Propagación y dispersión de ondas acústicas a través de nubes de microburbujas: aplicación al modelado de agentes de contraste ultrasónicos

Texto completo descargado desde TeseoLa posible aplicación de medios de contraste en la diagnosis médica por ultrasonidos fue descrita por primera vez hace 40 años. Hoy en día los agentes de contraste ultrasónicos (UCA), formados por microburbujas encapsuladas, son ampliamente utilizados para realizar diagnósticos clínicos especializados, de ahí la importancia del estudio de su comportamiento ante excitaciones acústicas. Por tanto, la optimización de las técnicas de contraste ecográfico, así como la generación y caracterización de estos agentes, permitirá dotar finalmente a esta herramienta clínica de las ventajas necesarias que le harán convertirse en una solución eficaz para el diagnóstico, e incluso tratamiento, de diversas patologías. Así pues, en el Capítulo 1 se lleva a cabo una introducción a los UCA, exponiendo los objetivos y metodología seguida para el desarrollo de esta tesis doctoral. En el aprendizaje y estudio de los fenómenos acústicos y ondulatorios en general, es imprescindible el análisis de las ecuaciones que modelan la propagación de ondas en medios homogéneos. Bajo la hipótesis de acústica lineal, muy común en la práctica, las ecuaciones de Navier-Stokes revelan que cualquier perturbación en la presión local de un fluido se propaga como una onda a la velocidad del sonido. Este resultado nos permite modelar, en el Capítulo 2, la radiación producida por un transductor piezoeléctrico con el fin de simular la propagación de un haz acústico en un medio fluido. Cuando en este medio existe alguna región homogénea con unas propiedades diferentes, aparece el fenómeno de dispersión acústica. Como consecuencia, la energía del haz es dispersada en distintas direcciones y con distintas intensidades, las cuales vienen marcadas por la diferencia entre las propiedades de los dos medios. Los conceptos y métodos usados para resolver este tipo de problemas será utilizados posteriormente para analizar la propagación y dispersión de ondas acústicas a través de nubes de burbujas, las cuales constituyen medios no homogéneos. Antes de analizar la dispersión que producen estas nubes, es necesario estudiar la dinámica de las burbujas mientras son excitadas acústicamente. De esta manera, en el Capítulo 3 se realiza una revisión exhaustiva de la formulación y de las ecuaciones que modelan las oscilaciones radiales de estas burbujas. Por otro lado, mediante un análisis lineal, se ha estudiado el comportamiento termodinámico del gas en el interior de las burbujas, permitiéndonos finalmente plantear una formulación muy precisa para las oscilaciones de pequeña amplitud. Este resultado nos permite caracterizar el espectro de oscilaciones, las cuales se relacionan con las propiedades de dispersión acústica de las burbujas. Los resultados revelan que las burbujas oscilan de forma resonante para determinadas frecuencias, lo que implica una elevada dispersión de energía acústica. Es esta propiedad ecogénica la que convierte a las microburbujas en un excepcional agente de contraste ultrasónico para su uso en aplicaciones clínicas. No obstante, para que estas burbujas sean estables en cuanto a su tamaño, necesitan ser recubiertas para evitar su disolución y eventual desaparición. Una vez analizada la dinámica de las microburbujas y justificada su aplicabilidad clínica, se ha llevado a cabo, en el Capítulo 4, la caracterización experimental de sus propiedades acústicas. Así pues, usando técnicas microfluídicas, se han generado suspensiones de microburbujas monodispersas recubiertas con fosfolípidos. El montaje experimental propuesto permite medir la frecuencia de resonancia de estas burbujas a través del espectro de atenuación de las muestras. Con este resultado, y conocido el tamaño medio de burbuja, podremos inferir las propiedades mecánicas que caracterizan al recubrimiento de forma experimental. Hasta ahora, las burbujas han sido tratadas como entes aislados, cuya dinámica es independiente de la presencia de burbujas vecinas. Sin embargo, para suspensiones o nubes con una densidad de burbujas elevada, aparecen efectos de acoplamiento que modifican sus oscilaciones, y por tanto, la dispersión total. Estos efectos de interacción burbuja-burbuja dan lugar a fenómenos colectivos, los cuales son analizados de forma rigurosa en el Capítulo 5. Así pues, en determinadas condiciones, las nubes pueden oscilar de forma colectiva siguiendo unos modos resonantes concretos. Para el cálculo de las oscilaciones acopladas y de la dispersión múltiple que aparece en consecuencia, se ha desarrollado un código numérico que permite obtener el campo de oscilaciones y presiones acústicas en todos los puntos de la nube. Esto nos permite caracterizar la dispersión acústica total en función de los parámetros que la definen. Finalmente, considerando a la nube como un medio efectivo homogéneo, y aplicando las técnicas afianzadas en la primera parte del desarrollo de este trabajo, se han calculado analíticamente las oscilaciones colectivas y la dispersión acústica total, resultados que han sido comparados con los obtenidos numéricamente. Sin embargo, el comportamiento real de las burbujas dentro de una nube es más