Raffaella Righetti

Bio: Raffaella Righetti is an academic researcher from Texas A&M University. The author has contributed to research in topics: Elastography & Poromechanics. The author has an hindex of 20, co-authored 75 publications receiving 2097 citations. Previous affiliations of Raffaella Righetti include Houston Methodist Hospital & Texas A&M University System.

Elastography: Imaging the elastic properties of soft tissues with ultrasound

Abstract: 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 characterization of HIFU-induced lesions in canine livers.

Abstract: The elastographic visualization and evaluation of high-intensity focused ultrasound (HIFU)-induced lesions were investigated. The lesions were induced in vitro in freshly excised canine livers. The use of different treatment intensity levels and exposure times resulted in lesions of different sizes. Each lesion was clearly depicted by the corresponding elastogram as being an area harder than the background. The strain contrast of the lesion/background was found to be dependent on the level of energy deposition. A lesion/background strain contrast between 22.5 dB and 23.5 dB was found to completely define the entire zone of tissue damage. The area of tissue damage was automatically estimated from the elastograms by evaluating the number of pixels enclosed inside the isointensity contour lines corresponding to a strain contrast of 22.5, 23 and 23.5 dB. The area of the lesion was measured from a tissue photograph obtained at approximately the same plane where elastographic data were collected. The estimated lesion areas ranged between approximately 10 mm 2 and 110 mm 2 . A high correlation between the damaged areas as depicted by the elastograms and the corresponding areas as measured from the gross pathology photographs was found (r 2 5 0.93, p value < 0.0004, n 5 16). This statistically significant high correlation demonstrates that elastography has the potential to become a reliable and accurate modality for HIFU therapy monitoring. © 1999 World Federation for Ultrasound in Medicine & Biology.

The Future of Biomedical UltrasoundElastographic imaging

Abstract: The mechanical attributes of soft tissues depend on their molecular building blocks (fat, collagen etc.), on the microscopic and macroscopic structural organization of these blocks (Fung 1981), and on the boundary conditions involved. These mechanical attributes may include the shear or elastic moduli (Young’s modulus), the Poisson’s ratio, or any of the longitudinal or shear strains that occur in tissues as a response to an applied load. In the normal breast, for example, the glandular structure may be firmer than the surrounding fibrous connective tissue, which in turn is firmer than the subcutaneous adipose tissue. Pathological changes are generally correlated with changes in tissue stiffness as well. Many cancers, such as scirrhous carcinomas of the breast, seem much stiffer and less mobile than benign (fibroadenoma) tumors (Anderson 1953). In many cases, in spite of the difference in stiffness or mobility, the small size of a pathological lesion and/or its location deep in the body impede its detection and/or evaluation by palpation. Moreover, lesions may or may not possess echogenic attributes that make them ultrasonically detectable. Because the echogenicity and the mechanical attributes of tissue are generally uncorrelated, it is expected that imaging some of the latter will provide new information that is related to tissue structure and/or pathology. For example, tumors of the prostate or the breast may be invisible or barely visible in standard ultrasound examinations, yet are much stiffer than the embedding tissue. Diffuse diseases such as cirrhosis of the liver are known to increase the stiffness of the liver tissue significantly (Anderson 1953), yet they may seem normal in conventional ultrasound examination. A clear understanding of tissue stress/strain relationships is necessary for the interpretation of any of these imaged mechanical attributes. Tissue may exhibit viscoelastic and poroelastic behavior such as hysteresis, fluid flow, stress relaxation and creep (Fung 1981). When all these factors are combined, it is evident that describing the mechanical behavior of tissue mathematically requires considerable simplification, if the model is to be useful in a real-time or nearly real-time situation. To a first approximation, most soft tissues have been assumed to be isotropic on the scale of interest (Krouskop et al. 1987; Parker et al. 1990; Sarvazyan et al. 1991), although there is evidence of anisotropic ultrasonic and mechanical attributes in some soft tissues such as muscle (Levinson 1987). Even for relatively small strains (less than 10%), tissue may exhibit nonlinear viscoelastic behavior (Krouskop et al. 1998; Parker et al. 1990). Thus, the mechanical attributes of tissue are often better defined if they are specified over the strain or stress ranges of interest in a specific application (Krouskop et al. 1987; Levinson 1987; Parker et al. 1990). Quantitative measurements of tissue mechanical parameters reported in the past show a wide range of values (Fung 1981; Parker et al. 1990). Most of the research has been done for tissues that undergo tensile loading (muscles, arteries, lung, tendons, bone, skin, ureter). In contrast, very little quantitative information has been collected on the compressive attributes of the tissue in organs. A limited set ofin vitro measurements of the elastic moduli of prostate and liver tissues was described by Parker et al. (1990). In a presentation by Sarvazyan (1993), shear modulus measurements indicated that normal breast tissue is approximately 4 times less stiff than fibroadenoma. Breast cancer showed a wide range of shear moduli that can be up to 7 times higher than those of normal tissue. Walz et al. (1993) presented results of Address correspondence to: Dr. Jonathan Ophir, Ultrasonics Laboratory, Radiology Department, The University of Texas, Medical School, 6431 Fannin Street, Suite 6.168, Houston, TX 77030, USA. E-mail: jonathan.ophir@uth.tmc.edu. Ultrasound in Med. & Biol., Vol. 26, Supplement 1, pp. S23–S29, 2000 Copyright © 2000 World Federation for Ultrasound in Medicine & Biology Printed in the USA. All rights reserved 0301-5629/00/$–see front matter

