derived from an earlier developed model of shear wave generation by radiation force of focused ultrasound. The study revealed significant dependence of shear strain on the medium viscosity. The complex oscillatory character of shear strain relaxation in viscoelastic phantom and muscle tissue in vitro was observed.

Monitoring thermally-induced lesions with supersonic shear imaging

Thermally-induced lesions are generally stiffer than surrounding tissues. We propose here to use the supersonic shear imaging technique (SSI) for monitoring high-intensity focused ultrasound (HIFU) therapy. This new elasticity imaging technique is based on remotely creating shear sources using an acoustic radiation force at different locations in the medium. In these experiments, an HIFU probe is used to generate lesions in fresh tissue samples. A diagnostic transducer, controlled by our ultrafast scanner, is located in the therapeutic probe focal plane. It is used for both generating the shear waves and imaging the resulting propagation at frame rates reaching 5,000 images/s. Movies of the shear wave propagation can be computed off-line. The therapeutic and imaging sequences are interleaved and a set of wave propagation movies is performed during the heating process. From each movie, elasticity estimations have been performed using an inversion algorithm. It demonstrates the feasibility of detecting and quantifying the hardness of HIFU-induced lesions using SSI.

Impact of gas bubbles generated during interstitial ablation on elastographic depiction of in vitro thermal lesions

OBJECTIVE

Artifacts from gas bubble formation during radio frequency ablation along with the poor intrinsic contrast between normal and treated regions (zone of necrosis) are considerable problems for the visualization of the necrotic region on conventional sonography. Sonographic elastography is very effective for visualizing the zone of necrosis, but it uses the same echo signals to estimate strain as those used to form gray scale images. Thus, the impact of gas bubbles on strain images or elastograms must be investigated.

METHODS

Radio frequency ablation was performed in vitro on liver tissue samples, approximately 40 x 40 x 20 mm, encased in 80-mm cubed gelatin phantoms. Elastograms generated at different instants during the ablation procedures were obtained on a real-time scanner with a 5-MHz linear array. Sequences of elastograms illustrate the growth of the thermal lesion.

RESULTS

Degradation of the distal boundary of the thermal lesion was observed. The degradation was confined to the lower-fifth quadrant of the thermal lesion. However, accurate estimates of lesion areas could still be obtained by extrapolation of the thermal lesion boundary.

CONCLUSIONS

Elastograms of thermal lesions in vitro can be obtained during radio frequency ablation. Some loss of thermal lesion boundary information on strain images was observed in regions where attenuation due to gas bubbles reduced the signal-noise ratio of the echo signals.

Shear stiffness estimation using intravoxel phase dispersion in magnetic resonance elastography

Dynamic MR elastography (MRE) is a phase-contrast technique in which the periodic shear motion of an object is encoded as variations in the phase of the reconstructed images. An alternative MRE method is presented whereby waves are depicted as intensity variations in the magnitude images due to intravoxel phase dispersion (IVPD). A theoretical framework is developed to model how the IVPD magnitude data are related to the underlying shear wave motion, and how they can be used to estimate shear stiffness. The results are shown in a series of phantom experiments to demonstrate that IVPD MRE complements phase-contrast MRE.

AAPM/RSNA physics tutorial for residents: topics in US: beyond the basics: elasticity imaging with US

A new mode of imaging with ultrasonography (US) is under development in several laboratories around the world. This technique allows estimation of some measure of the viscoelastic properties of tissue. The information displayed in the images is a surrogate for that obtained with manual palpation. Fundamental concepts in elasticity imaging include stress, strain, and the elastic modulus; strain imaging has received the most attention from researchers. A system for elasticity imaging is under development that produces images of mechanical strain in real time by means of a freehand scanning technique. This system is integrated into a clinical US system without any external equipment and involves software changes only. Data obtained with this system demonstrate that the relative stiffness of many fibroadenomas changes as they and the surrounding tissue are deformed. At elasticity imaging of in vivo breast lesions, invasive ductal carcinomas appear, on average, more than twice as large on the elasticity image than on the B-mode image, but fibroadenomas and cysts are nearly equal in size on the two image types. The usefulness of this technology and the new information it provides suggest that it might soon be available on commercial US systems.

Evaluation of renal parenchymal disease in a rat model with magnetic resonance elastography

Alterations in the mechanical properties or "hardness" of tissues allow physicians to detect disease by palpation. Recently, attempts have been made to quantitate and image these tissue properties with the use of magnetic resonance elastography (MRE). This technique has been validated in ex vivo specimens, including kidney, breast, and prostate. In this study, in vivo MRE imaging of rat renal cortex is demonstrated and validated with a disease model that will facilitate further studies. Normal rats and rats with nephrocalcinosis induced with either 2 or 4 weeks of ethylene glycol exposure were studied with MRE. Histology in the diseased rats documented the presence of nephrocalcinosis. MRE measurements and images of shear stiffness were highly reproducible in individual rats. The shear stiffness of the renal cortex in normal rats was 3.87 kPa (95% CI 2.84-4.90 kPa). The shear stiffness increased to 5.02 kPa (95% CI 3.34-6.70 kPa) after 2 weeks of exposure, and to 6.49 kPa (95% CI 4.84-8.14 kPa) after 4 weeks of exposure (P = 0.0302, alpha < 0.05). MRE is capable of detecting alterations in the tissue mechanical properties of kidneys in vivo. It is a promising noninvasive technique that might have pathologic and prognostic significance.

