Doctor of Philosophy

dissertationMagnetic resonance-guided focused ultrasound surgery (MRgFUS) is a noninvasive means of causing selective tissue necrosis using high-power ultrasound and MR temperature imaging. Inhomogeneities in the medium of propagation can cause significant distortion of the ultrasound beam, resulting in changes in focal-zone amplitude, location and shape. Current ultrasound beam simulation techniques are either only applicable to homogeneous media or are relatively slow in calculating power deposition patterns in inhomogeneous media. Further, these techniques use table-value estimates of the acoustic parameters for predicting ultrasound beam propagation in inhomogeneous media, resulting in at best an approximate power deposition pattern. This work improves numerical analysis of ultrasound beam propagation by developing techniques for: 1) fast, accurate predictions of ultrasound beam propagation in inhomogeneous media, 2) noninvasive estimation of acoustic parameters (speed of sound and attenuation coefficient) of tissue types present in inhomogeneous media, 3) noninvasive determination of changes in tissue acoustic properties due to treatment. These beam simulation techniques utilizing subject-specific tissue parameters will rapidly predict power deposition patterns in real patient geometries and estimate changes in tissue acoustic parameters during treatment, leading to treatment-responsive patientspecific treatment plans that will improve the safety, efficacy and effectiveness of MRgFUS

Phased electromagnetic acoustic transducer array for Rayleigh wave surface defect detection

A phased electromagnetic acoustic transducer (EMAT) array system has been developed for detection and characterisation of surface breaking defects. An array of four linear coils which are individually controlled are used to generate a Rayleigh wave. The high current electronics combined with the coil designs enables the array to generate either narrowband or broadband signals, and controlling the phase delay between the channels makes it possible to change the ultrasound wavelength without requiring the physical separation of the coils to be changed. Experimental results show that the four-coil phased array is able to generate a wavelength range from 3.0 mm to 11.7 mm. Surface breaking defects were characterised using a transmit-receive set-up with a broadband EMAT detector being used to detect the Rayleigh wave. Machined surface slots with different depths were used for technique validation. The results show that the array is sensitive to surface defects and that a wide depth sensitivity range for defect sizing can be easily achieved by applying phasing to tune the wavelength of operation. A large increase in detection flexibility is immediately shown

Simulating arbitrary-geometry ultrasound transducers using triangles

Calculation of ultrasound fields from medical transducers is often done by applying linear acoustics and using the Tupholme-Stepanishen method of calculation. Here the spatial impulse response is found and, together with the basic one-dimensional pulse, it is used to find both the emitted and pulse-echo field. The spatial impulse response has only been determined analytically for a few geometries and using apodization over the transducer surface generally makes it impossible to find the response analytically. A popular approach to find the general field is thus to split the aperture into small rectangles, and then sum the weighted response from each of these. The problem with rectangles is their poor fit to apertures which do not have straight edges, such as circular and oval shapes. The simulation thus introduces artifacts in the response, that necessitates the use of a large number of rectangles for a precise simulation. A triangle better fits these aperture shapes, and the field fr..

