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

472 16. Ultrasonicscurvilinear array. Because of its convex shape [Figure 16.8(b)], the curvilinear (orconvex) array scans a wider field of view than does a linear-array configuration.The curvilinear array operates in the same manner as the linear array in that thescan lines are directed normal to the transducer face. In the linear-phased array ofFigure 16.8(c), which may contain as many as 128 elements, each element is usedto emit and receive each line of data. In Figure 16.8(c), the scanner steers the beamthrough a sector-shaped region in the azimuth plane. These phased arrays can scana region considerably wider than the footprint of the transducer, thus renderingthem suitable for scanning through acoustically restricted windows. This is idealfor use in cardiac imaging, where the transducer must scan through a small windowin order to avoid obstruction by ribs and lungs.The 1.5D array is structurally similar to a 2D array but operates as 1D. The1.5D array consists of elements along both the azimuth and elevation directions.Dynamic focusing and phase correction can be implemented in both dimensionsto enhance image quality. Steering is not possible in the elevation dimension sincea 1.5D array contains a fairly limited number of elements in elevation (usually3 to 9 elements). Figure 16.8(d) shows a B-scan conducted with a 1.5D phasedarray. It is also possible to use linear sequential scanning with 1.5D arrays. In the2D phased array, a large number of elements are employed in both the azimuthand elevation. This permits focusing and steering of the acoustic beam along bothdimensions. With the application of parallel receive processing, a 2D array canscan a pyramidal volume in real time to yield a volumetric image, as illustrated inFigure 16.8(e).Linear-Array Transducer SpecificationsIn the design or selection of an ultrasound transducer, a number of compromisesare entailed. The ideal transducers have high sensitivity and SNR (signal-to-noiseratio), excellent spatial resolution and freedom from spurious signals. Additionallyan individual array element should possess wide angular response in steeringdimensions, low cross-coupling with another element, and an electrical impedancematching the transmitter.16.7 Basic Instrumentation: Scanning MethodsFigure 16.9 illustrates a “generic ultrasonic instrument” in block diagram format.The synchronization generator establishes a repetition rate (typically 1,000 repetitionsper second) for a pulse input and a display module. The pulser provideselectrical energy to the transducer through a pulse limiter, which clips the voltageexperienced by the amplifiers down to a tolerable threshold but allows the fullvoltage to impact the transducer. The transducer generates the ultrasonic wavesand receives the echoes. The resulting voltage of the echoes, not clipped by thepulse limiter because their voltages are usually below the clipping threshold, goeson directly to the amplifier and then onward to a display. The computer (whichmay or may not be present) acts to process algorithms.