Elastographic imaging of thermal lesions in liver in-vivo using diaphragmatic stimuli

Radiofrequency or microwave ablations are interstitial focal ablative therapies that can be used in a percutaneous fashion for treating tumors in the liver, kidney, and prostate. These modalities provide in situ destruction of tumors. We present a method for in-vivo elastographic visualization of the ablated regions in the liver during and after thermal therapy. In-vivo elastographic imaging uses compressions of the liver due to movement of the diaphragm during the respiratory cycle. Elastography of the liver and other abdominal organs has not been attempted previously due to the difficulty in providing controlled compressions. Gating of the data acquisition to the respiratory waveform would provide access to data where the compression increments are similar in both magnitude and direction, thereby enabling reproducible imaging of the thermal lesion or tumor. Comparison of elastograms with gross-pathology of ablated tissue illustrates the correspondence between elastographic image features and pathology. Ultrasound is routinely used to guide the rf ablation procedure, so the same imaging system could be used for elastographic imaging. Since the technique utilizes physiological motion of the diaphragm due to respiration, it may also be employed in the visualization of cancerous tumors in the liver.

A novel and robust method for rapid strain estimation in elastography

In elastography, change in signal shape from tissue deformation and nonaxial tissue motion reduce correlation between the pre- and postcompression echo signals. Appropriate global temporal stretching of postcompression signals can reduce the decorrelation. Adaptive stretching performs a search for the stretch factor that maximizes the correlation between the pre- and postcompression echo signal segments at each data window location. Adaptive stretching is robust but computation intensive. In contrast, global stretching is fast but performs well only in areas where local strains are close to the applied strain. We developed a method that strikes a balance between the speed of global stretching and the performance of adaptive stretching. In this method, several strain maps are computed by performing global stretching with a range of different stretch factors. The area in each computed strain image with strain values closely corresponding to the uniform stretch factor will contain 'good quality' strain estimates. To produce a single elastogram at the end, we identify the strain map with the maximum correlation at each location and the strain value in that strain map at that location is chosen for the combined map. Results from data generated by finite-element simulation and phantom experiments demonstrate that the described strain estimator is significantly less susceptible to signal degradation than conventional strain estimators.

Measurement of elastic nonlinearity of soft solid with transient elastography

Transient elastography is a powerful tool to measure the speed of low-frequency shear waves in soft tissues and thus to determine the second-order elastic modulus mu (or the Young's modulus E). In this paper, it is shown how transient elastography can also achieve the measurement of the nonlinear third-order elastic moduli of an Agar-gelatin-based phantom. This method requires speed measurements of polarized elastic waves measured in a statically stressed isotropic medium. A static uniaxial stress induces a hexagonal anisotropy (transverse isotropy) in solids. In the special case of uniaxially stressed isotropic media, the anisotropy is not caused by linear elastic coefficients but by the third-order nonlinear elastic constants, and the medium recovers its isotropic properties as soon as the uniaxial stress disappears. It has already been shown how transient elastography can measure the elastic (second-order) moduli in a media with transverse isotropy such as muscles. Consequently this method, based on the measurement of the speed variations of a low-frequency (50-Hz) polarized shear strain waves as a function of the applied stress, allows one to measure the Landau moduli A, B, C that completely describe the third-order nonlinearity. The several orders of magnitude found among these three constants can be justified from the theoretical expression of the internal energy.

Observation of shock transverse waves in elastic media

We report the first experimental observation of a shock transverse wave propagating in an elastic medium. This observation was possible because the propagation medium, a soft solid, allows one to reach a very high Mach number. In this extreme configuration, the shock formation is observed over a distance of less than a few wavelengths, thanks to a prototype of an ultrafast scanner (that acquires 5000 frames per second). A comparison of these new experimental data with theoretical predictions, based on a modified Burger's equation, shows good agreement.

