DK2985_C000 1..28 - AlSharqia Echo Club

Transducers 47Figure 3.10 Beam focusing. (A) Focusing reduces beam diameter in the near zone, increases beam diameter in the far zone and movesthe transition zone closer to the origin of the transducer. (B–D) Examples of two images with different focus location. Note the differencein (B) when the focus zone is not centered on the structure of interest, the aortic valve (AoV), compared with (C), where it is positionedover the AoV. The detail resolution is improved at that level (LA, left atrium; RVOT, right ventricular outflow tract).shorter than the near-zone length of the unfocused beam.The beam width decreases from the transducer to thefocal region but increases beyond in the far zone(Fig. 3.10). Beam focusing can be achieved by eithermechanical or electronic means.MechanicalWith simple disk transducers, two methods of focusinghave been described. Internal focusing is achieved byapplying a radius of curvature to the piezoelectriccrystal. In the external focusing method, the piezoelectricelement is kept flat while a concave acoustic lens is placedin front of the crystal (Fig. 3.11).Electronic Focusing by PhasingUsing multiple piezoelectric elements organized in atransducer array, focusing is achieved by electronicphasing. Electronic phasing consists of the successiveactivation of the different elements of the transducerarray following a specific time delay pattern. Varying activationtime pattern creates different specific focus regionsand allows the generation of multiple foci regions withoutmodifying the curvature of a mechanical element. Thediagram on the top in Fig. 3.12 shows a long delay in activationof the elements. This results in a greater curvaturemoving the focus region closer to the transducer. In theadjacent diagram, the delay between element activationis decreased, producing a narrower curvature andmoving the focus region further away.It is important to note that beam width changes as thefocal length is increased. In order to maintain the samefocal width, the number of elements activated has tochange according to the focal length. For short focallengths, a small number of the elements are activated,while for a focus placed further from the transducer, alarger percentage of the elements must be activated. Theability to change the number of activated elements is