New Absorbing Boundary Conditions for the Finite-Difference Modeling of Elastic Waves

The successful development of an effective NDE technique depends on a full understanding of the physics of ultrasound interaction with the test material. This can be best achieved through numerical simulation of the wave propagation and scattering in the test material. A good numerical model can supplement the information provided by laboratory measurements on manufactured faulty bonds. Numerical models provide controllable quantitative information, subject to their own limits on accuracy, but free from the uncertainties attached to many practical ultrasonic measurements

Numerical Model of Elastic Wave Interactions with a Diffusion Bond

Solid state bonds are being extensively developed and used in aerospace structures. As with any other bonding technology, a reliable method for estimating bond strength is needed in order to establish confidence levels in the process routes and the finished bonded products. Ultrasonic nondestructive evaluation (NDE) methods are currently being investigated for detection and characterisation of anomalies at diffusion bonds between similar or dissimilar metallic layers [1], There is large gap in the understanding of the nature and behaviour of imperfect bonds. These are diffusion bonds which do not show up as good reflectors of ultrasonic energy, but at the same time have very low bond strengths. A purely experimental approach to developing an NDE method is often hampered by the difficulties in fabricating samples with controlled defects, and other experimental uncertainties. The objectives of this study are thus to develop a computer model to determine whether ultrasound could be used for characterisation of imperfections at a diffusion bond, and what would be the optimum set-up for obtaining the most reliable information about the condition of the bond line

Image processing techniques for improved porosity estimation

The work reported here is on the first phase of the development of an automated image analysis package. It is intended that the initial application should be to the problem of porosity estimation in powder metals. Although a great deal of work has been concentrated recently on using ultrasonic velocity changes and attenuation for porosity estimation, the problem can also be addressed through acoustical imaging

Wave Propagation in a Fractional Viscoelastic Tissue Model: Application to Transluminal Procedures

In this article, a wave propagation model is presented as the first step in the development of a new type of transluminal procedure for performing elastography. Elastography is a medical imaging modality for mapping the elastic properties of soft tissue. The wave propagation model is based on a Kelvin Voigt Fractional Derivative (KVFD) viscoelastic wave equation, and is numerically solved using a Finite Difference Time Domain (FDTD) method. Fractional rheological models, such as the KVFD, are particularly well suited to model the viscoelastic response of soft tissue in elastography. The transluminal procedure is based on the transmission and detection of shear waves through the luminal wall. Shear waves travelling through the tissue are perturbed after encountering areas of altered elasticity. These perturbations carry information of medical interest that can be extracted by solving the inverse problem. Scattering from prostate tumours is used as an example application to test the model. In silico results demonstrate that shear waves are satisfactorily transmitted through the luminal wall and that echoes, coming from reflected energy at the edges of an area of altered elasticity, which are feasibly detectable by using the transluminal approach. The model here presented provides a useful tool to establish the feasibility of transluminal procedures based on wave propagation and its interaction with the mechanical properties of the tissue outside the lumen.University College London, United KingdomTalentia scholarship (grant C2012H-75146405T-1) from the regional government of Andalusia, Spainthe Ministry of Education and Science, Spain, grants DPI2017-83859-R, EQC2018-004508-P and UNGR15-CE3664Andalusia, Spain, grants SOMM17/6109/UGR, B-TEP-026-UGR18, IE2017-5537 and P18-RT-165

Reverse Time Migration and Genetic Algorithms combined for reconstruction in transluminal shear wave elastography: An in silico case study

