Abstract

The quality of phased array and synthetic aperture ultrasonic images is limited by several factors determined by the sound propagation physics and diffraction laws. Image quality is mainly determined by: 1. Resolution, which depends on signal bandwidth (in the axial direction) and on the aperture extent (in the lateral direction). 2. Dynamic range, which is bounded by the ratio of the main to the sidelobes level and is related to the smallest features detection capability. 3. Contrast, which is the capability to differentiate among subtle changes in the acoustic impedance (i.e., different tissues in medical imaging). 4. Signal-to-noise ratio, where noise can be electrical (i.e., thermal, EMI, etc.) or speckle, also called clutter or grain noise, depending of the application field. Speckle results from interferences among unresolved scatterers in a range cell. 5. Artifacts, such as reverberations, grating lobes and others which blur the image and reduce the dynamic range. Along the years, many research efforts have been devoted to find techniques that increase the image quality by addressing the above factors. Frequently, some characteristics are improved at the expenses of losses in some others. A typical example is apodization, used to reduce the sidelobe level, with an adverse effect in the lateral resolution [Szabo, 2004]. In medical imaging, where contrast is essential, this function is quite useful; however, in the NDT field, where resolution is more relevant, apodization provides marginal or no benefits at all. As another example, lateral resolution is improved with larger apertures, which may be obtained with increased array inter-element pitch d. However, when d>┣/2 (sparse apertures) grating lobe artifacts appear. The condition d>┣/2 also arises with 2D arrays or in high frequency ultrasound imaging due to manufacturing constraints. Sparse apertures with randomly distributed elements reduce the grating lobes by spreading their energy among the sidelobes, whose level increases [Turnbull & Foster, 1991]; [Gavrilov et al., 1997]. In other cases, the concept of effective aperture allows to reduce the grating lobe levels by using different element distributions in emission and in reception, so that the compound radiation pattern equals that of a dense aperture [Lockwood et al., 1998]; [Nikolov & Jensen, 2000]; [Nikolov & Behar, 2005]. When a single focus is set in emission (as in phased array), some residual grating lobe artifacts remain in the image [Lockwood et al., 1996]. Furthermore, these approaches produce a lower signal-to-noise ratio.

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