Realtime-Elastographie

It is well known that some diseases, such as cancer, lead to a change of tissue hardness (i.e. the so-called elasticity modulus). The reconstruction of tissue elasticity provides the sonographer with important additional information which can be applied for the diagnosis of these diseases. Elasticity imaging has recently attracted attention as a technique which directly reveals the physical property of tissue and enables us to determine the change of tissue hardness caused by diseases. The elasticity modulus, i.e. the tissue elasticity distribution can be calculated from the strain and the stress of the examined structures. While the strain field can be estimated from the RF signals returned from tissue structures before and after compression, it is impossible to measure the stress field directly within the tissue. Another problem is that the compression of harder tissue structures is often followed by a lateral displacement of these structures. It is nearly impossible to represent the volume of this sideslip with conventional 2D methods but its calculation is indispensable for an accurate determination of the tissue elasticity of the examined structures. To overcome these problems, we propose the so-called Extended CA-method (Extended Combined Autocorrelation Method) which allows the reconstruction of the tissue elasticity of the examined structures on the basis of the 3-dimensional finite element model. The new technique enables a highly accurate estimation of the tissue elasticity distribution and the adequate compensation of sideslips. The realtime elasticity imaging described in this article, can easily be performed with the SonoElastography module that can be integrated into the platform of the HITACHI EUB-8500 system. Like colour Doppler examinations, tissue elasticity imaging can easily be performed with conventional ultrasound probes and does not require additional instruments (e.g. for measuring pressure or vibrations). The calculation of tissue elasticity distribution is performed in realtime and the examination results are represented in colour over the conventional B-mode image. The results of the simulations and phantom experiments performed verify that with the information obtained by the new realtime elasticity imaging method, lesions can be detected and represented more rapidly and with higher accuracy than with conventional methods based on the 2D Model, and that even lesions invisible on B-mode images can be detected.

Detection of lesions using differential rates of speckle decorrelation

Breast lesions that may be cancerous generally possess a higher elastic modulus than the surrounding tissue. Thus, they are sensed to move differently from the surrounding tissue during finger-based palpation. Since the palpation is subjective in nature, a method using ultrasound for detection of the differential motion associated with hard lesions is proposed. It is intended that regions of detected 'lump' be highlighted on top of a conventional B-mode image so that the user can positively associate palpated lumps with specific regions on the anatomical B-mode image. We detect the differential motion associated with a lesion by detecting a local abnormality in a map of image-to-image correlations as the transducer is swept in an elevational direction. Experiments were performed with three different tissue mimicking phantoms. The results indicate that the location of the mobile lesion can be successfully estimated whereas immobile lesions are not very detectable. This approach is simple and requires only a short period of time for each scan. It has practically no theoretical incremental cost.

Correction for simultaneous catheter eccentricity and tilt in intravascular elastography

Intravascular elastography can provide significant new information about the elastic properties of vascular tissue and plaque, useful for the diagnosis of disease and appropriate selection of interventional methods. Knowledge of the plaque composition, vulnerability and its elastic properties can assist the clinician in selecting appropriate interventional techniques. However, several noise sources have to be addressed to obtain quality intravascular elastograms. Misalignment of the vessel lumen and the ultrasound beam can produce erroneous strain estimates in elastography. Errors in the strain estimate are introduced due to the eccentricity and tilt of the intravascular transducer within the vessel lumen. Previous work in this area has provided theoretical expressions for the correction of eccentricity and tilt errors when they occur independent of each other. However, under most imaging conditions, both eccentricity and tilt errors are simultaneously present. In this paper, we extend the theoretical correction factor by accounting for the influence of both of these errors occurring simultaneously in the positioning of the catheter within the vessel lumen.

Elastographic measurement of the area and volume of thermal lesions resulting from radiofrequency ablation: pathologic correlation

OBJECTIVE

Elastography is a promising tool for visualizing the zone of necrosis in liver tissue resulting from radiofrequency tumor ablation. Because heat-ablated tissues are stiffer than normal untreated tissue, elastography may prove useful for following up patients who undergo radiofrequency ablative therapy. We sought to report the initial evaluations of the reliability of elastography for delineating thermal lesion boundaries in liver tissue by comparing lesion dimensions determined by elastography with the findings at whole-mount pathology.

MATERIALS AND METHODS

Radiofrequency ablation was performed in vitro on liver tissue specimens encased in gelatin phantoms. The imaging plane for elastography was perpendicular to the axis of the radiofrequency electrode so that the ablated region was around the center of the plane. To obtain three-dimensional visualization of thermal lesions, we reconstructed the lesions from multiple elastograms by linearly translating the elastographic scanning plane. Pathology photographs were obtained in the same image plane used for elastography by slicing through the gelatin and tissue phantom using external markers. We used digitized gross pathology photographs obtained at a specified slice thickness to compute the areas and volumes of the lesions. These measurements were then compared to the measurements obtained from the elastograms.

RESULTS

In a sample of 40 thermal lesions, we obtained a correlation between in vitro elastographic and pathologic measurements of r = 0.9371 (p < 0.00001) for area estimates and r = 0.979 (p < 0.00001) for volume estimates.

CONCLUSION

We found excellent correlation between the measurements of the dimensions, areas, and volumes of thermal lesions that were based on elastographic images and the measurements that were based on digitized pathologic images. When compared with digitized pathologic measurements, elastographic measurements showed a tendency to slightly underestimate both the areas and volumes of lesions. Nevertheless, elastography is a reliable technique for delineating thermal lesions resulting from radiofrequency ablation.

