THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

470 16. UltrasonicsFigure 16.7. The phased array illustrated above provides for steering and focusing of anultrasonic beam. In (a) the six-element linear array is shown in the transmit mode. In thereceive mode of (b), dynamic focusing allows the scanner focus to track the returningechoes.In the receive mode, a dynamic adjustment can be made with a focal point so thatit coincides with the range of returning echoes. After the transmission of a pulse,the initial echoes return from targets nearest the transducer. The scanner thereforefocuses the phased array on these targets, located at focus point 1 as shown inFigure 16.7(b). As echoes return from the more distant targets, the scanner focusesat a greater depth (e.g., focus point 2). Focal zones are achieved with adequatedepth of field so that the targets always remain in focus in receive mode. This isthe dynamic receive focusing process.Array-Element ConfigurationsThe dynamic receive focusing process is repeated many times to form an ultrasonicimage in the scan of a 2D or 3D region of tissue. In defining the 2D image, thescanning plane is the azimuth dimension; the elevation dimension is normal tothe azimuth scanning plane. In linear sequential arrays, as many as 512 elementsconstitute a sequential linear array in currently available scanners. A subaperturecontaining as many as 128 elements is selected to function at a given time.In Figure 16.8(a), the scanning lines are directed perpendicularly to the transducerface; the acoustic beams is focused but not steered. The advantage is that