ShearWaveTM Elastography

2 One of the key medical methods for detection and characterization of pathologies is the assessment of tissue stiffness by palpation. The importance of clinical assessment of tissue stiffness has been known since ancient times: In Pharonic Egypt, more than 5000 years ago, physicians practiced palpation of body parts to determine tissue stiffness. They knew that a hard mass within an organ was often a sign of an abnormality. Since then, palpation has been used for screening and diagnosis, but is also used during interventional procedures to guide the surgeon to the area of pathology. Today, a new diagnostic imaging modality has emerged, called elastography, which uses ultrasound to assess tissue differences in Tissue stiffness is generally measured by a physical quantity called Young's modulus and expressed in pressure units-Pascals, or more commonly kilo Pascals (kPa). Young's modulus can be defined through a simple experiment illustrated in Figure 1. An external uniform compression (or stress S) is applied to a solid tissue and this induces a deformation, or strain (e) inside the tissue. As one might expect, the induced strain is smaller in harder tissue than in softer tissue. The relationship between stress and strain is expressed in the equation (1) below. The Young's modulus is defined simply as the ratio between the applied stress and the induced strain: Young's modulus, or elasticity E, quantifies tissue stiffness. Simply put, hard tissues have a higher Young's modulus than soft ones. Typical values of elasticity in different tissue types have been