Analytic modeling of breast elastography

The elastic moduli of tumors change during their pathological evolution. Elastographic imaging has potential for detecting and characterizing cancers by mapping the stiffness distribution in tissues. In this paper a micromechanics-based analytical method was developed to detect the location, size, and elastic modulus of a tumor mass embedded in a symmetric two-dimensional breast tissue. A closed-form solution for the strain elastograms (forward problem) was derived. A computational algorithm for the inverse problem was developed for the detection, localization, and characterization of a heterogeneous mass embedded in a breast tissue. Numerical examples were presented to evaluate the proposed method's performance. The detectability of a tumor mass was estimated with respect to lesion location, size, and modulus contrast ratio. It was shown that the micromechanics theory provides a powerful tool for the diagnosis of breast cancer.

Measuring the elastic modulus of ex vivo small tissue samples

Over the past decade, several methods have been proposed to image tissue elasticity based on imaging methods collectively called elastography. While progress in developing these systems has been rapid, the basic understanding of tissue properties to interpret elastography images is generally lacking. To address this limitation, we developed a system to measure the Young's modulus of small soft tissue specimens. This system was designed to accommodate biological soft tissue constraints such as sample size, geometry imperfection and heterogeneity. The measurement technique consists of indenting an unconfined small block of tissue while measuring the resulting force. We show that the measured force-displacement slope of such a geometry can be transformed to the tissue Young's modulus via a conversion factor related to the sample's geometry and boundary conditions using finite element analysis. We also demonstrate another measurement technique for tissue elasticity based on quasi-static magnetic resonance elastography in which a tissue specimen encased in a gelatine-agarose block undergoes cyclical compression with resulting displacements measured using a phase contrast MRI technique. The tissue Young's modulus is then reconstructed from the measured displacements using an inversion technique. Finally, preliminary elasticity measurement results of various breast tissues are presented and discussed.

[Prostate cancer diagnosis using ultrasound elastography. Introduction of a novel technique and first clinical results]

During the last decade screening has improved prostate cancer detection. The main reason for this development is a better understanding of the margins of prostate-specific antigen (PSA) serum levels and the classification of PSA subtypes. In contrast, the introduction of transrectal ultrasound has not led to a measurable change in the prostate cancer detection rate. Our aim was to develop a novel ultrasound system for the acquisition of elastographic images of the prostate and evaluate the system regarding its clinical applicability. We used a technically modified conventional ultrasound system and analyzed the high-frequency ultrasonic data with a computer program. The first patient-based results suggest that elastography allows an accurate measurement of tumor size and localization in contrast to conventional transrectal ultrasound. Elastography visualizes different tissue elasticities to distinguish benign and cancerous tissue. Thus, we were able to even correctly classify prostate cancer lesions which are iso- or hyperechoic in B-mode sonography.

Intravascular elastography: from bench to bedside

An unstable lesion may rupture and cause an acute thrombotic reaction. These lesions contain a large lipid pool covered by a thin fibrous cap. The stress in the cap increased with decreasing thickness and increasing macrophage infiltration. Intravascular ultrasound (IVUS) elastography might be an ideal technique to assess the presence of lipid pools and identify high stress regions. Elastography assesses the local mechanical properties of tissue using its deformation caused by the intraluminal pressure. The technique was validated in vitro using diseased human coronary and femoral arteries. These experiments demonstrated that the strain in the three plaque types is different (P < 0.001). Especially between fibrous and fatty tissue, a highly significant difference (P = 0.0012) was found. Additionally, the predictive value for identifying the vulnerable plaque was investigated. A high strain region at the lumen vessel wall boundary has 88% sensitivity and 89% specificity for identifying these plaques. In vivo, the technique is validated in an atherosclerotic Yucatan mini-pig animal model. This study also revealed higher strain values in fatty than fibrous plaques (P < 0.001). The presence of a high strain region at the lumen plaque interface has a high predictive value for identifying macrophages. Patient studies revealed high strain values (1-2%) in soft plaques. Calcified material shows low strain values (0-0.2%). With the development of three-dimensional elastography, identification of weak spots over the full length of a coronary artery becomes possible. In conclusion, intravascular elastography is a unique tool to assess lesion composition and vulnerability. The development of three-dimensional elastography provides a technique that may develop into a clinical available tool for identifying the rupture-prone plaque.

Thresholds for detecting and characterizing focal lesions using steady-state MR elastography

An objective contrast-detail analysis was performed in this study to assess the low contrast detectability of a clinical prototype harmonic magnetic resonance elastographic imaging system. Elastographic imaging was performed on gelatin phantoms containing spherical inclusions of varying size and modulus contrast. The results demonstrate that lesions as small as 5 mm can be detected with a minimum modulus contrast of 14 dB. However, the shear modulus of such small lesions was not accurately recovered. In general, the shear modulus of larger focal lesions was accurately (i.e., within 25% of the true value) recovered. The minimum modulus contrast needed to detect focal lesions was observed to decrease with increasing lesion size.

