Page 281 - Biomedical Engineering and Design Handbook Volume 2, Applications

INSTRUMENTATION DESIGN FOR ULTRASONIC IMAGING 259

9.4.2 Beam Steering
There are a number of different methods of steering an acoustic beam currently in use. These can be
grouped into three categories:
1. Mechanical
2. Element selection
3. Phased array
The major implication of the selection of the beam-steering approach is in the cost of the
instrument. Mechanically steered systems tend to be the simplest and hence the least expensive
while the phased arrays the most expensive. The great majority of recent vintage scanners have
the latter two types of beam steering. The following paragraphs will discuss the relevant features
of each of the three types.
Mechanical Steering. The simplest method of beam steering is to use a mechanism to reorient a
transducer (usually a circular aperture) to a predetermined set of orientations so as to capture the
required two-dimensional data set. This approach was dominant during the 1970s; however, in the last
15 years electronically steered systems have become, by far, the most popular and driven the mechan-
ical systems to near extinction. Mechanically systems usually use either a single-element transducer or
an annular array transducer. The former will have a fixed focus, while the latter does allow the focal
point to be moved electronically. A very interesting application for mechanical scanners today is that
of extremely low-cost systems (Richard, 2008) and the use in the acquisition of 3D data sets. In the
latter case, a conventional linear or curvilinear array is mechanically oscillated rapidly and a real-time
3D volume is acquired. The resulting surface-rendered images are sometimes referred to as 4D images,
time being the fourth dimension (GE, 2008).
There are a number of very attractive aspects to mechanical systems with their circular transduc-
ers. Among these are low cost and the ability to focus the sound beam electronically in all planes, in
other words, axisymmetrically. The low cost arises from the relatively low cost associated with the
mechanisms used to move the transducer in comparison to the multielement transducer arrays and
supporting electronics needed with electronic beam-steering. The ability to focus the acoustic energy
in all planes is a unique advantage since most mechanically steered systems use either single element–or
annular array–type transducers. With the annular arrays, one has the capability to move the focus
electronically in all planes as opposed to the electronically steered arrays that are usually rectangu-
lar in shape and will have electronic focusing only in one plane. The number of transducer elements
in an annular array is usually less than 12, typically 6 or 8. With electronically steered arrays, the
element count can go as high as 192 or more. As a consequence, the costs tend to be higher. Today,
mechanical scanners exist in niche markets such as intravascular imaging or in extremely low-cost
systems, or with 3D/4D scanners.
Some of the drawbacks associated with mechanical steering involve the inertia associated with
the transducer, the mechanism, and the fluid within the nosepiece of the transducer. The inertia
introduces limitations to the frame rate and clearly does not permit random access to look angles
as needed (the electronically steered approaches supply this capability). The ability to steer the
beam at will is important in several situations but most importantly in Doppler applications.
Further, electronic beam formation affords numerous advanced features to be implemented such
as the acquisition of multiple lines simultaneously and elimination of the effects due to variations
in speed of sound in tissue.

Steering by Element Selection. Another relatively low-cost beam-steering approach involves steering
of the beam by element selection. In this approach one doesn’t strictly steer the beam but rather
changes the location of its origin, thereby achieving coverage over a 2D tomographic slice. This
method is applied with both linear and curvilinear arrays. Figure 9.7 shows the application in the case
of curvilinear arrays. For this particular case, the 2D image will be sector shaped; with linear arrays it
will, of course, be rectangular. This is a relatively low-cost approach since aside from the multiplexing