Radiation force imaging of viscoelastic properties with reduced artifacts

It is well-known that changes in the mechanical properties of tissues are correlated with the presence of disease. In the eye, for example, the vitreous body undergoes dramatic changes in mechanical properties during age-related degradation. These changes may play a significant role in the formation of retinal detachment or other vitreoretinal diseases. We previously presented a noninvasive method called kinetic acoustic vitreoretial examination (KAVE), which may be used to detect these mechanical changes. KAVE uses acoustic radiation force as a means to produce small, localized displacements within the tissues. Returning echoes are processed using ultrasonic motion tracking so that the response of the tissue to the induced force can be evaluated. By repeating this process at a number of locations, images depicting viscoelastic properties of tissues can be formed. Through the combination of appropriate mechanical modeling and signal processing, we are able to generate images of parameters such as relative mass, relative elasticity, and relative viscosity. These parameters are called relative because they depend on the force applied, which is typically unknown. In this paper, we present new force-free images depicting the time constant tau, the damping ratio xi, and the natural frequency omega of the phantom material. These images are significant in that they lack the artifacts common in the relative property images. Experiments were conducted on a set of three acrylamide-based phantoms with varying gel concentrations. We present images depicting B-mode echogenicity, maximum radiation force-induced displacement, relative material parameters, and force-free characteristics of the series of phantoms. The presented force-free images depict mechanical properties without artifacts from local force variation due to acoustic reflection, refraction, and attenuation. Force-free images should prove particularly useful for in vivo imaging through inhomogeneous tissues.