Detecting stiff masses using strain-encoded (SENC) imaging

A method is proposed for detecting stiff masses using strain-encoded (SENC) magnetic resonance imaging (MRI). An object of interest is compressed to produce local strain distribution that depends on local elasticity, where intensities correlate with the local through-imaging-plane strain component. Because the strain is lower inside a stiff mass than in the surrounding soft tissue, an intensity contrast in the resulting images would enable direct detection of the mass without postprocessing. The technique was validated by a phantom experiment in which a gel phantom with a stiff region was used. The advantages of the proposed method include short imaging time and uncomplicated postprocessing. However, in its current form the technique does not measure elasticity.

Ultrasonic imaging of myocardial strain using cardiac elastography

Clinical assessment of myocardial ischemia based on visually-assessed wall motion scoring from echocardiography is semiquantitative, operator dependent, and heavily weighted by operator experience and expertise. Cardiac motion estimation methods such as tissue Doppler imaging, used to assess myocardial muscle velocity, provides quantitative parameters such as the strain-rate and strain derived from Doppler velocity. However, tissue Doppler imaging does not differentiate between active contraction and simple rotation or translation of the heart wall, nor does it differentiate tethering (passively following) tissue from active contraction. In this paper, we present a strain imaging modality called cardiac elastography that provides two-dimensional strain information. A method for obtaining and displaying both directional and magnitude cardiac elastograms and displaying strain over the entire cross-section of the heart is described. Elastograms from a patient with coronary artery disease are compared with those from a healthy volunteer. Though observational, the differences suggest that cardiac elastography may be a useful tool for assessment of myocardial function. The method is two-dimensional, real time and avoids the disadvantage of observer-dependent judgment of myocardial contraction and relaxation estimated from conventional echocardiography.

Comparison of quantitative shear wave MR-elastography with mechanical compression tests

The mechanical properties of in vivo soft tissue are generally determined by palpation, ultrasound measurements (US), and magnetic resonance elastography (MRE). While it has been shown that US and MRE are capable of quantitatively measuring soft tissue elasticity, there is still some uncertainty about the reliability of quantitative MRE measurements. For this reason it was decided to determine in vitro how MRE measurements correspond with other quantitative methods of measuring characteristic elasticity values. This article presents the results of experiments with tissue-like agar-agar gel phantoms in which the wavelength of strain waves was measured by shear wave MR elastography and the resultant shear modulus was compared with results from mechanical compression tests with small gel specimens. The shear moduli of nine homogeneous gels with various agar-agar concentrations were investigated. The elasticity range of the gels covered the elasticity range of typical soft tissues. The systematic comparison between shear wave MRE and compression tests showed good agreement between the two measurement techniques.

Elastography: Imaging the elastic properties of soft tissues with ultrasound

Elastography is a method that can ultimately generate several new kinds of images, called elastograms. As such, all the properties of elastograms are different from the familiar properties of sonograms. While sonograms convey information related to the local acoustic backscatter energy from tissue components, elastograms relate to its local strains, Young's moduli or Poisson's ratios. In general, these elasticity parameters are not directly correlated with sonographic parameters, i.e. elastography conveys new information about internal tissue structure and behavior under load that is not otherwise obtainable. In this paper we summarize our work in the field of elastography over the past decade. We present some relevant background material from the field of biomechanics. We then discuss the basic principles and limitations that are involved in the production of elastograms of biological tissues. Results from biological tissues in vitro and in vivo are shown to demonstrate this point. We conclude with some observations regarding the potential of elastography for medical diagnosis.

Elastographic imaging using staggered strain estimates

Conventional techniques in elastography estimate strain as the gradient of the displacement estimates obtained through crosscorrelation of pre- and postcompression rf A-lines. In these techniques, the displacements are estimated over overlapping windows and the strains are estimated as the gradient of the displacement estimates over adjacent windows. The large amount ofnoise at high window overlaps may result in poor quality elastograms, thus restricting the applicability of conventional strain estimation techniques to low window overlaps, which, in turn, results in a small number of pixels in the image. To overcome this restriction, we propose a multistep strain estimation technique. It computes the first elastogram using nonoverlapped windows. In the next step, the data windows are shifted by a small distance (small fraction of window size) and another elastogram is produced. This is repeated until the cumulative shift equals/exceeds the window size and all the elastograms are staggered to produce the final elastogram. Simulations and experiments were performed using this technique to demonstrate significant improvement in the elastographic signal-to-noise ratio (SNRe) and the contrast-to-noise ratio (CNRe) at high window overlaps over conventional strain estimation techniques, without noticeable loss of spatial resolution. This technique might be suitable for reducing the algorithmic noise in the elastograms at high window overlaps.

