Showing papers on "Elastography published in 2001"

Assessment of thermal tissue ablation with MR elastography.

Abstract: An important part of thermal ablation therapy is the assessment of the spatial extent of tissue coagulation. In this work, the mechanical properties of thermally-ablated tissue were quantitatively evaluated using magnetic resonance elastography (MRE). This study shows that the mechanical properties of focused ultrasound ablated tissue are significantly different from normal tissue, and the difference can be imaged and measured using MRE. Repeated experiments revealed a reproducible pattern of tissue mechanical property change during thermal ablation in ex vivo bovine muscle. This pattern may reflect changes in intrinsic tissue structure and could be used to evaluate tissue coagulation during thermal ablation therapy. Magn Reson Med 45:80–87, 2001. © 2001 Wiley-Liss, Inc.

Three-dimensional subzone-based reconstruction algorithm for MR elastography.

Abstract: Accurate characterization of harmonic tissue motion for realistic tissue geometries and property distributions requires knowledge of the full three-dimensional displacement field because of the asymmetric nature of both the boundaries of the tissue domain and the location of internal mechanical heterogeneities. The implications of this for magnetic resonance elastography (MRE) are twofold. First, for MRE methods which require the measurement of a harmonic displacement field within the tissue region of interest, the presence of 3D motion effects reduces or eliminates the possibility that simpler, lower-dimensional motion field images will capture the true dynamics of the entire stimulated tissue. Second, MRE techniques that exploit model-based elastic property reconstruction methods will not be able to accurately match the observed displacements unless they are capable of accounting for 3D motion effects. These two factors are of key importance for MRE techniques based on linear elasticity models to reconstruct mechanical tissue property distributions in biological samples. This article demonstrates that 3D motion effects are present even in regular, symmetric phantom geometries and presents the development of a 3D reconstruction algorithm capable of discerning elastic property distributions in the presence of such effects. The algorithm allows for the accurate determination of tissue mechanical properties at resolutions equal to that of the MR displacement image in complex, asymmetric biological tissue geometries. Simulation studies in a realistic 3D breast geometry indicate that the process can accurately detect 1-cm diameter hard inclusions with 2.5x elasticity contrast to the surrounding tissue.

Better breast cancer detection

Abstract: X-rays go digital, computers read film, and chemicals mark tumors, but will these new technologies make it in the clinic? The imaging technologies considered for breast cancer include film-screen mammography, full field digital mammography, ultrasound, magnetic resonance imaging, scintimammography, thermography, electrical impedance imaging, optical imaging, electric potential measurement, positron emission tomography, novel ultrasound techniques, elastography, magnetic resonance spectroscopy, thermoacoustic computed tomography, microwave imaging, Hall-effect imaging and magneto-mammography.

Power spectral strain estimators in elastography

Abstract: Elastography can produce quality strain images in vitro and in vivo. Standard elastography uses a coherent cross-correlation technique to estimate tissue displacement and tissue strain using a subsequent gradient operator. While coherent estimation methods generally have the advantage of being highly accurate and precise, even relatively small undesired motions are likely to cause enough signal decorrelation to produce significant degradation of the elastogram. For elastography to become more universally practical in such applications as hand-held, intravascular and abdominal imaging, the limitations associated with coherent strain estimation methods that require tissue and system stability, must be overcome. In this paper, we propose the use of a spectral shift method that uses a centroid shift estimate (Fig. 5) to measure local strain directly. Furthermore, we also show theoretically that a spectral bandwidth method can also provide a direct strain estimation. We demonstrate that strain estimation using the spectral shift technique is moderately less precise but far more robust than the cross-correlation method. A theoretical analysis as well as simulations and experimental results are used to illustrate the properties associated with this method.

New developments in the detection of vulnerable plaque.

