User's Manual

Clarius Ultrasound Scanner - HD3 Scanners Safety Topics
revision 1 47
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
undetermined, and the definition of “reasonable” is left to the judgment and insight of
qualified personnel. No set of rules can be formulated that would be sufficiently complete to
dictate the correct response to every circumstance. By keeping ultrasound exposure as low as
reasonably achievable as you obtain diagnostic images, you can minimize ultrasonic bioeffects.
Output display indices are designed to provide more quality information, to help guide the
sonographers using ultrasound technology, in applying the ALARA principle. Some variables
that affect the way output display indices can be used to implement the ALARA principle:
• index values
• body size
• location of the bone relative to the focal point
• attenuation in the body
• ultrasound exposure time (an especially useful variable, as it is controlled by you)