Temperature dependence of ultrasonic propagation speed and attenuation in canine tissue

Previously reported data on the temperature dependence of propagation speed in tissues generally span only temperature ranges up to 60 degrees C. However, with the emerging use of thermal ablative therapies, information on variation in this parameter over higher temperature ranges is needed. Measurements of the ultrasonic propagation speed and attenuation in tissue in vitro at discrete temperatures ranging from 25 to 95 degrees C was performed for canine liver, muscle, kidney and prostate using 3 and 5 MHz center frequencies. The objective was to produce information for calibrating temperature-monitoring algorithms during ablative therapy. Resulting curves of the propagation speed vs. temperature for these tissues can be divided into three regions. In the 25-40 degrees C range, the speed of sound increase rapidly with temperature. It increases moderately with temperature in the 40-70 degrees C range, and it then decreases with increasing temperature from 70-95 degrees C. Attenuation coefficient behavior with temperature is different for the various tissues. For liver, the attenuation coefficient is nearly constant with temperature. For kidney, attenuation increases approximately linearly with temperature, while for muscle and prostate tissue, curves of attenuation vs. temperature are flat in the 25-50 degrees C range, slowly rise at medium temperatures (50-70 degrees C), and level off at higher temperatures (70-90 degrees C). Measurements were also conducted on a distilled degassed water sample and the results closely follow values from the literature.

An efficient speckle tracking algorithm for ultrasonic imaging

An efficient speckle tracking algorithm is proposed for motion estimation in ultrasonic imaging. Speckle tracking involves a matching process and a searching process. The matching process of the proposed algorithm is based on a Block Sum Pyramid algorithm that significantly reduces the computational complexity while maintaining the same accuracy as the conventional sum of absolute difference approach. The searching process, on the other hand, is based on a multilevel search strategy rather than the full-search strategy used by most conventional tracking methods. Both simulated speckle images and clinical breast images were used to test the performance of the proposed algorithm. The results show that the computation efficiency is improved by up to a factor of five over the conventional approach. The improved efficiency enables real-time or near-real-time implementation of motion estimation in ultrasonic imaging, which is particularly beneficial in areas such as blood flow estimation, elasticity imaging, speckle image registration, and strain compounding.

An ultrasonic measurement for in vitro depth-dependent equilibrium strains of articular cartilage in compression

The equilibrium depth-dependent biomechanical properties of articular cartilage were measured using an ultrasound-compression method. Ten cylindrical bovine patella cartilage-bone specimens were tested in compression followed by a period of force-relaxation. A 50 MHz focused ultrasound beam was transmitted into the cartilage specimen through a remaining bone layer and a small hole at the centre of a specimen platform. The ultrasound echoes reflected or scattered within the articularcartilage were collected using the same transducer. The displacements of the tissues at different depths of the articular cartilage were derived from the ultrasound echo signals recorded during the compression and the subsequent force-relaxation. For two steps of 0.1 mm compression, the average strain at the superficial 0.2 mm thick layer (0.35 +/- 0.09) was significantly (p < 0.05) larger than that at the subsequent 0.2 mm thick layer (0.05 +/- 0.07) and that at deeper layers (0.01 +/- 0.02). It was demonstrated that the compressive biomechanical properties of cartilage were highly depth-dependent. The results suggested that the ultrasound-compression method could be a useful tool for the study of the depth-dependent biomechanical properties of articular cartilage.

A modified block matching method for real-time freehand strain imaging

This manuscript reports a technical innovation that has been developed for real-time, freehand strain imaging. This work is based on a well-known block-matching algorithm with two significant modifications. First, since displacements are estimated row-by-row, displacement estimates from the previous row are used to predict the displacement estimates in the current row, thereby drastically reducing the search-region size and increasing computational efficiency. Second, a displacement error detection and correction method is developed to overcome the local tracking errors that may be more severe with freehand scanning and thereby improve the robustness of the algorithm. This algorithm has been implemented on a clinical ultrasound imaging system, and with real-time imaging feedback, long sequences of high quality strain images are observed using freehand compression. Displacement estimation errors with this method are experimentally measured and compared with results from simulation. We report only a specific implementation, with no comparison to other displacement estimators in the literature and no optimization of this specific technique. Images of tissue-mimicking phantoms with small spherical targets are used to test the ability to detect small lesions using the strain imaging technique. In vivo strain images of breast and thyroid are also shown.

Microcalcifications in breast tissue phantoms visualized with acoustic resonance coupled with power Doppler US: initial observations

Calcium carbonate particles embedded in gelatin and turkey breast tissues were visualized with acoustic resonance imaging and power Doppler ultrasonography. Sonography revealed that the region of color level detection corresponded to the location of the calcium carbonate particles. Correlation between color level detection and the location of the particles was confirmed on radiographs of the specimens obtained at core needle biopsy performed through the region of color level detection.

Quantitative elasticity imaging: what can and cannot be inferred from strain images

We examine the inverse problem associated with quantitative elastic modulus imaging: given the equilibrium strain field in a 2D incompressible elastic material, determine the elastic stiffness (shear modulus). We show analytically that a direct formulation of the inverse problem has no unique solution unless stiffness information is known a priori on a sufficient portion of the boundary. This implies that relative stiffness images constructed on the assumption of constant boundary stiffness are in error, unless the stiffness is truly constant on the boundary. We show further that using displacement boundary conditions in the forward incompressible elasticity problem leads to a nonunique inverse problem. Indeed, we give examples in which exactly the same strain field results from different elastic modulus distributions under displacement boundary conditions. We also show that knowing the stress on the boundary can, in certain configurations, lead to a well-posed inverse problem for the elastic stiffness. These results indicate what data must be taken if the elastic modulus is to be reconstructed reliably and quantitatively from a strain image.

