Clarius Scanner HD3 Safety Topics
revision 9 49
Mechanical (Non-Thermal)
Mechanical bioeffects are threshold phenomena, such as cavitation, that occur when the
output exceeds a certain level. This threshold varies by tissue type.
Cavitation is the interaction of ultrasound with gas bubbles, causing rapid and potentially large
changes in bubble size. These bubbles originate within materials at locations termed
nucleation sites, the exact nature and source of which are not well understood in a complex
medium such as tissue or blood. The change in bubble size may increase temperature and
pressure within the bubble, causing mechanical stress on surrounding tissues, precipitate fluid
microjet formation, and generate free radicals. Gas-containing structures, such as lungs, are
most susceptible to the effects of acoustic cavitation; however, such higher frequency
ultrasounds do not provide sufficient time for significant bubble growth; therefore, cavitation
is unlikely to occur under these circumstances. Factors that produce cavitation include
pressure (compressional, rarefactional), frequency, focused/unfocused beam, pulsed/
continuous waves, degree of standing waves, boundaries, and the nature and state of material.
Scientific evidence suggests that the onset of transient cavitation is a threshold phenomenon.
There's a combination of rarefactional pressure values, ultrasonic frequency, and cavitation
nuclei that are required for inertial cavitation to occur. If inertial cavitation is a threshold
phenomenon, then exposure to pressure levels below the threshold will never induce such
events, regardless of the length of exposure.
There are two categories of cavitation:
• Stable: Stable cavitation is associated with vibrating gas bodies. In stable cavitation, a gas
body oscillates or pulsates continuously around its equilibrium size. As the oscillations
become established, the liquid-like medium around the gas body begins to flow or stream;
we call this microstreaming. Microstreaming has been shown to produce stress sufficient
to disrupt cell membranes.
• Inertial: During inertial (transient) cavitation, pre-existing bubbles or cavitation nuclei
expand because of the rarefactional pressure of the ultrasonic field and then collapse in a
violent implosion. The whole process takes place in a time span on the order of
microseconds. The implosion can produce huge local temperature rises that may be
thousands of degrees Celsius and pressures equal to hundreds of atmospheres, all in a
volume of less than 1 µm
3
. The implosion can damage cells and tissue, ultimately leading
to cell death. In addition, bubble implosion can generate highly reactive chemical species.
All of these effects, microstreaming, implosion, and generation of reactive chemicals,
occur in a very small space around the bubble, affecting only a few cells.
Exposure of the lung can produce small, localized hemorrhages under some conditions in
laboratory animals. These lesions resolve naturally and are without lasting effects in normal
subjects, but their possible significance in compromised individuals has not been studied.
ALARA Principle
The guiding principle for the use of diagnostic ultrasound is defined by the ALARA (as low as
reasonably achievable) principle. The threshold for diagnostic ultrasound bioeffects is
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