Lamb: a simulation tool for air-coupled lamb wave based ultrasonic NDE systems

La técnica de las ondas de Lamb acopladas por aire representa un importante avance en el área de los Ensayos No Destructivos (END) de materiales laminares.Sin embargo la compleja naturaleza de las vibraciones mecánicas encontradas en acústica, hacen que el análisis y el estudio de esta área del conocimiento sea un tema muy complejo. De allí que la posibilidad de contar con una herramienta de simulación de software que permita la evaluación y prueba de diferentes configuraciones de excitación y recepción acústica utilizando la flexibilidad de un modelo de computadora sea de una gran utilidad y ayuda.El objetivo de la presente tesis es proveer al área de los END con un software de simulación gratuito: The LAMB Matlab® toolbox basado en el modelo del software libre de la GNU.El software es capaz de simular el comportamiento de sistemas de END basados en ondas de Lamb acopladas por aire en láminas isótropas simples utilizando transductores tipo array.El programa se basa en un arreglo tipo C-scan de un sistema de END y está compuesto por tres bloques principales: 1) Excitación, 2) Propagación y 3) Recepción.La verificación individual del funcionamiento de dichos módulos se presenta a lo largo de la tesis mediante una serie de comparaciones entre simulaciones y datos experimentales provenientes de diferentes pruebas. Por otro lado, la validación del programa completo se llevo a cabo por medio de experimentos en láminas de cobre y aluminio; utilizando un sistema real de END por ondas de Lamb acopladas en aire mediante arrays cóncavos.La influencia negativa en el desempeño general de dicho sistema de END real basado en este tipo de transductores se comprobó efectivamente mediante el simulador desarrollado. Esto se debió fundamentalmente al efecto de directividad de los sensores individuales en los transductores y a la simetría cóncava de los arrays.Para emular este comportamiento la tesis presenta un modelo geométrico bidimensional simple de un filtro espacial, junto a las simulaciones de un nuevo tipo de array plano propuesto.El programa desarrollado comprobó así mismo la naturaleza coherente de los campos acústicos emitidos en aire por las láminas sujetas a vibraciones de Lamb. Esto se realizó mediante la implementación de un conformador de haz simple de suma y demora; constituyéndose así la etapa inicial de procesamiento de señal del bloque de recepción del programa.El objetivo principal del presente trabajo fue contribuir con un modelo operativo de simulación y prueba de nuevos diseños de arrays e implementación de estrategias de procesado de señal útiles en sistemas de END basados en ondas de Lamb acopladas por aire.Finalmente, si bien el objetivo de la calibración del programa no se pudo conseguir; si se logró efectivamente un notable grado de similitud con un sistema de END real.Air-coupled ultrasonic Lamb waves represent an important advance in Non- Destructive Testing and Evaluation (NDT & NDE) techniques of plate materials and structures. Examples of these advances are the characterization and quality assessment of laminate materials in manufacturing processes, the location of damaged parts in aircrafts and structure monitoring in the aerospace industry.However the rich and complex nature of mechanical vibrations encountered in acoustics make the subject of analysis and study of these systems a very complex task. Therefore a simulation tool that permits the evaluation and testing of different configuration scenarios using the flexibility of a computer model is an invaluable aid and advantage.The objective of this thesis is to provide the field of NDT with free open source software i.e. the LAMB Matlabrtoolbox. The toolbox is capable of simulating the behaviour of Lamb wave based NDE systems for single ideal isotropic laminates using air-coupled ultrasonic arrays. The programme usesa pitch-catch type of a Cscan NDE arrangement and is composed of three integrated sections each individually modelling a feature in the system: 1) Excitation, 2) Propagation, and 3) Reception.For assessment of the individual modules of the toolbox the thesis presents comparisons between each section simulations and the data obtained from different acoustic experiments. The validation of the complete simulator was carried out by evaluation tests on the copper and aluminium plates by use of a real hardware prototype of a Lamb wave based NDE system with aircoupled concave arrays.The negative impact on the performance of the real air-coupled NDE systembased on concave arrays was effectively confirmed by the programme. This was produced by the inherent directivity of the individual sensors as well as their concave arrangement. To emulate this behaviour the thesis introduces a simple two-dimensional geometric model for the inclusion of the spatial filtering effect of the sensors plus a group of simulations for a new proposed air-coupled plane array transducer.The software also verified the spatial coherent nature of the Lamb wave fields emitted by a plate in air. This was demonstrated by the implementation of a delay and sum beamformer to constitute an initial signal processing stage in the reception section