Analysis of an adaptive strain estimation technique in elastography

Elastography is based on the estimation of strain due to tissue compression or expansion. Conventional elastography involves computing strain as the gradient of the displacement (time-delay) estimates between gated pre- and postcompression signals. Uniform temporal stretching of the postcompression signals has been used to reduce the echo-signal decorrelation noise. However, a uniform stretch of the entire postcompression signal is not optimal in the presence of strain contrast in the tissue and could result in loss of contrast in the elastogram. This has prompted the use of local adaptive stretching techniques. Several adaptive strain estimation techniques using wavelets, local stretching and iterative strain estimation have been proposed. Yet, a quantitative analysis of the improvement in quality of the strain estimates overconventional strain estimation techniques has not been reported. We propose a two-stage adaptive strain estimation technique and perform a quantitative comparison with the conventional strain estimation techniques in elastography. In this technique, initial displacement and strain estimates using global stretching are computed, filtered and then used to locally shift and stretch the postcompression signal. This is followed by a correlation of the shifted and stretched postcompression signal with the precompression signal to estimate the local displacements and hence the local strains. As proof of principle, this adaptive stretching technique was tested using simulated and experimental data.

[Illustration of elasticity differences using MR-elastography]

Differences of elasticity in tissue phantoms with inclusions of different elasticity were mapped by means of MR elastography (MRE). This new magnetic resonance imaging technique is based on the phase shift of the MR signal by switching a motion sensitizing magnetic field gradient simultaneously with the coupling of a shear wave. Wave patterns showing snapshots of the shear wave that propagates through the investigated substance were depicted in tomographic phase images. It was investigated wether a visualization of differences in elasticity of soft tissues was possible on the basis of differences in the wavelength. For this purpose, tissue phantoms with cylindrical inclusions were produced from agar gels, with agar concentrations between 1.0 and 1.5%. The diameters of the inclusions were of the order of a few centimetres. For diameters as small as 4 cm, there were still distinct differences in the wavelength between the matrix and the inclusion. The results of our study suggest that this technique has the potential for future application as an additional imaging method for tumor detection.

Acoustic radiation force impulse imaging: in vivo demonstration of clinical feasibility

The clinical viability of a method of acoustic remote palpation, capable of imaging local variations in the mechanical properties of soft tissue using acoustic radiation force impulse (ARFI) imaging, is investigated in vivo. In this method, focused ultrasound (US) is used to apply localized radiation force to small volumes of tissue (2 mm(3)) for short durations (less than 1 ms) and the resulting tissue displacements are mapped using ultrasonic correlation-based methods. The tissue displacements are inversely proportional to the stiffness of the tissue and, thus, a stiffer region of tissue exhibits smaller displacements than a more compliant region. Due to the short duration of the force application, this method provides information about the mechanical impulse response of the tissue, which reflects variations in tissue viscoelastic characteristics. In this paper, experimental results are presented demonstrating that displacements on the order of 10 microm can be generated and detected in soft tissues in vivo using a single transducer on a modified diagnostic US scanner. Differences in the magnitude of displacement and the transient response of tissue are correlated with tissue structures in matched B-mode images. The results comprise the first in vivo ARFI images, and support the clinical feasibility of a radiation force-based remote palpation imaging system.

MR validation of soft tissue mimicing phantom deformation as modeled by nonlinear finite element analysis

A study of the applicability of nonlinear finite element analysis (FEA) to predict soft tissue deformation was validated with phase contrast magnetic resonance velocity imaging. A phantom of varying stiffness was placed in a special purpose, computer controlled MR compatible compression apparatus which provided precise, time varying compression with surface deformations on the order of 11%. The resulting motion was measured with MR velocity images acquired throughout the cycle of compression. The phantom geometry was modeled with a finite element mesh and the mechanical properties of the phantom material were measured and incorporated in the FEA model. The motion as calculated by the FEA model was compared to the motion measured with MRI and the results were found to vary with the material's Poisson's ratio and the coefficient of friction. A minimum difference was reached when the Poisson's ratio and coefficient of friction were set to 0.485 and 0.3, respectively. Under these conditions, the root mean square difference was found to be 14.4%.

Elastographic imaging using a handheld compressor

Elastography is an emerging imaging modality that allows noninvasive imaging of tissue stiffness changes and stiffness values associated with pathology or as a result of therapy. However, many currently-used systems for elastography rely on a fixed geometry transducer and compressor system for imaging. This configuration is disadvantageous for imaging difficult-to-reach regions that are currently accessible with conventional ultrasound. In this paper, we describe a handheld, portable stepper motor controlled system for elastography. This system may reduce motion and jitter errors that are prevalent in completely 'freehand elastography' that employs hand-induced compressions using the transducer. The latter also requires collection of large amounts of data and use of strain estimation algorithms that may not be sensitive to phase changes or use additional preprocessing to minimize decorrelation effects. The stepper motor controlled handheld system provides controlled compressions and synchronized data acquisition. Our technique yields elastograms of a low-contrast phantom that have contrast levels and contrast-to-noise ratios that are comparable to those obtained with a fixed geometry system.

