Gregorio Godoy

2D array design based on Fermat spiral for ultrasound imaging

The main challenge faced by 3D ultrasonic imaging with 2D array transducers is the large number of elements required to achieve an acceptable level of quality in the images. Therefore, the optimisation of the array layout, in order to reduce the number of active elements in the aperture, has been a research topic in the last years. Nowadays, array technology has made viable the production of 2D arrays with larger flexibility on elements size, shape and position, allowing to study other configurations different to the classical matrix organisation, such as circular, archimedes spiral or polygonal layout between others. In this work, the problem of designing an imaging system array with large apertures and a very limited number of active elements (N(e)=128 and N(e)=256) using the Fermat spiral layout has been studied. As summary, a general discussion about the most interesting cases is presented.

Beam Steering with Segmented Annular Arrays

Two-dimensional (2-D) arrays of squared matrix have maximum periodicity in their main directions; consequently, they require half wavelength (lambda/2), interelement spacing to avoid grating lobes. This condition gives rise to well-known problems derived from the huge number of array elements arid from their small size. In contrast, 2-D arrays with curvilinear configuration produce lower grating lobes and, therefore, allow the element size to be increased beyond lambda/2. Using larger elements, these arrays have the advantage of reducing the number of elements arid of increasing the signal-to-noise ratio (SNR). In this paper, the beamforming properties of segmented annular phased arrays are theoretically analyzed and compared with the equivalent squared matrix array. In the first part, point-like elements are considered in order to facilitate the field analysis with respect to the array structure. Afterward, the effect of the element size on the steered beam properties also is presented. In the examples, it is shown that the segmented annular array has notably lower grating lobes than the equivalent squared matrix array and that it is possible to design segmented annular arrays with interelement distance higher than lambda whose beam characteristics are perfectly valid for volumetric imaging applications

Coarray Synthesis Based on Polynomial Decomposition

Synthetic aperture (SA) techniques have been frequently used to reduce the volume and complexity of the imaging systems. A useful tool for designing synthetic aperture configurations is the coarray. This is the virtual aperture that produces in one way the same beam pattern as the SA system in emission and reception. In this correspondence, we propose a new algorithm, based on the polynomial decomposition, that allows to obtain any wanted coarray on a linear array using whatever synthetic aperture configuration. With this fast and simple algorithm, the desired coarray is decomposed into a set of sub-apertures, whose length is determined by the requirements and resources of the system. The result is the set of weights that have to be applied on the sub-apertures to get the desired coarray, and consequently, the wanted beam pattern.

Design of Curvilinear Array Apertures for 3D Ultrasonic Imaging

The development of ultrasonic volumetric imaging is closely linked to the development of systems that are able to operate bidimensional array transducers. These arrays are useful for ultrasonic volumetric imaging, because they produce steered and focused beams throughout a volume of interest. Typical 2-D arrays are based on a Squared Matrix (SM) configuration, where the array elements are the matrix cells. Their performance is determined by their width in terms of wavelenght. Resolution and the dynamic range are determined by wavelenght/aperture diameter ratio and number of elements and the wavelenght/ interelement distance ratio respectively (Smith et al. (1991)). In SM apertures, since element distribution is uniform, the composition of the secondary lobes is determined by interelement distance. All elements contribute to its formation. These lobes are known as grating lobes and produce image artifacts that can reduce the signal-to-noise ratio. Nowadays avoiding image artifacts is key to array design. For matrix and linear arrays the composition of these lobes can only be avoided by limiting the interelement distance to λ/2. In practice, it means that for 1o of lateral resolution a 60λ diameter aperture with 14400 elements is needed. Then, several problems can be identified: