tailieunhanh - Adaptive beamforming by phase coherence processing

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: Resolution, which depends on signal bandwidth (in the axial direction) and on the aperture extent (in the lateral direction); Dynamic range, which is bounded by the ratio of the main to the sidelobes level and is related to the smallest features detection capability. | 4 Adaptive Beamforming by Phase Coherence Processing J. Camacho and C. Fritsch Consejo Superior de Investigaciones Cientificas CSIC Madrid Spain 1. Introduction 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 . different tissues in medical imaging . 4. Signal-to-noise ratio where noise can be electrical . 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 x 2 sparse apertures grating lobe artifacts appear. The condition d X 2 also arises with 2D arrays or in high frequency ultrasound imaging due to manufacturing constraints.