Abstract: Failure of coronary angiography (luminography) in prediction of future acute coronary syndromes has cast a shadow of doubt over the value of this old gold-standard technique. The fact that angiographically invisible or nonsignificant lesions cause the majority of acute coronary syndromes has driven scientists to develop new diagnostic methods. In this article, we review the ongoing worldwide research on both invasive techniques (such as intravascular angioscopy and colorimetry, ultrasound, thermography, optical coherence tomography, near infrared spectroscopy, Raman spectroscopy, fluorescence emission spectroscopy, elastography, magnetic resonance imaging and spectroscopy, nuclear immunoscintigraphy, electrical impedance imaging, vascular tissue doppler, and shear stress imaging) and noninvasive techniques (such as MRI, contrast-enhanced MRI with and without immunolabeled agents, electron beam computed tomography, multi-slice spiral / helical computed tomography, and nuclear imaging, including positron emission tomography). Each of these techniques and their potential combination holds promise for characterization of plaques responsible for acute coronary syndromes, namely vulnerable plaque.

Axial strain imaging of intravascular data: results on polyvinyl alcohol cryogel phantoms and carotid artery.

Abstract: Mapping the local elastic properties of an atherosclerotic artery is of major interest for predicting the disease evolution or an intervention outcome. These properties can be investigated by elastography, which estimates the strain distribution within a medium in response to a stress. But because diseased arteries are highly heterogeneous, a small global deformation may result in high local strains in the softest regions. For those reasons, we use in this paper the strain estimation method we recently developed to compute elastograms of original vessel-mimicking cryogel phantoms and a fresh excised human carotid artery. This adaptive method has been effectively proved to be accurate in a wider range of strains (0 -7%) than commonly used gradient-based methods, and very adapted for investigating highly heterogeneous tissues. Resulting elastograms cover a wider range of strains (0 -3.5%) than all previously reported intravascular elastograms, improving the discrimination between healthy and diseased regions. (E-mail: elisabeth.brusseau@creatis.insa-lyon.fr) © 2002 World Feder- ation for Ultrasound in Medicine & Biology.

Imaging prostate cancer: current and future applications.

Abstract: Various treatment options are available for adenocarcinoma of the prostate--the most common malignant neoplasm among men in the United States. To select an optimum management strategy, we must be able to identify an organ-confined disease (in which local therapy such as surgery or radiation may be beneficial) vs prostate cancer beyond the confines of the gland (for which other treatment approaches may be more appropriate). At present, no standard imaging modality can by itself reliably diagnose and/or stage adenocarcinoma of the prostate. Standard transrectal ultrasound, magnetic resonance imaging (MRI), computed tomography, bone scans, and plain x-ray are not sufficiently reliable when used alone. Fortunately, advances in imaging technology have led to the development of several promising modalities. These modalities include color and power Doppler ultrasonography, ultrasound contrast agents, intermittent and harmonic ultrasound imaging, MR contrast imaging, MRI with fat suppression, MRI spectroscopy, three-dimensional MRI spectroscopy, elastography, and radioimmunoscintigraphy. These newer imaging techniques appear to improve the yield of prostate cancer detection and staging, but are limited in availability and thus require further validation. This article reviews the status of current imaging modalities for prostate cancer and identifies emerging imaging technologies that may improve the diagnosis and staging of this disease.

Deformation correction in ultrasound images using contact force measurements

Abstract: During an ultrasound scan, contact between the probe and the skin deforms the underlying tissue. This can be considered a feature (as in elastography), but is in general undesirable, particularly for 3D scanning. In this paper we propose a novel system to correct this deformation by measuring the contact force at the time of the ultrasound scan and then using an elastic model to predict the tissue deformation. The inverse of this deformation is then applied to the image, generating the image that would have been seen had there been no contact with the probe. A prototype system has been implemented using an a priori finite element model to predict the deformation. This has been tested on gelatine phantoms and shown to remove the contact deformation and so give improved 3D reconstructions.

Assessment of regional myocardial strain using cardiac elastography: distinguishing infarcted from non-infarcted myocardium

Abstract: Estimation of the regional mechanical properties of the cardiac muscle has been shown to play a crucial role in the detection of cardiovascular disease. Current echocardiography-based cardiac motion estimation techniques, such as Doppler Myocardial Imaging (DMI), are limited due to angle dependence. By contrast, elastography, a method designed and used for the detection of tumors, measures displacement and strain by comparing echoes before and after (not during) a deformation and thus is not angle-dependent. Therefore, the feasibility of cardiac elastography to provide reliable and reproducible displacement and strain estimates from multiple sonographic views was recently demonstrated utilizing RF data from a normal human heart in vivo. In this paper, we demonstrate this technique utilizing 2D B-scan data in a patient with a known myocardial infarction. Envelope-detected sonographic data was used to estimate regional wall motion and deformation. Displacement and strain estimates were obtained in both non-infarcted, normally contracting and infarcted regions. By obtaining cine-loop and M-Mode elastograms from both regions, the ischemic regions could be identified. In conclusion, elastography may be a clinically viable method for detection of abnormalities of regional wall motion throughout the cardiac cycle.

