Biomedical Engineering Reference
In-Depth Information
can be formed into thin membranes that can be used as pressure
sensors that disturb an ultrasound field only minimally.
applications, such as HIFU, the gap is lined with an insulating
layer to prevent short circuiting during operation. On the appli-
cation of voltage, the top membrane is attracted to the substrate
by an electrostatic force. A mechanical restoring force resists this
movement. When high-frequency sinusoidal voltage is applied,
an ultrasound pressure wave is produced. Conversely, the appli-
cation of pressure to the surface at a constant bias voltage leads
to a capacitance change and a current whose amplitude depends
on the frequency, the bias voltage, and the capacitance. An elec-
tric field in excess of 10
8
V cm
−1
must be maintained across the
gap. Pressure amplitudes of ~1 MPa peak to peak have been
achieved at the transducer surface.
CMUTs provide a number of advantages over more conven-
tional transducer technologies. They have a wide bandwidth
and are not subject to self-heating. CMUT arrays are low cost
and lend themselves to miniaturization. Basic microlithography
techniques allow considerable flexibility in array design, and
they have been used, for example, in catheter-based devices.
5.2.1 piezocomposite transducers
While PZT materials can be used to provide reliable single-
element transducers, their use for phased array transducers
involves cutting grooves into the ceramic, leaving residual
material of the required shape and size. While such transduc-
ers are usually highly efficient and capable of operating at high
power, they are often fragile, and difficult to manufacture as
large area arrays. Their construction is liable to cause signifi-
cant lateral vibration, which sets up parasitic waves, leading
to undesirable hotspots, and they are, by nature, narrow band
devices.
In order to circumvent these problems, the use of piezocom-
posite materials has been explored. These are made by inter-
leaving precut shaped pillars with passive polymer or epoxy
host matrix compound (Smith 1989, Berriet and Fleury 2007,
Chapelon et al. 2000). This gives considerable flexibility in shape,
size, and array geometry. The infill has a low glass transition
temperature. The 1-3 structure used in most piezocomposite
transducers reduces lateral modes and enhances thickness mode
vibration. There are a number of other advantages of this mate-
rial. The embedding matrix structure can be shaped into a bowl,
thus facilitating the production of focused beams. The backing
has a low impedance, and the composite material has low elec-
trical and mechanical losses, thus reducing transducer heating.
The composite has a high Curie temperature.
5.2.3 transducer Geometries
A variety of transducer geometries have been used for therapy
ultrasound, depending on the therapeutic application. Plane
discs are used when energy deposition starting from the skin
surface is required, as, for example, in some physiotherapy treat-
ments. Where energy deposition is required more locally such as
selectively, at depth, focused beams are required. Here, focusing
may be achieved in a number of ways. Plane transducers may be
combined with appropriate lenses, single piezoelectric elements
may be shaped to form spherical bowls, or multielement-phased
arrays may be driven to provide electronic focusing and beam
steering. Figure 5.2 shows a multielement spherical bowl trans-
ducer with multiple connections that allow each element to be
driven independently.
For therapy purposes, the ultrasound transducer element is
usually held at the rim and has an air backing. This provides
the greatest efficiency and a high Q. Imaging elements, on the
other hand, have a block of backing material designed to damp
out any resonances and provide a more uniform frequency
response in reception mode, and to allow short pulse transmis-
sion. The amount of energy transmitted into the medium in
front of the transducer (the coupling medium) depends on the
relative acoustic impedance of the transducer front face and the
medium. Acoustic impedance
Z
is the product of the density and
the speed of sound (
Z
=
ρ
c
) (see Section 5.3).
5.2.2 CMUts
Capacitive micromachined ultrasound transducer arrays (CMUTs)
rely on microelectromechanical systems (MEMS) technology.
These arrays are formed with many adjacent cells arranged in
parallel. Originally developed for industrial use in air, their adap-
tation to use in liquids has led to potential medical applications,
initially for imaging but more recently for therapies such as high
intensity focused ultrasound (HIFU; Haller and Khuri-Yakub
1994, Khuri-Yakub and Oralkan 2011, Wong et al. 2008). A single
CMUT cell is a capacitor formed from a silicon substrate. The
principle is shown in Figure 5.1.
A thin membrane is placed over a submicron vacuum gap.
A metal layer is placed on top of the membrane to provide one
electrode. The second electrode is provided by the conductive
substrate that forms the base of the vacuum gap. For high-power
Insulating layer
Metallized membrane
(top electrode)
~1 µm
vacuum gap
Insulating layer
Silicon substrate
FIGURE 5.1
A CMUT cell.
