Safety
The following results are contained in the AIUM-report too:
•
Other acting mechanisms
- no other acting mechanisms for bioeffects are known, that are caused
by exposure to diagnostic ultrasound.
•
Epidemiology
- in more than 25 years of extensive clinical ultrasound application no harmful
effects due to exposure to diagnostic ultrasound have become known.
•
Bioeffects in vivo with mammals
- no significant bioeffects in vivo independently confirmed could
be determined under the following conditions:
•
Unfocussed exposure with an SPTA-intensity of less than 100 mW/cm2 in an exposure period of
less than 500 seconds (measured in water).
•
Focused exposure with an SPTA-intensity of less than 1 W/cm2 in an exposure period of less than
50 seconds (measured in water).
•
Product from intensity (measured in water) and time of exposure less than 50 joule/cm2.
2.13.2 Intensities Measured in Water and recalculated In Situ
All intensity parameters are determined by measurement in water. As water does not absorb the
acoustic energy, these measurements in water represent the most unfavorable value. In biological
tissue however the acoustic intensity is absorbed. The "real" value in a given position depends on the
amount and type of tissue through which the ultrasound beam passes and on the ultrasound frequency.
The value in tissue (
in situ
) can be approximately determined with the following formula:
in situ = water [ e -(0.23dlf) ]
Whereas:
in situ =
value in situ
water
=
value in water
e
=
2.7183
d
=
attenuation
coefficient
l
=
distance from skin surface to measuring depth (cm)
f
=
mean frequency of combination probe/system/operation
mode (MHz)
tissue
brain
heart
kidney
liver
muscle
d
(dB/cm/MHz)
0.53
0.66
0.79
0.43
0.55
As the ultrasound generally crosses tissue layers of different thickness and different types on its way
through the body during an examination, it is very difficult to estimate the real intensity
in situ
.
For reports generally an impedance coefficient of 0.3 dB/cm/MHz is assumed. The value in situ that is
generally indicated in reports is calculated according to the following formula:
in situ (recalculated ) = water [e -(0.69lf) ]
As this value must not be considered as the real intensity in situ, the term "recalculated" is used
hereunder.
Voluson
®
730 - Instruction Manual
105838 Rev. 3
2-11