Darstellung von Tumoren der weiblichen Brust mit MR-Elastographie

Abstract: Purpose Imaging of breast tumors using MR-Elastography. Material and method Low-frequency mechanical waves are transmitted into breast-tissue by means of an oscillator. The local characteristics of the mechanical wave are determined by the elastic properties of the tissue. By means of a motion-sensitive spin-echo-sequence these waves can be displayed within the phase of the MR image. Subsequently, these images can be used to reconstruct the local distribution of elasticity. In-vivo measurements were performed in 3 female patients with malignant tumors of the breast. Results All patients tolerated the measurement set-up without any untoward sensation in the contact area of skin and oszillator. The waves completely penetrated the breast, encompassing the axilla and regions close to the chest wall. All tumors were localized by MRE as structures of markedly stiffer tissue when compared to the surrounding tissue. Furthermore, in one patient, a metastasis in an axillary lymph node was detected. In all patients, local regions of increased elasticity were found in the remaining parenchyma of the breast, which, however, did not reach the high levels of elasticity found in the tumors. Conclusion MRE is an imaging modality enabling adjunct tissue differentiation of mammary tumors.

Investigation of real-time remote palpation imaging

Abstract: We are investigating a novel ultrasonic method for remote palpation, which provides images of local variations in tissue stiffness. Acoustic radiation force is applied to small volumes of tissue, and the resulting displacement patterns are imaged using ultrasonic correlation based techniques. Tissue displacements are inversely proportional to tissue stiffness, thus a stiffer region of tissue exhibits smaller displacements than a more compliant region. This method also provides information about tissue recovery after force cessation. We will present in vivo experimental results demonstrating the feasability of this method. Using intensities ranging from 120 to 300 W/cm2, peak displacements of up to 50 microns were observed after 1.4 milliseconds of force application. The tissue moved to its peak displacement within 3 milliseconds of force application, and the time constants for tissue recovery varied with tissue type. Tissue displacements appeared to be correlated with tissue structure in matched B-mode images. To our knowledge, these results represent the first in vivo soft tissue images generated using radiation force. These findings support the feasibility of Remote Palpation imaging. We will discuss the technical, safety, and clinical challenges of implementing a real-time Remote Palpation imaging system on a commercial diagnostic scanner.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A signal/noise analysis of quasi-static MR elastography

Abstract: In quasi-static magnetic resonance elastography, strain images of a tissue or material undergoing deformation are produced. In this paper, the signal/noise (S/N) ratio [SNR] of elastographic strain images, as measured by a phase-contrast technique, is analyzed. Experiments are conducted to illustrate how diffusion-mediated signal attenuation limits maximum strain SNR in small displacement cases, while the imaging point-spread function limits large displacement cases. A simple theoretical treatment agrees well with experiments and shows how an optimal displacement encoding moment can be predicted for a given experimental set of parameters to achieve a maximum strain SNR. A further experiment demonstrates how the limitation on strain SNR posed by the imaging point-spread function may potentially be overcome.

Time-resolved 2D pulsed elastography: experiments on tissue-equivalent phantoms and breast in vivo

Abstract: Time-resolved 2D Pulsed Elastography is a new elastographic technique for imaging the shear modulus of soft tissues. A low-frequency transient shear wave is sent in the medium while an ultra-fast ultrasonic imaging system acquires frames at a very high frame rate (up to 10,000 frames/s). This ultra-fast ultrasonic imaging system has been specifically developed for this application. It is based on a time-reversal mirror of 128 channels sampled at 50 MHz and having 2 Mbytes random access memory. Displacements induced by the slowly propagating shear wave are measured using the standard cross-correlation technique. The low-frequency excitation is obtained with a device composed of two rods that are placed around the ultrasonic transducer linear array. The rods vibrate perpendicularly to the surface of the tissues. They may be either parallel or perpendicular to the active surface of the array. With this device, large amplitude displacements are observed in the ultrasonic image area. We have measured the spatio- temporal evolution of the displacements induced by the low- frequency (20-100 Hz) shear wave in tissue-equivalent phantoms and breast in-vivo. A direct local inversion is used to recover the shear modulus distribution map in phantoms containing hard regions.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