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.

The feasibility of elastographic visualization of HIFU-induced thermal lesions in soft tissues

Abstract: The potential for visualizing high-intensity focused ultrasound (HIFU)-induced thermal lesions in biological soft tissues in vitro using elastography was investigated. Thermal lesions were created in rabbit paraspinal skeletal muscle in vivo. The rabbits were sacrificed 60 h following the treatment and lesioned tissues were excised. The tissues were cast in a block of clear gel and elastographic images of the lesions were acquired. Gross pathology of the tissue samples confirmed the characteristics of the lesions.

Introduction to the Finite Element Method

Abstract: This chapter introduces the finite element method (FEM) as a tool for solution of classical electromagnetic problems. Although we discuss the main points in the application of the finite element method to electromagnetic design, including formulation and implementation, those who seek deeper understanding of the finite element method should consult some of the works listed in the bibliography section.

In Vivo Endoscopic Optical Biopsy with Optical Coherence Tomography

Abstract: Current medical imaging technologies allow visualization of tissue anatomy in the human body at resolutions ranging from 100 micrometers to 1 millimeter. These technologies are generally not sensitive enough to detect early-stage tissue abnormalities associated with diseases such as cancer and atherosclerosis, which require micrometer-scale resolution. Here, optical coherence tomography was adapted to allow high-speed visualization of tissue in a living animal with a catheter-endoscope 1 millimeter in diameter. This method, referred to as "optical biopsy," was used to obtain cross-sectional images of the rabbit gastrointestinal and respiratory tracts at 10-micrometer resolution.

High-intensity focused ultrasound in the treatment of solid tumours.

Abstract: Traditionally, surgery has been the only cure for many solid tumours. Technological advances have catalysed a shift from open surgery towards less invasive techniques. Laparoscopic surgery and minimally invasive techniques continue to evolve, but for decades high-intensity focused ultrasound has promised to deliver the ultimate objective - truly non-invasive tumour ablation. Only now, however, with recent improvements in imaging, has this objective finally emerged as a real clinical possibility.

EFSUMB Guidelines and Recommendations on the Clinical Use of Ultrasound Elastography. Part 1: Basic Principles and Technology

Abstract: The technical part of these Guidelines and Recommendations, produced under the auspices of EFSUMB, provides an introduction to the physical principles and technology on which all forms of current commercially available ultrasound elastography are based. A difference in shear modulus is the common underlying physical mechanism that provides tissue contrast in all elastograms. The relationship between the alternative technologies is considered in terms of the method used to take advantage of this. The practical advantages and disadvantages associated with each of the techniques are described, and guidance is provided on optimisation of scanning technique, image display, image interpretation and some of the known image artefacts.