A Review of Theoretical and Experimental Aspects of Imaging the Elastic Attributes of Tissue In Vivo

Abstract

3 The literature describing the gross mechanical properties of tissues is primarily concerned with muscle (skeletal and cardiac) as well as with blood vessel walls [1]. Relatively little has been written about the mechanical properties of other normal and pathological tissues. Pathological changes are generally correlated with changes in tissue elastic modulus; in fact, the standard medical practice of soft tissue palpation is based on qualitative, low-resolution assessment of the static elastic modulus of tissue. Many cancers, such as scirrhous carcinoma of the breast, appear as extremely hard nodules [2]. In quite a few cases, despite the difference in elastic modulus, the small size of a pathological lesion and/or its location deep in the body precludes its detection and evaluation by palpation. In general, the lesion may or may not possess echogenic properties that could make it ultrasonically detectable. For example, tumors of the prostate or the breast could be invisible in standard ultrasound examinations, yet be much harder than the embedding tissue. Diffuse diseases such as cirrhosis of the liver are known to significantly reduce the elastic modulus of the liver tissue as a whole [2], yet they may appear normal in conventional ultrasound examination. Because the ultrasonic echogenicity and the elasticity of tissue are generally uncorrelated, it is expected that imaging tissue elastic modulus will provide new information that is related to tissue structure and/or pathology (Fig. 1). Biological tissues can be considered as approximating homogeneous gels [3]. Different modes of elastic wave propagation in such media are determined primarily by their bulk (K) and shear (G) elastic moduli. In biological soft tissues, the value of K far exceeds that of G. The bulk properties (and hence the ultrasonic properties) are determined by the molecular composition of the tissue [1], whereas the shear properties are determined by the higher level of tissue organization [3]. Because deformable soft tissues are essentially volume incompressible (i.e., Poisson ratio, ~0.5), their shear moduli are proportional to their longitudinal (Young’s) moduli [4]. It follows that estimation and imaging of the Young’s moduli of tissue should in principle convey information about their shear properties, and hence about the higher level of tissue organization.