Subresolution Displacements and Shear Shock Wave Tracking in the Human Brain

Highly realistic finite difference simulations of acoustical wave propagation can be used to describe ultrasound imaging in soft tissue. They have recently been shown to also model the backscattering physics from the subresolution motion of distributed scatterer fields. Here, we present a generalized impedance flow method that models displacements in heterogeneous tissue maps, specifically the human brain. The proposed method was used to i) model tissue motion due to shear shock wave propagation in the brain, which we hypothesize is a primary mechanism for traumatic brain injuries ii) obtain the backscattered signal from an imaging pulse, and iii) estimate the imposed displacements from the simulated RF data. The anatomical fidelity of the tissue maps combined with the high accuracy of the displacement model facilitate the validation of displacement tracking algorithms and the design of imaging sequences that can detect the odd harmonic spectral signature of shear shocks. An independent simulation tool based on a piecewise parabolic method was used to numerically calculate shear shock wave displacements in the brain medium. These displacements were then imposed by the proposed impedance flow method by varying the impedance of scatterers and tissue interfaces. The resulting phase shift was measured from the beamformed RF data using a custom correlation-based tracking algorithm. The average error in the displacement tracking was small compared to the wavelength λ/619. This method, validated here for a challenging nearly-discontinuous velocity model, can be applied to less complex motion, such as linear shear waves generated for shear wave elasticity imaging.