● Part I : Science and Technology ELASTOGRAPHIC IMAGING

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 of in 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: [email protected]. 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