complejo debido a que sus posiciones relativas varían como efecto de la excitación acústica. Las fuerzas que inducen este movimiento se conocen como fuerzas de Bjerknes, y son analizadas de forma teórica en el Capítulo 6. Así pues, se ha estudiado la interacción mutua entre pares de burbujas con el fin de calcular el movimiento relativo entre ellas. Finalmente se ha implementado un algoritmo que predice la formación de agrupaciones (clustering) de burbujas en nubes excitadas acústicamente, fenómeno ampliamente observado en la práctica.The potential application of contrast agents in ultrasound medical diagnosis was first described 40 years ago. Today, ultrasound contrast agents (UCA), which consist of encapsulated microbubbles, are widely used for specialized clinical diagnostics, hence the importance of the study of their behavior under acoustic excitations. Therefore, the optimization of medical techniques and procedures, as well as the generation and characterization of new agents, will provide to this clinical tool the advantages to become an effective solution for diagnosis and treatment of several pathologies. Thus, in Chapter 1 we carry out an introduction to UCA, stating the objectives and methodology that has guided the development of this dissertation. In the study of acoustic and wave phenomena, is essential to first analyze the equations that govern the wave propagation in homogeneous media. Under the hypothesis of linear acoustics, the Navier-Stokes equations show that any disturbance in the local pressure of a fluid propagates at the speed of sound. This finding allows us to model, in Chapter 2, the radiation generated by a piezoelectric transducer in order to simulate the propagation of an acoustic beam. When in this environment there exists a homogeneous region with different properties, the beam energy is scattered to different directions with different intensities. The concepts and methods employed to solve these kind of problems will be recover to analyze the propagation and scattering of acoustic waves through microbubble clouds. Before analyzing the scattering from these clouds, it is necessary to study the bubble dynamics while they are driven acoustically. Thus, in Chapter 3, a deep review of the formulation and the equations that model the radial oscillations of these bubbles has been performed. Furthermore, by using a linear analysis, we studied the thermodynamic behavior of the gas inside the bubbles. Such analysis brings a very accurate formulation for small amplitude oscillations. This finding allows us to characterize the spectrum of the oscillations, which is closely related to the acoustical properties of the bubbles. The results reveal that bubbles oscillate resonantly while they are driven at certain frequencies, which implies a very high acoustic energy scattering. Indeed, this echogenic property makes the microbubbles to be a unique contrast agent for clinical applications. However, these bubbles must be coated (stabilized) to prevent a rapid dissolution. Once analyzed the microbubble dynamics and justified their clinical applicability, we carry out the experimental characterization of its acoustic properties in Chapter 4. So, by using microfluidic techniques, we have generated monodisperse microbubbles suspensions coated with phospholipids. The proposed experimental setup enables the measurement of the bubbles’ resonance frequency through the attenuation spectrum of the samples. With this result, we can finally infer experimentally the mechanical properties which characterize the lipid coating. Until now, the bubbles have been treated as isolated entities whose dynamic is inde pendent of the presence of neighboring bubbles. However, when dealing with suspensions or clouds with a higher bubble density, the coupling effects could modify the oscillations and the total acoustic scattering. These bubble-bubble interactions give rise to collective phenomena within the cloud, which are rigorously analyzed in Chapter 5. Thus, under certain conditions, clouds can oscillate collectively following a specific resonant mode. In order to calculate the coupled oscillations and the multiple scattering, a numeric code has been developed. The code is able to get numerically the oscillations of each bubble and the sound pressure field in every point of the cloud. This allows us to characterize the total acoustic scattering as a function of the parameters that define the cloud and the excitation. Finally, considering that the cloud can be modeled as an effective homogeneous medium, we have calculated analytically both the collective oscillations and the total acoustic scattering with an excellent accuracy. However, the real behavior of the bubbles inside a cloud is more complex because their relative positions may vary under the effect of the acoustic excitation. The forces that drive this movement are known as Bjerknes forces, and they are theoretically analyzed in Chapter 6. Thus, we studied the mutual interaction between pairs of bubbles in order to calculate the relative displacement therebetween. To conclude this study, we have implemented a numerical algorithm that predicts the formation of bubble clusters within acoustically excited clouds, a phenomenon widely observed in experiments