A new reconstruction approach that combines Reverse Time Migration (RTM) and Genetic Algorithms (GAs) is proposed for solving the inverse problem associated with transluminal shear wave elastography. The transurethral identification of the first thermal lesion generated by transrectal High Intensity Focused Ultrasound (HIFU) for the treatment of prostate cancer, was used to preliminarily test in silico the combined reconstruction method. The RTM method was optimised by comparing reconstruction images from several cross-correlation techniques, including a new proposed one, and different device configurations in terms of the number and arrangement of emitters and receivers of the conceptual transurethral probe. The best results were obtained for the new proposed cross-correlation method and a device configuration with 3 emitters and 32 receivers. The RTM reconstructions did not completely contour the shape of the HIFU lesion, however, as planned for the combined approach, the areas in the RTM images with high level of correlation were used to narrow down the search space in the GA-based technique. The GA-based technique was set to find the location of the HIFU lesion and the increment in stiffness and viscosity due to thermal damage. Overall, the combined approach achieves lower level of error in the reconstructed values, and in a shorter computational time, compared to the GA-based technique alone. The lowest errors were accomplished for the location of HIFU lesion, followed by the contrast ratio of stiffness between thermally treated tissue and non-treated normal tissue. The homologous ratio of viscosity obtained higher level of error. Further investigation considering diverse scenarios to be reconstructed and with experimental data is required to fully evaluate the feasibility of the combined approach.Talentia scholarship C2012H- 75146405T-1 from the regional government of AndalusiaMinistry of Science and Innovation, grants PID2020-115372RB-I00 and PDC2021- 120945-I00Regional government of Andalusia, grants B-TEP-026-UGR18 and P18-RT-1653University of Granada PPJIA2022-2

New Developments and Challenges in Liver Transplantation

Liver disease is increasing in incidence and is the third most common cause of premature death in the United Kingdom and fourth in the United States. Liver disease accounts for 2 million deaths globally each year. Three-quarters of patients with liver disease are diagnosed at a late stage, with liver transplantation as the only definitive treatment. Thomas E. Starzl performed the first human liver transplant 60 years ago. It has since become an established treatment for end-stage liver disease, both acute and chronic, including metabolic diseases and primary and, at present piloting, secondary liver cancer. Advances in surgical and anaesthetic techniques, refined indications and contra-indications to transplantation, improved donor selection, immunosuppression and prognostic scoring have allowed the outcomes of liver transplantation to improve year on year. However, there are many limitations to liver transplantation. This review describes the milestones that have occurred in the development of liver transplantation, the current limitations and the ongoing research aimed at overcoming these challenges

Benchmarking preconditioned boundary integral formulations for acoustics

The boundary element method (BEM) is an efficient numerical method for simulating harmonic wave scattering. It uses boundary integral formulations of the Helmholtz equation at the interfaces of piecewise homogeneous domains. The discretisation of its weak formulation leads to a dense system of linear equations, which is typically solved with an iterative linear method such as GMRES. The application of BEM to simulating wave scattering at large-scale geometries is only feasible when compression and preconditioning techniques reduce the computational footprint. Furthermore, many different boundary integral equations exist that solve the same boundary value problem. The choice of preconditioner and boundary integral formulation is often optimised for a specific configuration, depending on the geometry, material characteristics, and driving frequency. On the one hand, the design flexibility for the BEM can lead to fast and accurate schemes. On the other hand, efficient and robust algorithms are difficult to achieve without expert knowledge of the BEM intricacies. This study surveys the design of boundary integral formulations for acoustics and their acceleration with operator preconditioners. Extensive benchmarking provide valuable information on the computational characteristics of several hundred different models for multiple scattering and transmission of acoustic wave fields

Frequency-robust preconditioning of boundary integral equations for acoustic transmission

The scattering and transmission of harmonic acoustic waves at a penetrable material are commonly modelled by a set of Helmholtz equations. This system of partial differential equations can be rewritten into boundary integral equations defined at the surface of the objects and solved with the boundary element method (BEM). High frequencies or geometrical details require a fine surface mesh, which increases the number of degrees of freedom in the weak formulation. Then, matrix compression techniques need to be combined with iterative linear solvers to limit the computational footprint. Moreover, the convergence of the iterative linear solvers often depends on the frequency of the wave field and the objects' characteristic size. Here, the robust PMCHWT formulation is used to solve the acoustic transmission problem. An operator preconditioner based on on-surface radiation conditions (OSRC) is designed that yields frequency-robust convergence characteristics. Computational benchmarks compare the performance of this novel preconditioned formulation with other preconditioners and boundary integral formulations. The OSRC preconditioned PMCHWT formulation effectively simulates large-scale problems of engineering interest, such as focused ultrasound treatment of osteoid osteoma