Biomedical Engineering Reference
In-Depth Information
5
The Physics of Ultrasound
Energy Sources
5.1 Introduction ...............................................................................................................................75
5.2 Ultrasound Transduction .........................................................................................................75
Piezocomposite Transducers ∙ CMU Ts ∙ Transducer Geometries ∙ Plane
Transducers ∙ Focused Fields
5.3 Acoustic Field Propagation ......................................................................................................79
Reflection and Refraction ∙ Non-Linear Propagation
5.4 Interactions of Ultrasound with Tissue .................................................................................81
Thermal Effects ∙ Mechanical Effects
5.5 Characterization and Calibration ...........................................................................................85
Introduction ∙ Field Parameters and Their Definitions ∙ Measurement Methods and Quality
Assurance
References .............................................................................................................................................. 90
Victoria Bull
Institute of Cancer Research
Gail R. ter Haar
Institute of Cancer Research
5.1 Introduction
5.2 Ultrasound transduction
Many people think of medical ultrasound as being used solely
for clinical diagnosis, and they are unaware of its therapeutic
potential unless they are sportsmen who have received ultra-
sound treatment for soft tissue injuries. In fact, the use of
therapeutic ultrasound predates its imaging applications. As
ultrasound travels through tissues, a number of physical mecha-
nisms take place. These include thermal effects arising from
energy absorption and mechanical effects that are caused by the
passage of an acoustic pressure wave through tissue. Both these
classes of mechanism can be harnessed for therapeutic benefit.
A range of biological responses may be sought from the bio-
effects achieved. For physiotherapy, where the goal is functional
modification for therapeutic reasons, the changes may be revers-
ible or irreversible. In cancer therapy, ultrasound may be used
to induce temperatures in the hyperthermic range (43-50°C)
for use in conjunction with radio or chemotherapy or may be
used on its own to create thermally ablated volumes when the
temperature is raised rapidly above 55°C, or, as in histotripsy
and drug delivery, to induce cavitation. As will be discussed in
the following section, the difference between these regimes lies
principally in the amount of power deposited in the tissue and
the mode of delivery.
The common feature of most ultrasound transducers is that they
are piezoelectric devices. Application of an electric field leads
to a change in the thickness of these materials, whereas an inci-
dent pressure pulse leads to an imbalance of electric charge and,
thus, voltage generation (inverse piezoelectric effect; Ballato 1995,
Silk 1984). Naturally occurring piezoelectric materials that are
used to generate ultrasound include quartz and lithium niobate.
Ferroelectric materials with a microcrystalline structure that
can be rendered permanently piezoelectric by the application of
a strong electric field are also used. Quartz crystals were used in
the early days of medical ultrasound, but as these have narrow
resonant peaks they have largely been replaced by ferroelectric
ceramics such as lead zirconate titanate (PZT) that have a wider
bandwidth. For therapy applications, the low loss material PZT4
is commonly used, whereas PZT5, which has higher sensitivity, is
chosen for imaging (O'Donnell et al. 1981, Foster et al. 1991, Ballato
1995). Piezoceramic probes have been used both as therapy ultra-
sound sources and as hydrophones for detecting and mapping
ultrasound fields. Polyvinyledene (di-)fluoride (PVDF) is a syn-
thetic polymer that becomes piezoelectric when polarized using
high electric fields at high temperature (Ohigashi 1976, Lewin and
Schafer 1988). While PVDF is a poor transmitter of ultrasound, it
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