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Instructions For Use
EN
Appendix 3 Ultrasound Intensity and Safety
A3.1 Ultrasound in Medicine
The use of diagnostic ultrasound has proved to be a valuable tool in medical practice.
Given its known benefits for non-invasive investigations and medical diagnosis,
including investigation of the human fetus, the question of clinical safety with regards to
ultrasound intensity arises.
There is no easy answer to the question of safety surrounding the use of diagnostic
ultrasound equipment. Application of the ALARA (As Low As Reasonably Achievable)
principle serves as a rule-of-thumb that will help you to get reasonable results with the
lowest possible ultrasonic output.
The American Institute of Ultrasound in Medicine (AIUM) states that given its track
record of over 25 years of use and no confirmed biological effects on patients or
instrument operators, the benefits of the prudent use of diagnostic ultrasound clearly
outweigh any risks.
A3.2 Ultrasound Safety and the ALARA Principle
Ultrasound waves dissipate energy in the form of heat and can therefore cause tissue
warming. Although this effect is extremely low with Doppler, it is important to know
how to control and limit patient exposure. Major governing bodies in ultrasound have
issued statements to the effect that there are no known adverse effects from the use of
diagnostic ultrasound, however, exposure levels should always be limited to As Low As
Reasonably Achievable (the ALARA principle).
A3.3 Explanation of MI/TI
MI (Mechanical Index)
Cavitations will be generated when ultrasound wave passes through and contacts
tissues, resulting in instantaneous local overheating. This phenomenon is determined
by acoustic pressure, spectrum, focus, transmission mode, and factors such as states
and properties of the tissue and boundary. This mechanical bioeffect is a threshold
phenomenon that occurs when a certain level f ultrasound output is exceeded. The
threshold is related to the type of tissue. Although no onfirmed adverse mechanical
effects on patients or mammals caused by exposure at intensities typical of present
diagnostic ultrasound instruments have ever been reported, the threshold for cavitation
is still undetermined. Generally speaking, the higher the acoustic pressure, the greater
the potential for mechanical bioeffects; the lower the acoustic frequency, the greater
the potential for mechanical bioeffects.
The AIUM and NEMA formulate mechanical index (MI) in order to indicate the potential
for mechanical effects. The MI is defined as the ratio of the peak-rarefactional acoustic
pressure (should be calculated by tissue acoustic attenuation coefficient 0.3dB/cm/
MHz) to the acoustic frequency.
C
MI
= 1 (MPa / MHz )
f
awf
<?> C
MI
MI = Pr,
α
