Page 275 - Biomedical Engineering and Design Handbook Volume 2, Applications

INSTRUMENTATION DESIGN FOR ULTRASONIC IMAGING 253

and analog forms of beam formers, it is also possible to use charge-coupled devices for this purpose
and these, in fact, are receiving increased attention at the present time.
The following paragraphs introduce and describe briefly the functions of the most important blocks
in Fig. 9.1. The most important of these will receive greater attention in the remainder of the chapter.

9.3.1 B-Mode Transducers
As has been noted in the previous chapter, the mode of transduction in ultrasound systems takes
advantage of the piezoelectric characteristic of certain ceramics. There are several types of trans-
ducers currently in use, and the nature of processing of acoustic data is different among them. These
differences are highlighted in later sections as appropriate.
The piezoceramics can be fabricated in a number of ways to perform B-mode imaging. In its sim-
plest form, the B-mode transducer is a circular single-element transducer with a fixed geometric focus.
This type of a transducer has been replaced today by more sophisticated multielement transducers,
although some extremely low-cost systems based on mechanically driven single-element probes are
available. The next more complicated designs are annular arrays that are also circular but, as the name
implies, are composed of several (4 to 12) rings. These transducers can be used with mechanically
steered systems, although very few, if any, are currently available. Both the single-element and annular-
array transducers usually have concave-curved transmitting surface or an acoustic lens to focus the
acoustic energy at a given location. The next broad transducer category is that of linear arrays which
are built by dividing a piezoceramic strip into a large number of line-source-like elements. The num-
ber of elements in such arrays can exceed 200, although 128 is a typical number. A variation of the lin-
ear array is the curvilinear array that is built on a curved surface. With linear arrays the acquired image
is rectangular in shape, while with curvilinear arrays (and single element, annular arrays, and certain
linear arrays) it is sector shaped. Both linear and curvilinear arrays have either a lens or are curved to
focus the ultrasound in the plane perpendicular to the imaging plane. With both of these types of arrays,
focusing in the image plane is done electronically. Finally, in order to improve on slice thickness, some
form of electronic elevation focusing is being introduced into today’s systems. This is realized by the
use of multiple rows of elements that form a two-dimensional array of elements (Wildes, 1997). Arrays
which are connected to form a symmetrical aperture about the center line are sometimes referred to
either as 1.25-D or 1.5-D arrays, depending on whether improved slice thickness in the elevation direc-
tion is achieved by increasing the aperture size or by performing electronic focusing with those rows
of elements. Finally, in the last 5 years, two-dimensional (or 2D) arrays, which are capable of steering
the acoustic beam in two dimensions, have become commercially available. These are aimed largely at
the echocardiology market.
The transducer or transduction itself has received considerable attention from researchers due to
its critical position in the signal-processing sequence (Hunt, 1983; Szabo, 2004). Much progress has
been made in the areas of sensitivity, bandwidth, and the use of composite materials. Bandwidths of
today’s transducers can exceed 80 percent of the center frequency. This gives the system designer
additional flexibility with the available imaging frequencies and allows the optimization of image
quality among the various imaging modes as well as over a larger cross section of patients. There is
some potential of increasing the transmitted signal bandwidths to more than 100 percent with a new
class of transducers referred to as single-crystal relaxors. Also, the Khuri-Yakub Ultrasound Group
at Stanford University has shown that silicon-based MEMS (microelectromechanical systems)
devices can be made to perform transduction for ultrasound purposes (Johnson, 2002).

9.3.2 Pulser,T/R Switch, Variable Gain Stage
(or TGC Amplification)
This group of blocks is among the most critical from the analog signal-to-noise ratio point of view
(Analog Devices, 1999; Schafer, 1985; Wells, 1983). The piezoceramic array elements are energized
with the pulser and the transduction occurs as has been described. With the newer generation of
instruments, the excitation waveform is typically a short burst of one- to three-cycle duration. Earlier