Appendix 3 Ultrasound Intensity And Safety; A3.1 Ultrasound In Medicine; A3.2 Ultrasound Safety And The Alara Principle; A3.3 Explanation Of Mi/Ti - EDAN SD5 User Manual

SD5/SD6 Ultrasonic TableTop Doppler User Manual Ultrasound Intensity and Safety

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

A3.3.1 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
of ultrasound output is exceeded. The threshold is related to the type of tissue. Although no
confirmed 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.
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