Subresolution Displacements in Finite Difference Simulations of Ultrasound Propagation and Imaging

Pinton, Gianmarco

doi:10.1109/tuffc.2016.2638801

Cited by 6 publications

(8 citation statements)

References 15 publications

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“…These shear excitations result in inter-frame deformations that change the relative scatterer positions and re-duce the correlation between frames. The tracking algorithm uses a quality weighted three dimensional median filter to iteratively optimize the correlation values and it was calibrated to preserve the high frequency displacements necessary to characterize the sharp shock front (see appendix C for validation using simulated ultrasound imaging [31,32] and Rusanov solutions of cubically nonlinear shocks).Three snapshots of the resulting high frame-rate movie of particle velocity [25b] are shown in Fig. 2a for the case where the amplitude of the particle velocity at the plate was v 0 = 1m/s which produced a Mach number of M = c T /v 0 = 0.49.…”

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Shear Shock Waves Observed in the Brain

2017

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The internal deformation of the brain is far more complex than the rigid motion of the skull. An ultrasound imaging technique that we have developed has a combination of penetration, frame-rate, and motion detection accuracy required to directly observe, for the first time, the formation and evolution of shear shock waves in the brain. Experiments at low impacts on the traumatic brain injury scale demonstrate that they are spontaneously generated and propagate within the porcine brain. Compared to the initially smooth impact, the acceleration at the shock front is amplified up to a factor of 8.5. This highly localized increase in acceleration suggests that shear shock waves are a fundamental mechanism for traumatic injuries in soft tissue.PACS numbers: Valid PACS appear here Traumatic brain injuries (TBI's) are a major source of death and disability worldwide. Falls and motor vehicle related accidents are the largest contributors. Current biomechanical predictive criteria for TBI are based on measurements of skull motion such as linear and rotational acceleration [1,2]. Although the relationship between skull motion and injury has been extensively tested [2][3][4][5][6][7], mechanisms relating the two have not been conclusively established [8], due to the complexity of the deformation of the brain [9][10][11] which behaves as a nonlinear viscoelastic medium. In situ measurements of the rapid transient motion of the whole brain during a traumatic event may establish a more accurate biomechanical description of injury.Shear vibration experiments on small brain samples have shown that the stress-strain relationship behaves nonlinearly for amplitudes as low as 1% [12,13], which is well below the strain threshold for injury [7,14,15]. Therefore, even mild injuries typically occur within the nonlinear elastic regime. Furthermore, the brain's shear wave speed (c t ∼2 m/s) is three orders of magnitude smaller than the compressional wave speed (c p ∼1500 m/s) therefore the deformation from an impact is almost entirely in shear mode.Outside of ultrasound, current methods that measure brain motion cannot capture this nonlinear behavior of shear waves due to limitations in either, frame rate, penetration, or motion detection accuracy. For instance, magnetic resonance elastography (MRE) has been used to investigate shear wave propagation in the brain, typically at frame rates on the order of tens of images/second. Furthermore the temporal sampling of MRE is fundamentally limited to a few ms by the finite integration time over the spin relaxation [16,17]. The harmonics generated by nonlinear shear wave propagation exceed these sampling capabilities. Optical methods have larger frame rates and they can measure motion in optically trans- * david.espindola@unc.edu † gfp@unc.edu parent materials [18][19][20]. However, they are limited to shallow penetration depths in soft tissue (∼ 2 mm) [21]. The only experimental corroboration of nonlinear shear waves and its characteristic harmonic signature was made over a decade ago in a homo...

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Shear Shock Waves Observed in the Brain

2017

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“…A previously described impedance flow method [23] was generalized to represent subresolution motion on finite grids and so that the motion of the brain during shear shock wave propagation can be described within ultrasound imaging simulations. The generalization enables implementing displacement in an arbitrary distribution of impedance values, without the restriction of assuming a two-pixel scatterer model.…”

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confidence: 99%

“…The motion generated by a shear wave, e.g. in elastography, can be accurately modeled by finite difference simulations [23]. Displacements that were much smaller than the grid size (up to λ/6000) have been previously modeled in Fullwave simulations, using an impedance flow method [23].…”

Section: B Incorporation Of Brain Motion For Fdtd Simulationsmentioning

confidence: 99%

“…in elastography, can be accurately modeled by finite difference simulations [23]. Displacements that were much smaller than the grid size (up to λ/6000) have been previously modeled in Fullwave simulations, using an impedance flow method [23]. This method consist ed of generating scatterers in the acoustic simulation field, with each scatterer being composed of two spatial pixels.…”

Section: B Incorporation Of Brain Motion For Fdtd Simulationsmentioning

confidence: 99%

“…5(c) and (d) were obtained by displacing the brain map shown in Fig. 3(c-d) by 0.5 pixel and 0.7 pixel respectively, by the scatterer-based impedance flow method as implemented in homogenous maps in [23]. By assuming each scatterer to be composed of two pixels, impedance is transferred between the constituent pixels to describe motion, while keeping the total scatterer impedance as conserved.…”

Section: Subresolution Displacement In the Heterogeneous Tissue Map U...mentioning

confidence: 99%

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Modeling ultrasound propagation in the moving brain: applications to shear shock waves and traumatic brain injury

Chandrasekaran¹,

Tripathi²,

Pinton³

2020

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Traumatic Brain Injury (TBI) studies on the living human brain are experimentally infeasible due to ethical reasons and the elastic properties of the brain degrade rapidly postmortem. We present a simulation approach that models ultrasound propagation in the human brain while it is moving due to the complex shear shock wave deformation from a traumatic impact. Finite difference simulations can model ultrasound propagation in complex media such as human tissue. Recently, we have shown that the Fullwave finite difference approach can also be used to represent displacements that are much smaller than the grid size, such as the motion encountered in shear wave propagation from ultrasound elastography. However, this subresolution displacement model, called impedance flow, was only implemented and validated for acoustical media composed of randomly distributed scatterers. Here we propose a generalization of the impedance flow method that describes the continuous subresolution motion of structured acoustical maps, and in particular of acoustical maps of the human brain. It is shown that the average error in the subresolution displacement method is small compared to the wavelength (λ/1702). The method is then applied to acoustical maps of the human brain with a motion that is imposed by the propagation of a shear shock wave. This motion is determined numerically with a custom piecewise parabolic method that is calibrated to ex vivo observations of shear shocks in the porcine brain. Then the Fullwave simulation tool is used to model transmit-receive imaging sequences based on an L7-4 imaging transducer. The simulated radiofrequency data is beamformed using a conventional delay-and-sum method and a normalized cross-correlation method designed for shock wave tracking is used to determine the tissue motion. This overall process is an in silico reproduction of the experiments that were previously performed to observe shear shock waves in fresh porcine brain. It is shown that the proposed generalized impedance flow method accurately captures the shear wave motion in terms of the wave profile, shock front characteristics, odd harmonic spectrum generation, and acceleration at the shear shock front. We expect that this approach will lead to improvements in image sequence design that takes into account the aberration and multiple reflections from the brain and in the design of tracking algorithms that can more accurately capture the complex brain motion that occurs during a traumatic impact. These methods of modeling ultrasound propagation in moving media can also be applied B.B. Tripathi, D. Espíndola and G. Pinton are with the

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