Medical Design Briefs - August 2024 - 13

At the front, these annular arrays have an electrode that
covers the entire surface. An electronic focus control of annular
arrays is possible along the ultrasound beam axis by a
phase-shift control of the individual electrodes. Annular
arrays are, for example, used in handpieces for prostate tumor
ablation.
Curved phased arrays are the most complex sound transducers
used for therapeutic ultrasound. They are mainly used for thermal
or histotripsy-induced tissue ablation, like of breast cancer, in
the abdomen or for the therapy of tremor, as well as epilepsy. In
order to generate a directed ultrasound wave, piezoelectric elements
- a few to over several thousand elements depending on
the application - are arranged on a concave spherical dome and
excited synchronously (see Figure 7). This focuses the energy in
the shock wave. Commonly used shapes are plates or disks.
Although piezo plates make use of the entire space in the transducer,
they still lead to distortions in the sound field and a softening
of the sound focus. In contrast, piezo disks have a more homogenous
single sound field and are thus better suited for generating
the most accurate focusing possible. Special shapes such as pentagons
or hexagons are also suitable for lining a HIFU phased array
transducer with piezo elements. The electrodes of these sound
transducers can e.g. be Ag thick films or thins films of Cu or CuNi.
Structuring the electrode surfaces with wraparound contacts
is the best solution in order to lead any electrical contact
to the rear of the transducer. The numerous piezo elements
can individually be soldered or glued with stranded wires or
cables. Furthermore, customized flexible circuit boards can
be used for contacting these complex transducers. The sound
focus is controlled in most cases by using the phase offset of
the individual signals in the drive electronics used. Concave
HIFU phased array transducers are always used in conjunction
with a water bellow since this is the only way to overcome
the impedance jump between the piezo elements and the surrounding
environment with as little loss as possible.
Transducers in Minimally Invasive Catheters
Minimally invasive catheters use ultrasonic sound waves for
therapy, like for removal of blood vessel deposits or for targeted
tissue ablation for the treatment of atrial fibrillation of the
Fig. 9 - Miniaturized piezo elements can be integrated into minimally invasive catheters.
(Credit: PI)
heart (see Figure 8). In this case, individual piezo components,
such as miniaturized piezo tubes or plates, are used
(see Figure 9).
Freely defined shapes with dimensions of only a few millimeters
also offer high effective force in a minimal installation
space. For transducer applications, it is furthermore possible
to assemble several elements in a catheter by soldering or gluing
them together. It is important that the piezo elements do
not come into direct contact with tissues or bodily fluids. An
electrically insulating protective layer made of silicon or parylene,
for example, protects against an electrical short circuit
and prevents the piezo elements from corrosion.
Therapeutic ultrasound opens up numerous new fields of
application for the treatment of neurodegenerative, cardiovascular
or cancer diseases.
This article was written by M.Sc. Annemarie Oesterle, Segment
Marketing Manager, Medical Technology at PI Ceramic,
Physik Instrumente (PI) GmbH. For more information, contact
Stefan Vorndran, VP Marketing, Physik Instrumente LP,
832-3456 or e-mail stefanv@pi-usa.us, or visit www.pi-usa.us.
508Medical
Design Briefs, August 2024
Watch the video to learn more about piezoelectric ultrasound transducers for medical applications.
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Medical Design Briefs - August 2024

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