Correlations of the acoustic fields of mudejar-gothic churches

This paper studies the statistical correlations between monaural acoustic parameters withthemselves and with architectural parameters from an acoustical survey undertaken in 12Mudejar-Gothic churches in Seville, southern Spain. Objective room-acoustic parameters havebeen measured using impulse response analysis through a maximum-length-sequencemeasurement system in each church. Measurements cover time parameters (reverberation time,early decay time); levels (sound strength); energy-based parameters (centre time, clarity forspeech, clarity, definition, early lateral energy fraction); and speech intelligibility (rapid speechtransmission index) and the analysis takes into account both spatially averaged values andindividual positions. The aim of the study is to identify the main architectural factors that haverelevant influence on the acoustic fields of the churches, and the minimum number of roomacoustic parameters which are able to describe adequately this acoustic field, thus the possibleexcess of information is evaluated. Significant correlations have been found that, in the bestcases, correspond to fits using pairs of independent variables instead of one single variable. Thebest option to describe the six subjective attributes of sound perception in these buildings consistsof three acoustic parameters formed by two different pairs plus the early lateral energy fractio

Measured acoustic parameters versus predicted ones in two mudejar-gothic churches

In this work we have undertaken an acoustic simulation of the sound field in two churches in the historical centre of the city of Seville. The simulation has been carried out through the CATT-Acoustic v7.2f programme, which has been based on the reverberation times measured in those spaces. We have focused on Sound Pressure Levels Lp, Clarity C80, Definition D50, and RASTI indices data in relation to their spectral variations and spatial distributions and their comparison with the equivalent experimental result

Method for the Economic Profitability of Energy Rehabilitation Operations: Application to Residential Dwellings in Seville

This work proposes a method based on a computer model to assess the economic profitability of energy rehabilitation operations in the envelope of a block of residential dwellings (Virgen del Carmen) in Seville (Southern Spain). The work evaluates the influence that certain hypotheses of interventions in the opaque part of the envelope exert on the annual energy demand: better insulation of the façade, interior partitions, roof and ground floors, in addition to its semi-transparent part: improvements in the airtightness of the building openings, the glass windows, and the thermal conductivity of its frame. These interventions arise from strict compliance with the Spanish regulatory framework. This model has been designed for the context of a Mediterranean climate, (mild winters and hot summers). The simulation tool Design Builder 3.4.0.041, which uses the calculation engine Energy Plus 8.1, has been selected to generate the computing model and establish its energy demand. The amortization of the economic costs of rehabilitation is quantified by the net present value (NPV) index, in accordance with the savings in the bills of energy consumption due to the reductions in demand for heating and cooling in the building, thereby obtaining an amortization period which exceeds 22 year

Method for the Economic Profitability of Energy Rehabilitation Operations: Application to Residential Dwellings in Seville

This work proposes a method based on a computer model to assess the economic profitability of energy rehabilitation operations in the envelope of a block of residential dwellings (Virgen del Carmen) in Seville (Southern Spain). The work evaluates the influence that certain hypotheses of interventions in the opaque part of the envelope exert on the annual energy demand: better insulation of the façade, interior partitions, roof and ground floors, in addition to its semi-transparent part: improvements in the airtightness of the building openings, the glass windows, and the thermal conductivity of its frame. These interventions arise from strict compliance with the Spanish regulatory framework. This model has been designed for the context of a Mediterranean climate, (mild winters and hot summers). The simulation tool Design Builder 3.4.0.041, which uses the calculation engine Energy Plus 8.1, has been selected to generate the computing model and establish its energy demand. The amortization of the economic costs of rehabilitation is quantified by the net present value (NPV) index, in accordance with the savings in the bills of energy consumption due to the reductions in demand for heating and cooling in the building, thereby obtaining an amortization period which exceeds 22 year

Comparative study of various techniques to measure speech intelligibility

The aim of this paper is to compare two different methods of measuring the Modulation Transfer Functions (MTF) and the corresponding derived STI-RASTI indices to assessing intelligibility. The first method uses, as test signal, a band filtered (500 and 2000 Hz) amplitude modulated noise. The second one uses MLS signals to obtain the impulse response of the system; further post processing allows us to calculate the MTF and STI-RASTI indices from this filtered respons

Clarity and Definition in Mudejar-Gothic Churches

This paper studies the C80 and D50 objective acoustic indices in relation to source receiver distance for fivedifferent octave bands inside Mudejar-Gothic churches. This type of church, eight examples of which we havestudied in the city of Seville, is characterised by good acoustic conditions as far as this can be indicated byreverberation times. This article aims to supply more data on these values and to explore the dependence of these measurements on receiver position. Finally, the experimental results have been compared with expected resultsderived from a semiempirical analytical model proposed

Dissolution of a CO2 spherical cap bubble adhered to a flat surface in air-saturated water

Bubbles adhered to partially hydrophobic flat surfaces often attain a spherical cap shape with a contact angle much greater than zero. We address the fundamental problem of the diffusion-driven dissolution of a sessile spherical cap bubble (SCB) adhered to a flat smooth surface. In particular, we perform experiments on the dissolution of CO2 bubbles (with initial radii similar to 1 mm) immersed in air-saturated water adhered to two substrates with different levels of hydrophobicity. It is found that the contact angle dynamics plays an important role in the bubble dissolution rate. A dissolution model for a multicomponent SCB in an isothermal and uniform pressure environment is then devised. The model is based on the quasi-stationary approximation. It includes the effect of the contact angle dynamics, whose behaviour is predicted by means of a simplified model based on the results obtained from adhesion hysteresis. The presence of an impermeable substrate hinders the overall rate of mass transfer. Two approaches are considered in its determination: (a) the inclusion of a diffusion boundary layer-plate interaction model and (b) a finite-difference solution. The model solutions are compared with the experimental results, yielding fairly good agreement.The authors gratefully acknowledge the support of Total E&P Recherche et Développement through study agreement FR00006995, and the Spanish Ministry of Economy and Competitiveness through grant DPI2014-59292-C3-1-P

Identifying acoustically coupled volumes in the Cathedral of Toledo, Spain

Cathedrals are large monuments combined with great geometrical complexity due to different architectural styles, commonly featuring modifications resulting from additions and adaptations to liturgical practice in various historical periods. In the so-called "Spanish mode", the choir is situ-ated in the centre of the main nave and generally constitutes a more absorbent volume connected by an acoustically transparent opening with the rest of the cathedral. This space, together with the presbytery and the lateral chapels, can present the phenomenon of acoustic coupling. Taking advantage of a recent acoustic survey carried out in Toledo cathedral, the presence of such non-uniform acoustic energy distribution is investigated. By means of standardised parameters and the application of Bayesian methods, an analysis is performed of the non-linearity of the energy decay curves of the impulse responses registered in the cathedralLas catedrales son grandes monumentos con una gran complejidad geométrica debido a los diferentes estilos arquitectónicos, consecuencia de las modificaciones resultantes de adiciones y adaptaciones a diferentes períodos históricos y a la evolución de la práctica litúrgica. En el llamado "modo español" el coro está situado en el centro de la nave principal y, generalmente, constituye un volumen más absorbente conectado por una abertura acústicamente transparente con el resto de la catedral. Este espacio, junto con el presbiterio y las capillas laterales, pueden presentar el fenómeno del acoplamiento acústico. Aprovechando una reciente campaña acústica llevada a cabo en la catedral de Toledo, se investiga la presencia de dicho fenómeno. El análisis se realiza calculando parámetros normalizados y aplicando métodos bayesianos para caracteri-zar la no linealidad de las curvas de decaimiento de energía derivadas de las respuestas al im-pulso registradas en la catedra