Variational method for estimating the effects of continuously varying lenses in HIFU, sonography, and sonography-based cross-correlation methods

The effects of high intensity focused ultrasound (HIFU)-induced continuously varying thermal gradients on sound ray propagation were modeled theoretically. This modeling was based on Fermat's variational principle of least time for rays propagating in a continuously varying thermal gradient described by a radially symmetric heat equation. Such thermal lenses dynamically affect HIFU beam focusing, and simultaneously create ultrasonic geometric and intensity distortions and artifacts in monitoring devices. Techniques which are based upon ultrasonic cross-correlation methods, such as elastography and two-dimensional temperature estimation, also suffer distortion effects and generate artifacts.

Magnetic resonance elastography: non-invasive mapping of tissue elasticity

Magnetic resonance elastography (MRE) is a phase-contrast-based MRI imaging technique that can directly visualize and quantitatively measure propagating acoustic strain waves in tissue-like materials subjected to harmonic mechanical excitation. The data acquired allows the calculation of local quantitative values of shear modulus and the generation of images that depict tissue elasticity or stiffness. This is significant because palpation, a physical examination that assesses the stiffness of tissue, can be an effective method of detecting tumors, but is restricted to parts of the body that are accessible to the physician's hand. MRE shows promise as a potential technique for 'palpation by imaging', with possible applications in tumor detection (particularly in breast, liver, kidney and prostate), characterization of disease, and assessment of rehabilitation (particularly in muscle). We describe MRE in the context of other recent techniques for imaging elasticity, discuss the processing algorithms for elasticity reconstruction and the issues and assumptions they involve, and present recent ex vivo and in vivo results.

A focused ultrasound method for simultaneous diagnostic and therapeutic applications--a simulation study

Similar to other therapeutic methods, ultrasound surgery requires an imaging modality to monitor the extent of tissue damage during treatment. In this paper, we have considered the method of ultrasound-stimulated acoustic emission (USAE) that uses two ultrasonic beams at high frequency (1.7 MHz) (same as that used for ablation) to locally excite the tissue by generating a low-frequency (1-50 kHz) radiation force. Recording of the tissue response at several locations yields an image. The amplitude of the tissue response depends on the mechanical and acoustic tissue properties, namely its stiffness and absorption. These two properties were initially hypothesized to have counteractive effects on the response amplitude, i.e., the amplitude should increase with absorption and decrease with stiffness. To check this hypothesis as well as the degree to which these properties influence the response, finite-element simulations of a uniform lesion formed inside a homogeneous medium were used. The results show that, as expected, the displacement amplitude decreased with increasing lesion stiffness at lower frequencies (except at resonance) while, contrary to our initial hypothesis, it increased with stiffness at relatively higher frequencies (>22 kHz). At resonance, a frequency upshift occurred with increasing stiffness but was found to be highly spatially variant and system dependent, i.e., not yielding a uniform lesion response when imaged. On the other hand, the absorption increase led to a uniform linear increase of the mechanical response amplitude of the lesion. Therefore, at higher frequencies, increase of the two parameters had a synergistic effect on the tissue response to the applied radiation force. This study showed that relatively higher frequencies constitute the optimal range in the use of USAE for coagulation monitoring. A preliminary experimental verification in vitro is also provided.

A freehand elastographic imaging approach for clinical breast imaging: system development and performance evaluation

A prototype freehand elastographic imaging system has been developed for clinical breast imaging. The system consists of a fast data acquisition system, which is able to capture sequences of intermediate frequency echo frames at full frame rate from a commercial ultrasound scanner whilst the breast is deformed using hand-induced transducer motion. Two-dimensional echo tracking was used in combination with global distortion compensation and multi-compression averaging to minimise decorrelation noise incurred when stress is applied using hand-induced transducer motion. Experiments were conducted on gelatine phantoms to evaluate the quality of elastograms produced using the prototype system relative to those produced using mechanically induced transducer motion. The strain sensitivity and contrast-to-noise ratio of freehand elastograms compared favourably with elastograms produced using mechanically induced transducer motion. However, better dynamic range and signal-to-noise ratio was achieved when elastograms were created using mechanically induced transducer motion. Despite the loss in performance incurred when stress is applied using hand-induced transducer motion, it was concluded that the prototype system performed sufficiently well to warrant clinical evaluation.

Processing algorithms for tracking speckle shifts in optical elastography of biological tissues

Parametric and nonparametric data processing schemes for analyzing translating laser speckle data used to investigate the mechanical behavior of biological tissues are examined. Cross-correlation, minimum mean square estimator, maximum likelihood, and maximum entropy approaches are discussed and compared on speckle data derived from cortical bone samples undergoing dynamic loading. While it was not the purpose of this paper to demonstrate that one processing technique is superior to another, maximum likelihood and maximum entropy approaches are shown to be particularly useful when the observed speckle motion is small.