Medical Design Briefs - August 2024 - 10

Therapeutic Ultrasound
Left: Fig. 1 - Frequency
range of sound. (Credit: PI))
Infrasound
Hearing range of the human ear
Ultrasound
Speech
Compression
16
10
100
300
1.000
3.150
20.000
10.000
Frequency (f) (Hz)
Rarefaction
Rarefaction
Rarefaction
5000 °C
2000 atm
Right: Fig. 2 - Cavitation in liquids caused
by ultrasonic waves. (Credit: PI)
Bubble
forms
Bubble
grows
Bubble
reaches
unstable
size
Bubble
collapses
violenty
Compression
Compression
they finally implode. In this case, very high forces and pressures
are released in the form of shock waves and extremely high temperatures
of several thousand °C, as well as flashes of light can
occur temporarily (see Figure 2).
These physical effects of cavitation are used for various medical
applications. For example, the stable cavitation is applied
in medical imaging when gas-filled microspheres are used as an
ultrasound contrast medium. In addition to cavitation, power
ultrasound can cause changes in material or even destroy it,
which is used, for example, in medical applications, but also in
industrial material processing or ultrasonic cleaning.
Therapeutic Ultrasound Methods
Ultrasound opens up new and innovative therapeutic possibilities
that improve or even substitute many established procedures:
Minimally invasive, and gentle treatment methods with
improved therapy success and fewer side effects, can be realized
using piezo components.
Tissue Ablation with Ultrasound
It is possible to selectively remove certain tissue areas with
the help of focused ultrasound - and with a completely contactless
procedure (see Figure 3). Ultrasound-operated tools
make this minimally- or noninvasive surgery technique, which
is often also called incisionless, possible today.
The ultrasound-based ablation of tissue, for example the
removal of tumors in the prostate or uterus, is carried out
extracorporeal and thus noninvasive. For this form of therapy,
high-intensity focused ultrasound (HIFU) is projected
into the body with the help of piezoelectric elements. The
acoustic waves lead to a thermal ablation since the absorption
of the ultrasonic wave energy generates heat above 42
°C due to friction, so that the proteins of the cell structures
denaturize and entire tissue areas within the focus of the
sound waves coagulate. HIFU can also be used to treat conditions
such as tremor or epilepsy without medication by targeting
specific brain structures with heat. Monitoring in
these cases is performed simultaneously by magnetic resonance
imaging (MRT).
Fig. 3 - Ablation of tissue structures with HIFU. (Credit: PI)
10
An alternative method for tissue ablation is histotripsy. In
this case, HIFU generates cavitation bubbles, whose high mechanical
energy causes cells to burst without any significant
heating of the affected tissue.
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Medical Design Briefs, August 2024
In this case, therapeutic ultrasound refers to a group of
methods in which ultrasound is not only a means of diagnosis
but also the core element of therapy itself. By using piezoceramic
ultrasonic transducers, minimally or noninvasive and
gentle treatment methods, with improved therapy success,
can be realized. These include methods which use focused,
but also unfocused, ultrasound - for example tissue ablation,
targeted drug delivery, the ultrasound-assisted dissolution of
blood clots (thrombolysis), or the fragmentation of kidney
stones (lithotripsy). Further application examples using therapeutic
ultrasound are cartilage therapy, the administration
of drugs through the skin and cosmetic treatments.
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Medical Design Briefs - August 2024

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