Equivalent-source acoustic holography for projecting measured ultrasound fields through complex media

Holographic projections of experimental ultrasound measurements generally use the angular spectrum method or Rayleigh integral, where the measured data is imposed as a Dirichlet boundary condition. In contrast, full-wave models, which can account for more complex wave behaviour, often use interior mass or velocity sources to introduce acoustic energy into the simulation. Here, a method to generate an equivalent interior source that reproduces the measurement data is proposed based on gradient-based optimisation. The equivalent-source can then be used with full-wave models (for example, the open-source k-Wave toolbox) to compute holographic projections through complex media including nonlinearity and heterogeneous material properties. Numerical and experimental results using both time-domain and continuous-wave sources are used to demonstrate the accuracy of the approach

Molecular insights into the effects of focused ultrasound mechanotherapy on lipid bilayers:Unlocking the keys to design effective treatments

Administration of focused ultrasounds (US) represents an attractive complement to classical therapies for a wide range of maladies, from cancer to neurological pathologies, as they are non-invasive, easily targeted, their dosage is easy to control, and they involve low risks. Different mechanisms have been proposed for their activity but the direct effect of their interaction with cell membranes is not well understood at the molecular level. This is in part due to the difficulty of designing experiments able to probe the required spatio-temporal resolutions. Here we use Molecular Dynamics (MD) simulations at two resolution levels and machine learning (ML) classification tools to shed light on the effects that focused US mechanotherapy methods have over a range of lipid bilayers. Our results indicate that the dynamic-structural response of the membrane models to the mechanical perturbations caused by the sound waves strongly depends on the lipid composition. The analyses performed on the MD trajectories contribute to a better understanding of the behavior of lipid membranes, and to open up a path for the rational design of new therapies for the long list of diseases characterized by specific lipid profiles of pathological membrane cells.</p

Experimental validation of k-Wave: Nonlinear wave propagation in layered, absorbing fluid media

Models of ultrasound propagation in biologically relevant media have applications in planning and verification of ultrasound therapies and computational dosimetry. To be effective, the models must be able to accurately predict both the spatial distribution and amplitude of the acoustic pressure. This requires that the models are validated in absolute terms, which for arbitrarily heterogeneous media should be performed by comparison with measurements of the acoustic field. In this study, simulations performed using the open-source k-Wave acoustics toolbox, with a measurement-based source definition, were quantitatively validated against measurements of acoustic pressure in water and layered absorbing fluid media. In water, the measured and simulated spatial peak pressures agreed to within 3% under linear conditions and 7% under non-linear conditions. After propagation through a planar or wedge shaped glycerol-filled phantom, the difference in spatial peak pressure was 8.5% and 10.7%, respectively. These differences are within or close to the expected uncertainty of the acoustic pressure measurement. The -6 dB width and length of the focus agreed to within 4% in all cases, and the focal positions were within 0.7 mm for the planar phantom and 1.2 mm for the wedge shaped phantom. These results demonstrate that when the acoustic medium properties and geometry are well known, accurate quantitative predictions of the acoustic field can be made using k-Wave

Mesh adaptation for pseudospectral ultrasound simulations

High-intensity focussed ultrasound (HIFU) is an emerging cancer therapy that holds great promise, as it is minimially invasive, requires no ionising radiation, and can treat small volumes precisely. However, currently therapies are hindered by an inadequate capacity for treatment planning, as the interactions between the sound waves and tissue are complex and difficult to simulate. The Fourier pseudospectral method is one way of efficiently performing these simulations, as it can provide high accuracies with low computational costs. However, it is typically used with uniform computational meshes, wasting resolution in regions of the simulation where only low frequencies are present, and typically under-resolving the acoustic field in the focal region. This thesis addresses this problem in two ways: First, a bandwidth-based measure of the spatial resolution requirements for a model solution is developed and integrated into a moving mesh method. This allows spatially and temporally-varying resolution requirements to be met. Bandwidth-based meshes are shown to perform very well when compared with current mesh adaptation approaches. Second, a technique is presented for discretising arbitrary acoustic source distributions that does not rely on the source's region of support coinciding with the mesh. This not only allows sources to be represented with adaptive meshes, but greatly improves the accuracy of source discretisations for uniform meshes as well. These two contributions are of vital importance in the context of HIFU simulation, and can easily be applied to the many other problems for which the Fourier pseudospectral method is used