Ultrasonic measurements of breast viscoelasticity

In vivo measurements of the viscoelastic properties of breast tissue are described. Ultrasonic echo frames were recorded from volunteers at 5 fps while applying a uniaxial compressive force ( 1 – 20 N ) within a 1 s ramp time and holding the force constant for up to 200 s . A time series of strain images was formed from the echo data, spatially averaged viscous creep curves were computed, and viscoelastic strain parameters were estimated by fitting creep curves to a second‐order Voigt model. The useful strain bandwidth from this quasi‐static ramp stimulus was10 − 2 ⩽ ω ⩽ 10 0rad ∕ s( 0.0016 – 0.16 Hz ) . The stress‐strain curves for normal glandular tissues are linear when the surface force applied is between 2 and 5 N . In this range, the creep response was characteristic of biphasic viscoelastic polymers, settling to a constant strain (arrheodictic) after 100 s . The average model‐based retardance time constants for the viscoelastic response were 3.2 ± 0.8 and 42.0 ± 28 s . Also, the viscoelastic strain amplitude was approximately equal to that of the elastic strain. Above 5 N of applied force, however, the response of glandular tissue became increasingly nonlinear and rheodictic, i.e., tissue creep never reached a plateau. Contrasting in vivo breast measurements with those in gelatin hydrogels, preliminary ideas regarding the mechanisms for viscoelastic contrast are emerging.