User Manual
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FloPatch FP120
Rev 1.0
CONFIDENTIAL
UNCONTROLLED UNLESS OTHERWISE SIGNED AND/OR STAMPED.
Page 29 / 33
Appendix D
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Ultrasound Intensity and Safety
9.5
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 As Low As
Reasonably Achievable (ALARA) 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.
9.6
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).
9.7
Explanation of MI/TI
9.7.1
Mechanical Index (MI)
Scientific evidence suggests that mechanical or nonthermal bioeffects, such as cavitation, are
threshold phenomena, occurring only when a certain level of output is exceeded. The phenomena
is determined by acoustic pressure, spectrum, focus, transmission mode, and factors such as states
and properties of tissue and boundary. The threshold level varies depending on the tissue. The
potential for mechanical effects is thought to increase as the peak pressure increases but to
decrease as the ultrasound frequency increases. Although, no confirmed adverse effects on
patients and mammals caused by exposure at intensities typical of present diagnostic ultrasound
instruments have ever been reported, the threshold for cavitation is still undetermined.
The AIUM and NEMA (The Association of Electrical Equipment and Medical Imaging Manufacturers)
formulate mechanical index (MI) in order to indicate the potential for mechanical effects. The MI
is defined as the ratio of the peak-rarefaction acoustic pressure (should be calculated by tissue
acoustic attenuation coefficient 0.3 dB/cm/MHz to acoustic frequency).
9.7.2
TI (Thermal Index)
TI is an estimate of the increase in temperature that occurs in the region of the ultrasound scan.
It is defined as the ratio of the total acoustic power to the acoustic power required to rause the
tissue temperature by 1°C (1.8 °F).
There are 3 kinds of TI:
1.
TIS (Soft Tissue Thermal Index): It provides an estimate of potential temperature rise in soft
or similar tissues.
2.
TIB (Bone Thermal Index): It provides an estimate of potential temperature rise when the
ultrasound beam passes through soft tissue and focal region is in the immediate vicinity of
bone.
