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Nowadays ultrasonic imaging is one of the methods more frequently employed in clinical diagnosis and NDT applications. Ultrasonic commercial systems are mainly based on bidimensional images. Development of volumetric imaging systems based on array transducers has started at R&D laboratories a decade ago. An array transducer is electronically controlled in order to steer the ultrasonic beam in any direction, avoiding mechanical scanning systems. Mainly rectangular MxN monolithic arrays have been proposed for 3D imaging. Unfortunately, these apertures need a large amount of elements (they can reach several thousands) to produce good quality images. For this reason, a great research effort has been addressed to develop subsampling techniques in order to reduce the number of active elements. But these techniques produce a drastic reduction of the emitted energy, so the image contrast is seriously limited.
In this thesis 2D annular-segmented arrays are proposed as an alternative to the more common rectangular patterns. Annular segmented arrays have two main advantages: first, they present axial symmetry so the ultrasonic field shows almost invariant properties at any steered azimuth direction and a great symmetry with respect to the steering axis. Second, elements have a lower periodicity degree than rectangular arrays, and therefore grating lobes level is reduced. Consequently, we can increase the elements size holding at the same time a good quality in the acoustic field. With the purpose of reducing the number of elements with a full-active
aperture, one of the objectives of this work is to find a criterion for array designing, in order to determine which is the aperture geometry that gives a good image quality with minimum electronic resources.
Due to the scarce previous work devoted to 2D annular-segmented arrays, this thesis proposes innovative methods in the following aspects:
• Computational methods valid for complex apertures. An exact solution based on the spatial impulse response, for the ultrasonic field radiated by annular sector transducers is presented. Nevertheless, due to the high computational cost associated to the exact
solution, other approximated methods and algorithms are developed that reduce the computational cost in more than one order of magnitude.
• Innovative methods to describe the field of annular-segmented arrays for several conditions of the aperture geometry, radiated pulse, field point position, etc. Quantitative and qualitative rules have been obtained to describe lateral response, side lobes and
grating lobes in these conditions.
• Designing rules for annular-segmented arrays. The influence of several array-designing parameters on the field quality is studied. The following parameters are studied: the elements interspacing, the aperture dimensions, the number and shape of the elements, the UT pulse, etc.
From previous results, a theoretical comparative study between annular-segmented arrays and 2D rectangular apertures is presented. And finally, experimental work is done which validates theoretical results. |
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