Placental Elastography reveal Viscoelastic Signatures ex-vivo with Single-Track Location Maximum a Posteriori Probability Spectroscopy

Viscoelasticity assessments in placenta is an emerging marker of fetal health. Non invasive bio-rheological measurements with shear waves have so far relied on ideal elastic assumptions for mapping material properties. However, shear waves in soft tissues undergo losses due to dispersion and attenuation while propagation. We present a framework of viscoelasticty reconstruction in frequency domain that incorporates both dispersion and attenuation of the propagating wave from its spatio-temporal profile. Such a framework, in contrast to conventional dispersion curve fitting, is able to exploit the full shear wave data set for rheological parameter mapping. Our shear wave excitation-detection scheme, known as Single Track Location Acoustic radiation Force Impulse (STL-ARFI), minimizes bias due to differential speckle at tracking zones, which enable creating a robust cross-spectral density estimate of the wave and aid in including absorption in dynamic elastography. We demonstrate feasibility of the technique on ex-vivo human placenta by creating quasi-local estimates of viscoleasticity over a broad bandwidth (10–350 Hz) mapped by a two time constant Generalized Maxwell Model (GMM). Furthermore, we demonstrate that our measurements are invariant when probed under different imaging configurations, reinforcing their robustness.