Page 284 - Biomedical Engineering and Design Handbook Volume 2, Applications

262 DIAGNOSTIC EQUIPMENT DESIGN

delay lines cannot compensate with a simple linear increment as in Fig. 9.8. It can be shown easily
that the differential time delay between channels can be determined from the law of cosines.
As the process of steering and focusing is repeated for a sequence of look angles, a sector-shaped
image data set is acquired. The line density available from phased array scanners is not as restricted
as with curvilinear arrays, but some limitations exist in certain systems due to the relatively large size
of delay increments available in tapped delay lines. Heterodyned systems and digital beam formers
have less limitations in this area.
All linear and curvilinear array systems have limitations or design constraints associated with the
existence of grating lobes that are due to leakage of acoustic energy in unwanted angles. It turns out
that for certain larger center-to-center array element spacings, there will be constructive interference at
look angles other than the main beam. This difficulty is particularly serious for the case of phased-array
systems due to the need for beam steering. It turns out that the grating lobes move with the steering
angle and can be brought into the visible region by the simple act of beam steering.
Grating lobes can be completely avoided by keeping the center-to-center spacing at one-half of
the wavelength at the highest contemplated operating frequency. (It turns out this is completely anal-
ogous to the familiar sampling theorem which states the temporal sampling has to occur at a fre-
quency that is twice that of the highest spectral component of the signal being processed (Steinberg,
1976). This has the drawback of forcing the use of a larger number of array elements and their pro-
cessing channels. This, and the expensive processing required for each channel, causes the phased-
array systems to be more expensive than the other types.

9.4.3 Harmonic Imaging
A recent development in the area of B-mode imaging is that of imaging of the harmonics generated
during propagation of acoustic waves in tissue (Averkiou, 1997; Jiang, 1998; Wells, 2006; Kollmann,
2007). While all the discussion so far has assumed that the propagation of these waves is linear, this is
actually not the case. There is a difference in the speed of sound in the compressional and rarefactional
parts of the acoustic pressure wave. As a consequence, the positive half of a propagating sine wave will
move faster than the negative half; this results in the formation of harmonic energy. An image formed
from such harmonics will be superior to that from the fundamental part of the spectrum due to reduced
reverberant energy and narrower main beam. The acceptance of this form of imaging has been so rapid
that in certain clinical applications (e.g., echocardiology), harmonic imaging is the default operating
mode. From the point of view of beam former design, there is relatively little that needs to be done
differently other than developing the ability to transmit at a lower frequency while receiving at twice
the transmit frequency.

9.4.4 Compression, Detection, and Signal-Processing Steps
The sequence of the processing steps between the beam former and scan conversion is different among
the various commercial systems, but the goals of the steps remain the same. The beam former output
will be a wideband RF, an IF, or a complex baseband signal, which will usually be bandpass filtered to
reduce out-of-band noise contributions. In systems with very wideband processing, frequency diversity
techniques (e.g., split spectrum processing) can be brought into play to try to reduce the impact of
coherent interference or speckle.
With most of today’s systems, there is a logarithmic compression of the amplified signal after
beam formation amplification. The goal of this is to emphasize the subtle gray level differences
between the scatterers from the various types of tissues and from diffuse disease conditions.
There are a number of ways that envelope detection has been implemented. In purely analog
approaches, simple full wave-rectification followed by a low-pass filtering has been shown to work
quite well. It is also possible to digitize the RF signals earlier in the processing chain, perform the
compression and detection processes digitally, and use quadrature detection to determine the signal
envelope.