DOSSIER IMAGERIE ACOUSTIQUE ET OPTIQUE DES MILIEUX BIOLOGIQUES OPTICAL AND ACOUSTICAL IMAGING OF BIOLOGICAL MEDIA Elastography

Abstract: We present a concise summary of some of the fundamentals and of our work in the field of elastography over the past 12 years. This summary is not exhaustive, since several recent reviews of this area are available. We begin by presenting some relevant background material from the field of biomechanics, which forms the foundation of this work. We then proceed to discuss the basic principles and limitations that are involved in the production of strain images (elastograms) of biological tissues. Early results as well as current results from biological tissues in vitro and in vivo are shown. We conclude with some thoughts regarding the potential of elastography for medical diagnosis.  2001 Academie des sciences/Editions scientifiques et medicales Elsevier SAS elastography / ultrasound / stress / strain / imaging / elasticity / shear modulus / Young's modulus Elastographie

Novel estimators for elastography

Abstract: In conventional elastography, strains are estimated by computing gradient of estimated displacement. However, gradient-based algorithms are susceptible to noise. We have developed two new strain estimators to overcome the common limitationsof elastography. The first estimator is based on a frequency-domain formulation; it estimates local strain by maximizing thecorrelation between the spectra of pre- and post-compression echo signals by iteratively frequency-scaling the latter. Wediscuss a variation of this algorithm that may be computationally more efficient. The second estimator is based on theobservation that an extremely stiff region will undergo virtually no strain when compressed, and will exhibit quasi-rigid bodymotion. As a result, an area with high similarity between the pre- and post-compression signals indicates low strain, and anarea with low similarity indicates large strain. We use a normalized 2-D correlation function to estimate this similarity. Thismethod offers significant advantages for detecting rigid tissues in the presence of large, irregular, non-axial motion. Both th eestimators exhibited promising results in simulation and experiments.Keywords: Correlation, elastogram, elastography, imaging, rigid-body motion, spectral estimation, spectral shift, strain,ultrasound, ultrasonic imaging.

A modified synthetic aperture focusing technique for the correction of geometric artefacts in intravascular ultrasound elastography

Abstract: Various elastography methods have been developed for imaging the elastic properties of coronary plaques with intravascular ultrasound (IVUS). In these methods the vessel tissue is compressed due to changes of the intraluminal pressure, i.e. by a force originating from the lumen center. If the transducer is not centered in the lumen, the ultrasound beams are not parallel to the direction of force, which leads to errors in the strain estimate. In this work a modified synthetic aperture focusing technique (SAFT) is presented, that allows the reconstruction of ultrasound beams parallel to the force direction. To verify this approach, IVUS data and a cylindrical phantom are simulated with various eccentric catheter positions.

Strain-encoded (SENC) harmonic phase imaging for fast magnetic resonance elastography

Abstract: A new method is proposed for imaging the elasticity of tissue using magnetic resonance (MR). Using an external actuator to deform the tissue, local strains way according to local elasticity. The method produces images shows the distribution of strains with different grades of intensity; thus, revealing the underlying elasticity distribution. The advantages of the method are that imaging is much faster than other magnetic resonance elastography (MRE) techniques, there is no need for complicated computations, and it produces high-resolution images.

