Ultrasound and optical methods for dynamic viscoelastic imaging

This chapter summarizes the approaches to tissue elasticity imaging based on ultrasonic and/or optical modalities that have been developed during the last three decades. We describe a wide range of techniques, from simply mapping soft-tissue stiffness to accurately estimating a complex modulus of viscoelastic tissues, in vivo. We describe how experimental parameters determine the spatiotemporal properties of the force that then couple to the properties of the imaging system used to track deformations and to specific tissue geometries. These experimental-design parameters determine the resolution and noise properties of the image, and they can also be set to select which elements of tissue structure contribute to the object contrast. Parameter selection depends on measurement goals, which might include anything from a basic-science investigation into mechanobiology of disease processes to diagnosis of focal diseases in a patient. Although there are many coupled parameters to consider, they provide a broad pallet of opportunities for discovery and diagnosis related to changes in the mechanical properties of tissues.