Ultrasparse, ultrawideband arrays

Abstract

This paper investigates the properties of highly thinned ultrawideband (UWB) arrays. The design aim is high resolution and very low side radiation levels (SL). One- and two-dimensional ultrasparse UWB arrays can be designed to achieve both. The minimum available pulse-echo SL is shown to approach N(-4) where N is the number of elements in the transmit and receive arrays. Periodic thinning is shown to be superior to random thinning, and amplitude taper is shown to raise the SL. Two-dimensional curvilinear deployment of elements are shown to outperform rectilinear designs, and different transmit and receive arrays in pulse-echo systems are shown to outperform systems that use the same array for transmit and receive. Very low SL is achievable in an ultrasparse UWB system with so few elements that echo signal-to-noise ratio (SNR) rather than SL becomes the constraint on the minimum number of elements required by the system for the array to be useful for imaging. For example, in ultrasonic pulse-echo breast imaging, SL approximately -70 dB is desired to distinguish small cysts from tumors. A 2-D randomly thinned array requires about 10,000 elements. A 2-D ultrasparse UWB periodic array requires less than 100 to satisfy SL, a reduction of 100:1, but provides insufficient SNR. A 500-element, 7.5 MHz array operating with 4 cm penetration depth satisfies both. Experimental results demonstrate the theory.

Beam Steering with Segmented Annular Arrays

“…The pulse-echo analysis of 2-D arrays can be simplified by applying the projection-slice theorem [12], [31] and the effective aperture concept [9], [10]. The projection-slice method considers that the CW response of a 2-D array at a given azimuth direction φ can be found by projecting the 2-D element distribution on a line oriented at the φ direction (both 1-D and 2-D arrays are equivalent).…”

“…As an alternative to these limitations, array apertures with curvilinear deployment have been proposed as they outperform rectilinear designs [12]- [15]. Due to their greater aperture spatial diversity, curvilinear arrays produce lower grating lobes and, therefore, layouts with interelement distance beyond λ/2 can be considered.…”

Design of Curvilinear Array Apertures for 3D Ultrasonic Imaging

“…Alternatively apertures with curvilinear deployment have been proposed (Schwartz & Steinberg (1998);Mendelsohn & Wiener-Avnear (2002); Ullate et al (2006); Bavaro et al (2008); Sumanaweera et al (1999)). Unlike matrix distributions, curvilinear apertures have no privileged direction.…”

“…To meet these challenges and reduce the cost and complexity of 3-D systems, the main design strategy is to break the periodicity in the aperture, using different thinning strategies to reduce the number of active elements and maintaining good field characteristics at the same time (Smith et al (1991); Schwartz & Steinberg (1998)). …”

“…Array designers then try to select those elements that produce the most appropriate beam pattern or image for a given set of constraints (Smith et al (1991); Hendricks (1991); Schwartz & Steinberg (1998); Lockwood & Foster (1996)). Most thinning solutions are based on random distribution but over the last decades some solutions that use the multiplicative nature of the pulse-echo process have been presented (Schwartz & Steinberg (1998); Lockwood & Foster (1996); Austeng & Holm (2002); Nikolov & Jensen (2000)). It has been proposed that an imaging system can be composed by two different apertures in emission and reception, where the grating lobes in emission and the grating lobes in reception appear at different positions and thus they are partly suppressed in the two-way response.…”