This work describes preliminary results of a two-modality imaging system aimed at the early detection of breast cancer. The first technique is based on compounding conventional echographic images taken at regular angular intervals around the imaged breast. The other modality obtains tomographic images of propagation velocity using the same circular geometry. For this study, a lowcost prototype has been built. It is based on a pair of opposed 128-element, 3.2 MHz array transducers that are mechanically moved around tissue mimicking phantoms. Compounded images around 360º provide improved resolution, clutter reduction, artifact suppression and reinforce the visualization of internal structures. However, refraction at the skin interface must be corrected for an accurate image compounding process. This is achieved by estimation of the interface geometry followed by computing the internal ray paths. On the other hand, sound velocity tomographic images from time of flight projections have been also obtained. Two reconstruction methods, Filtered Back Projection (FBP) and 2D Ordered Subset Expectation Maximization (2D OSEM), were used as a first attempt towards tomographic reconstruction. These methods yield useable images in short computational times that can be considered as initial estimates in subsequent more complex methods of ultrasound image reconstruction. These images may be effective to differentiate malignant and benign masses and are very promising for breast cancer screening.

The phenomenon of the displacement of the position of the pressure, intensity and acoustic radiation force maxima along the axis of focused acoustic beams under increasing driving amplitudes (nonlinear focal shift) is studied for the case of a moderately focused beam excited with continuous and 25 kHz amplitude modulated signals, both in water and tissue. We prove that in amplitude modulated beams the linear and nonlinear propagation effects coexist in a semi-period of modulation, giving place to a complex dynamic behaviour, where the singular points of the beam (peak pressure, rarefaction, intensity and acoustic radiation force) locate at different points on axis as a function of time. These entire phenomena are explained in terms of harmonic generation and absorption during the propagation in a lossy nonlinear medium both, for a continuous and an amplitude modulated beam. One of the possible applications of the acoustic radiation force displacement is the generation of shear waves at different locations by using a focused mono-element transducer excited with an amplitude modulated signal.

No abstract

En este trabajo se presenta un modelo matemático que describe el comportamiento dinámico de una cerámica piezoeléctrica comercial vibrando en modo espesor. Dicho modelo resuelve, mediante diferencias finitas, las ecuaciones piezoeléctricas que relacionan los fenómenos mecánicos y eléctricos que se producen en este tipo de materiales, y simula el comportamiento de algunas variables físicas puntuales -como campo eléctrico o velocidad de partícula-en el dominio de tiempo. Asimismo, se ha implementado un entorno gráfico en Matlab que permite realizar las simulaciones de forma sencilla y asequible a todos los usuarios. Esta interfaz contempla dos tipos de estudio: excitación de la cerámica en banda ancha y excitación a una sola frecuencia. Como valores de salida el programa facilita, una vez finalizada la simulación de las variables dependientes del tiempo, los distintos parámetros piezoeléctricos de la cerámica y su respuesta eléctrica en el dominio de la frecuencia (curvas de admitancia e impedancia). Los resultados obtenidos permiten, además de predecir los modos propios de vibración y la respuesta eléctrica, comprender el efecto de piezoelectricidad en la respuesta mecánica de un medio material.In this paper, a mathematical model describing the dynamic behavior of a commercial piezoelectric ceramic, vibrating in thickness, is presented. This model solves, by finite differences, the piezoelectric equations which relate he mechanical and electrical phenomena produced in this kind of material. The behavior in time domain of some physical variables, as the electric field and particle velocity, is simulated too. In addition, we have implemented a GUI in Matlab that allows simple and affordable simulations for all users. The implemented interface provides two kind of study: Broadband and single frequency excitation. The output parameters, obtained after time domain simulation, are piezoelectric constants of the ceramic and the electric response in frequency domain, i.e. admittance and impedance curves. The results allow predicting vibration modes, electrical response and understanding the effect of piezoelectricity on the mechanical response.