Medical Design Briefs - August 2024 - 12

Therapeutic Ultrasound
Fig. 6 - Annular phased transducer. (Credit: PI)
Fig. 8 - Intravascular lithotripsy with ultrasound. (Credit: PI)
HIFU Transducer with Fixed Focus Point. Transducers with a
device-specific fixed focus point (see Figure 5) are mainly used in
ultrasonic handpieces, like for cosmetic skin treatments or cartilage
therapy. The focus depth, which is usually in the range of a
few centimeters, is determined by the shape of the transducer. To
generate focused ultrasound, piezoceramic focus bowls can be
used. Their radius determines the ultrasound focus depth. Depending
on the application, ferroelectric soft or hard ceramics
such as PIC255, PIC155, and PIC151, or PIC181 are used.
The piezoceramic focus bowls can be customized from 2 to
130 mm outer diameter, in various designs and different quantities.
Special shape variations are possible - like collar, flattening,
phases, spheres, or application-specific designs. The electrode
of such a focus bowl can be provided with one or more
wraparound contacts in order to facilitate installation and cable
routing in the end device.
HIFU Transducer with Steerable Focus Point (Phased ArFig.
7 - Schematic representation of a curved HIFU phased array transducer.(Credit: PI)
applied for kidney stone fragmentation. The advantage here
lies in the contactless execution of therapeutic procedures in
patients. Ultrasound-operated medical devices are applied extracorporeal,
and are, therefore, even less stressful for the patient
than established minimally invasive procedures.
Which Transducers Are Suitable for Therapeutic
Ultrasound?
The selection of a suitable transducer for therapeutic ultrasound
strongly depends on the specifications of the potential device,
such as focus depth, focus size, maximum electrical voltage,
installation space, sound pressure and ambient conditions like
strong magnetic fields. In the following, various transducer types
that can be used for therapeutic ultrasound are explained.
12
rays). In order to move an ultrasound focus, so-called phased
array transducers are used. Such ultrasonic transducers are
usually made of many piezoelectric elements (array) that can
be controlled individually. Using a time-shift electrical control
(phase), interferences occur in the sound field, creating
a defined and steerable focus point. The motion of the ultrasound
focus in these phased array transducers can be controlled
by appropriate electronics. Transducers typically used
for therapeutic ultrasound are annular phased arrays or
curved phased arrays.
Annular phased arrays are group of transducers made of
a piezoelectric ceramic disk, ceramic plate, or focus bowl.
The applied electrode can be chosen from various material
systems, such as Ag thick films, or thin films of Cu or CuNi.
Annular phased arrays have a structured electrode that can,
for example, be structured using a laser. In this case, the
electrode is divided into concentric surfaces in the shape of
a central ring with surrounding rings. The width of the electrode
rings diminishes from the inner to the outer ring
since all electrode segments have the same surface area (see
Figure 6).
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Medical Design Briefs, August 2024
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Medical Design Briefs - August 2024

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