Wall influence on dynamics of a microbubble

The nonlinear dynamic behaviour of microscopic bubbles near a wall is investigated. The Keller-Miksis-Parlitz equation is adopted, but modified to account for the presence of the wall. This base model describes the time evolution of the bubble surface, which is assumed to remain spherical, and accounts for the effect of acoustic radiation losses owing to liquid compressibility in the momentum conservation. Two situations are considered: the base case of an isolated bubble in an unbounded medium; and a bubble near a solid wall. In the latter case, the wall influence is modeled by including a symmetrically oscillating image bubble. The bubble dynamics is traced using a numerical solution of the model equation. Subsequently, Floquet theory is used to accurately detect the bifurcation point where bubble oscillations stop following the driving ultrasound frequency and undergo period-changing bifurcations. Of particular interest is the detection of the subcritical period tripling and quadrupling transition. The parametric bifurcation maps are obtained as functions of non-dimensional parameters representing the bubble radius, the frequency and pressure amplitude of the driving ultrasound field and the distance from the wall. It is shown that the presence of the wall generally stabilises the bubble dynamics, so that much larger values of the pressure amplitude are needed to generate nonlinear responses.Comment: 25 pages, 14 figure

A biomechanical analysis of shear wave elastography in pediatric heart models

Early detection of cardiac disease in children is essential to optimize treatment and follow-up, but also to reduce its associated mortality and morbidity. Various cardiac imaging modalities are available for the cardiologist, mainly providing information on tissue morphology and structure with high temporal and/or spatial resolution. However, none of these imaging methods is able to directly measure stresses or intrinsic mechanical properties of the heart, which are potential key diagnostic markers to distinguish between normal and abnormal physiology. This thesis investigates the potential of a relatively new ultrasound-based technique, called shear wave elastography (SWE), to non-invasively measure myocardial stiffness. The technique generates an internal perturbation inside the tissue of interest, and consequently measures the propagation of the acoustically excited shear wave, of which the propagation speed is directly related to tissue stiffness. This allows SWE to identify regions with higher stiffness, which is associated with pathology. SWE has shown to be successful in detecting tumors in breast tissue and fibrosis in liver tissue, however application of SWE to the heart is more challenging due to the complex mechanical and structural properties of the heart. This thesis provides insights into the acoustically excited shear wave physics in the myocardium by using computer simulations in combination with experiments. Furthermore, these models also allow to assess the performance of currently used SWE-based material characterization algorithms

Elastography: modality-specific approaches, clinical applications, and research horizons

Manual palpation has been used for centuries to provide a relative indication of tissue health and disease. Engineers have sought to make these assessments increasingly quantitative and accessible within daily clinical practice. Since many of the developed techniques involve image-based quantification of tissue deformation in response to an applied force (i.e., "elastography”), such approaches fall squarely within the domain of the radiologist. While commercial elastography analysis software is becoming increasingly available for clinical use, the internal workings of these packages often remain a "black box,” with limited guidance on how to usefully apply the methods toward a meaningful diagnosis. The purpose of the present review article is to introduce some important approaches to elastography that have been developed for the most widely used clinical imaging modalities (e.g., ultrasound, MRI), to provide a basic sense of the underlying physical principles, and to discuss both current and potential (musculoskeletal) applications. The article also seeks to provide a perspective on emerging approaches that are rapidly developing in the research laboratory (e.g., optical coherence tomography, fibered confocal microscopy), and which may eventually gain a clinical foothol