Ultrasonic air-coupled capacitive arrays

A model is developed which is capable of predicting the pressure field of a rectangular source, as measured by a finite-sized receiver. This novel method treats the problem in a new way, which allows an integration to be performed over the area of the receiver. Previously it has only been possible to model two circular transducers coaxially aligned. The model is used to identify a receiver, which can be used to measure the highly focussed pressure field from a phased array, with only a negligible effect due to the receiver size. Productions from the model are compared to experimental data, and show a good correlation. A parabolic mirror used to focus the field from a circular device in air has been studied, and a model developed to predict the pressure field produced by this device. This is done by an approximation of the mirror surface to a grid of finely spaced points. The model correlates well with measured results. In addition, an image of a defect in a solid sample was produced. Arrays are then used to image solid samples in air. This is done using three techniques. The first is a combined phased source and receiver, which is shown to locate a wire accurately and to measure a step in the surface of a sample. A 2-D array is shown to image a defect in a composite plate, and the potential for a fast through-transmission air-coupled system is indicated. In addition, two post-processing techniques are used on data recorded using an array receiver, to locate an object in air. Of these two techniques, ellipse crossing is shown to have better results for large signal to noise ratios, and SAFT for lower ratios. The combination of theoretical modelling and experimental observations has indicated that the transducers and arrays constructed for use in air are well-understood, and that their characteristics can be predicted

Parallelization and improvement of beamforming process in synthetic aperture systems for real-time ultrasonic image generation

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Informática, Departamento de Arquitectura de Computadores y Automática, leída el 9-02-2016La ecografía es hoy en día uno de los métodos de visualización más populares para examinar el interior de cuerpos opacos. Su aplicación es especialmente significativa tanto en el campo del diagnóstico médico como en las aplicaciones de evaluación no destructiva en el ámbito industrial, donde se evalúa la integridad de un componente o una estructura. El desarrollo de sistemas ecográficos de alta calidad y con buenas prestaciones se basa en el empleo de sistemas multisensoriales conocidos como arrays que pueden estar compuestos por varias decenas de elementos. El desarrollo de estos dispositivos tiene asociada una elevada complejidad, tanto por el número de sensores y la electrónica necesaria para la adquisición paralela de señales, como por la etapa de procesamiento de los datos adquiridos que debe operar en tiempo real. Esta etapa de procesamiento de señal trabaja con un elevado flujo de datos en paralelo y desarrolla, además de la composición de imagen, otras sofisticadas técnicas de medidas sobre los datos (medida de elasticidad, flujo, etc). En este sentido, el desarrollo de nuevos sistemas de imagen con mayores prestaciones (resolución, rango dinámico, imagen 3D, etc) está fuertemente limitado por el número de canales en la apertura del array. Mientras algunos estudios se han centrado en la reducción activa de sensores (sparse arrays como ejemplo), otros se han centrado en analizar diferentes estrategias de adquisiciónn que, operando con un número reducido de canales electrónicos en paralelo, sean capaz por multiplexación emular el funcionamiento de una apertura plena. A estas últimas técnicas se las agrupa mediante el concepto de Técnicas de Apertura Sintética (SAFT). Su interés radica en que no solo son capaces de reducir los requerimientos hardware del sistema (bajo consumo, portabilidad, coste, etc) sino que además permiten dentro de cierto compromiso la mejora de la calidad de imagen respecto a los sistemas convencionales...Ultrasound is nowadays one of the most popular visualization methods to examine the interior of opaque objects. Its application is particularly significant in the field of medical diagnosis as well as non-destructive evaluation applications in industry. The development of high performance ultrasound imaging systems is based on the use of multisensory systems known as arrays, which may be composed by dozens of elements. The development of these devices has associated a high complexity, due to the number of sensors and electronics needed for the parallel acquisition of signals, and for the processing stage of the acquired data which must operate on real-time. This signal processing stage works with a high data flow in parallel and develops, besides the image composition, other sophisticated measure techniques (measure of elasticity, flow, etc). In this sense, the development of new imaging systems with higher performance (resolution, dynamic range, 3D imaging, etc) is strongly limited by the number of channels in array’s aperture. While some studies have been focused on the reduction of active sensors (i.e. sparse arrays), others have been centered on analysing different acquisition strategies which, operating with reduced number of electronic channels in parallel, are able to emulate by multiplexing the behavior of a full aperture. These latest techniques are grouped under the term known as Synthetic Aperture Techniques (SAFT). Their interest is that they are able to reduce hardware requirements (low power, portability, cost, etc) and also allow to improve the image quality over conventional systems...Depto. de Arquitectura de Computadores y AutomáticaFac. de InformáticaTRUEunpu