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Traditional 510(k) of Fetal & Maternal Monitor
F15 Series Fetal & Maternal Monitor User Manual
Appendix 4 Ultrasound Intensity and Safety
A4.1 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.
A4.2 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).
A4.3 Explanation of MI/TI
A4.3.1 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.
A4.3.2 TI (Thermal Index)
Heating of tissues is caused by absorption of ultrasound when the ultrasound energy is applied. The
temperature rise is determined by the acoustic intensity, exposed area and thermophysical properties
of the tissue.
In order to indicate the potential for temperature rise caused by thermal effects, the AIUM and NEMA
formulate thermal index (TI). It is defined as the ratio of the total acoustic power to the acoustic power
required to raise the tissue temperature by 1º C (1.8º F).
According to different thermophysical properties of the tissue, TI is divided into three kinds: TIS, TIB
and TIC.
TIS (Soft Tissue Thermal Index): It provides an estimate of potential temperature rise in soft or similar
tissues.
014_14.1_F15_Series_User_Manual
MI =
P
r, α
f
× C
MI
awf
C
= 1 (MPa / MHz )
MI
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Ultrasound Intensity and Safety
164／202
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