Force-controlled ultrasound robot for consistent tissue pre-loading: Implications for acoustic radiation force elasticity imaging

Acoustic radiation force (ARF)-based measurements of tissue elasticity require transmission of an acoustic pulse and ultrasound image-based tracking of the resulting tissue displacements. This technique provides diagnostic information about various disease states of tissue. One limitation, however, is the dependency on applied probe pressure, which is difficult to control manually and prohibits standardization of quantitative measurements. To overcome this limitation, we introduce a custom-built robot that controls probe contact forces. The robot was evaluated in an in vivo canine prostate and ex vivo bovine liver. Markers implanted in the prostate were visualized with 3D probe contact forces (i.e. tissue pre-loading) that ranged 10-11 N. The resulting displacement of the markers were evaluated to estimate the variability in pre-loading strains that could exist prior to making an ARF-based elasticity measurement. One standard deviation of corresponding strains ranged 0-2%. In the ex vivo liver, differences in speckle-tracked tissue displacements were observed when the probe sustained tissue contact as it returned to its initial position, indicating that there is a potential benefit in losing tissue contact prior to taking measurements that will be used for standardization (e.g. to avoid differences in pre-loading and corresponding tissue elasticity). Results are promising for the introduction of robotic systems to control the applied probe pressure for ARF-based measurements of tissue elasticity.