Biomedical Engineering Reference
In-Depth Information
structure of the material. This alignment of molecules will cause the material to
change dimensions. This phenomenon is known as electrostriction. In addition, a
permanently polarized material such as quartz (SiO
2
), barium titanate (BaTiO
3
),
lead zirconate titanate (PZT), or piezo-polymers produce an electric field when the
material changes dimensions as a result of an imposed mechanical force. This phe-
nomenon is known as the piezoelectric effect. The active element of most acoustic
transducers used is a piezoelectric ceramic, which can be cut in various ways to
produce different wave modes. Piezoelectric ceramics have become the dominant
material for transducers due to their good piezoelectric properties and their ease of
manufacture into a variety of shapes and sizes. The thickness of the transducer is
determined by the desired frequency of the transducer by the relation
nk
(8.33)
f
=
x
x
l
where
f
x
is the frequency in the
x
direction,
n
is the harmonic order,
l
is the thick-
ness of the crystal in the
x
-direction, and
k
x
is known as the “frequency constant”
in the
x
-direction. A thin wafer element vibrates with a wavelength that is twice
its thickness. Therefore, piezoelectric crystals are cut to a thickness that is 1/2 the
desired radiated wavelength. The higher the frequency of the transducer, the thinner
the active element. The primary reason that high frequency contact transducers are
not produced is the necessary thin element, which is too fragile to handle. An active
area of research is developing novel materials with better performance.
Traditional 2D ultrasound visualization produces planar tomographic slices
through the object of interest. However, 3D images are formed by volume visu-
alization methods, significantly improving the spatial relationships. To create 3D
images, transducers containing piezoelectric elements arranged in 2D arrays are
developed. To aid in construction of complex arrays, simulations are performed to
analyze and optimize their beam pattern for a particular application. Array success
is based on the
radiation pattern,
which determines how well the ultrasound energy
is focused, the lateral resolution, and the contrast, and the
pulse-echo response
,
which determines the axial resolution.
However, high frequency arrays are very difficult to build because of fragility.
8.5.4.2 Effects of Ultrasonic Waves on Tissues
Ultrasound interacts with tissues in a few different ways including cavitation, chem-
ical, mechanical, and thermal interactions. Cavitation is unique to ultrasound due
to the mechanical nature of the ultrasound wave, which uses a change in a pressure
of the medium to propagate. This change in pressure can cause the tissue to collapse
when the mechanical wave cycles too far negative. It can also change the chemical
composition of a tissue. For example, exposure to ultrasound can cause the depoly-
merization (or breakdown of long chains of polymers). Ultrasound can also cause
oxidation, hydrolysis, and changes in crystallization. Ultrasound has been linked to
some increased permeability of tissues, due to the rise in temperature of the tissue
as the mechanical waves interact with the tissue at the different interfaces and add