Feasibility of monitoring HIFU prostate cancer therapy using elastography

Abstract: The objective of this study is to investigate the feasibility of elastographic monitoring of High Intensity Focused Ultrasound (HIFU) therapy of prostate cancer. Elastography is an imaging technique based on strain estimation in soft tissues under quasi-static compression. Since pathological tissues and HIFU-induced lesions exhibit different elastic properties than normal tissues, elastography is potentially able to achieve these goals. An ultrasound scanner was connected to a PC to acquire RF images. This setup is compatible with a HIFU device used for prostate cancer therapy by transrectal route. The therapy transducer and the biplane-imaging probe are covered with a balloon filled with a coupling liquid. Compression of the prostate is applied by inflating the balloon, while imaging sector scans of the prostate. In-vivo elastograms of the prostate were acquired before HIFU treatment. Problems inherent to in-vivo acquisitions are reported, such as undesired tangential displacements during the radial compression. This study shows the potential for in-vivo elastogram acquisition of HIFU-induced lesions in the human prostate.

Optical elastography. Discussion

Abstract: A maximum likelihood estimator for quantifying shifts in speckle patterns resulting from an applied stress to biological tissue is described. The estimator is then applied to calculating the strains and instantaneous strain rates in chicken skeletal muscle subjected to a dynamic acoustic (2 Hz) sinusoidal loading.

Experiments with 3-D Mammography and their Possible Applications in Telemammography

Abstract: he three-dimensional (3-D) breast imaging is an attempt to overcome limitations related to 2-D nature of conventional projection mammography, which is the leading method for the breast imaging today. Conventional mammograms are 2-D projection images of the breast, which is a 3-D body. Thus, the spatial arrangement of tissues cannot be preserved, causing loss of morphologic information [1]. In addition, superimposed projections of breast tissues, which are not spatially adjacent cause loss of the tissue interrelationship. Figure 1 shows two calcified masses, one malignant and one benign, with similar mammographic appearances. Enhancing the difference between these two types of masses can be an important diagnostic achievement. Methods for 3-D mammography can potentially be used for both early detection and diagnosis of breast cancer. In early detection, they may be suitable replacements for two-view mammography, since they can eliminate the superposition artifacts. However, they might not include as much breast tissue as the two views combined. The 3-D mammography can also be used as an adjuvant diagnostic technique, with a goal of reducing the number of breast biopsies. This paper describes the 3-D mammographic techniques, since the diagnostic quality of the x-ray based images is better than those obtained by other modalities. Other approaches to 3-D breast imaging include breast MRI, ultrasound-mammography, SPECT and PET based imaging, transillumination, elastography, electrical impedance imaging, etc. [2]. The primary concern in the 3-D mammography is the received dose. Dose for the screening should not exceed the dose currently used in mammography (approx. 2.3 mGy of mean glandular dose, per image) unless the sensitivity is greatly improved. This is why the full computed tomography (CT) reconstruction is not considered to be clinically acceptable, since in order to achieve resolution appropriate for early detection, CT requires large number of slices and high dose. Another concern is duration of the exam, due to the number of screening patients, and the amount of the tissue visualized. Imaging geometry must allow rapid imaging and include all glandular tissue. The 3-D mammography is based upon digital mammography and utilizes its advantages of separate optimization of image acquisition, reconstruction, visualization, and storing. Reconstruction methods include stereoscopy, tomosynthesis, and a modified CT algorithm with small number of views for dose reduction. Limited view reconstruction algorithms use reduced amount of image data, which is more appropriate for

Uncertainty relation in elastography: new approach to explain for errors of in vivo elastography

Abstract: Since its invention in 1991 by Ophir et al. ultrasonic strain imaging (elastography) has been investigated by many researchers, including theoretical analysis, in vitro studies and clinical experiments in vivo. However, the clinical relevance of the method concerning an improvement of diagnostic sensitivity and specifity of cancerous lesions has not been shown yet. We want to present a model that explains the loss of strain image quality in case of clinical examinations due to lateral displacements, which are not present in theoretical experiments and in vitro studies, respectively.

System response compensation for high-resolution ultrasound elastography

Abstract: This paper examines a phenomenon suspected of limiting the spatial resolution of ultrasound elastography, and pursues a technical development to reduce the error introduced by this phenomenon. Elastography determines tissue strain by mapping post-motion image data onto pre-motion image data. An error arises in this mapping due to the fact that spatial warping of image data implies a warping of both the tissue and the measurement system response. An approach to data processing is studied to compensate for this system response related error. Underlying principals are first examined in one dimension, followed by presentation of at three dimensional implementation, suitable for application to volumetric ultrasound data.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.