Beyond Linear Elasticity Imaging with Acoustic Radiation Force Ultrasound Shearwave Elastography

Introduction: : Acoustic Radiation Force (ARF) Ultrasound Shearwave Elastography Imaging refers to mapping spatial variations of stiffness within tissues non-invasively using diagnostic ultrasonic imaging. However, conventional elastography scans are limited to the linear elasticity assumption and account only for variations in the linear shear modulus of the medium.
As shown by several studies the viscoelastic modulus, however, is a better representative parameter of the mechanical behavior of tissue and therefore must be accounted in elastography estimators. The viscoelastic behavior of tissue is represented by its Storage (μ_S) and Loss (μ_L) modulus, which constitute the overall complex viscoelastic modulus (μ^*).
In this work we report a viscoelastic estimator of ultrasound elastography imaging using a Planewave Single Track Location (p-STL) sequence to interrogate and track shear waves in a medium. Our approach uses two spatially offset ARF pushes and tracks the wave at several locations within the imaging field-of-view to produce spatial parametric images of variations of tissue’s complex modulus. The viscoelastic modulus maps are resolved over multiple reconstruction frequencies, as opposed to a single estimate of the viscoelasticity that is averaged over a region-of-interest.

Materials and

Methods: : Planewave Single Track Location (p-STL) complex modulus imaging is performed on several ex-vivo organs (rabbit kidney, liver, and brain), full-term, post-delivery human placenta. Ultrasound-derived values are compared with measurements from rotating-disk rheometer. We also report in-vivo measurements on a cohort of healthy (C57BL/6J) and obese mice (B6.Cg-Lepob/J) (n=5).
Elasticity scans were performed with a Vantage 64LE ultrasound system. Tissue displacement through time was measured via high frame rate imaging. Shear wave dispersion and attenuation were estimated from the cross-power spectral density (CPSD) measurement of the spatiotemporal shear wavefield generated by two ARF pushes for each spatial displacement location. Spectroscopic estimation and image reconstructions were performed pixel-wise to produce shear wave dispersion and attenuation maps resolved over 50-1000 Hz. We report the Magnitude Squared Coherence (MSC) estimate to assess the measurement reliability of the tracked shearwave dataset and provide insights into the time-dependent mechanical behavior of interrogated tissues observed over a few milliseconds.

Results, Conclusions, and Discussions:: Figure 1 shows representative multi-frequency shear modulus parametric images acquired in-vivo for an obese (B6.Cg-Lep OB/J) mouse. The top panel in both plots indicates the elastic modulus, the right panel displays storage, the middle panel shows loss, and the magnitude of the dynamic modulus is followed by the magnitude-squared coherence image at several different frequencies (100, 200, and 500 Hz respectively). The average dynamic modulus value is also plotted for a few ROIs across the frequency range.
We compared the viscoelastic moduli across different frequency components for two groups as shown in Figure 2. We observed that across lower frequencies (50–400 Hz), the obese group exhibits significantly different modulus values than the healthy group, suggesting increased tissue stiffness.
However, at higher frequencies (≥ 700 Hz), the differences are statistically not significant, implying that the viscoelastic modulus estimated from high-frequency bands is prone to noise in the reconstructions which leads to lower statistical variations across groups.
In contrast to previously described methods that produce a single estimate for shear wave spectral parameters over an extended ROI, we have demonstrated pixel-wise shear wave speed and attenuation imaging using an p-STL spectral reconstruction approach to reconstruct the viscoelastic moduli obtained from shearwave dispersion and attenuation measurements. Importantly, this method reveals tissue contrast not visible in images of linear shear elastic modulus derived from the shear wave group speed.

Acknowledgements and/or References (Optional)::