Acoustic Power Principle
14-2
Users must take responsibility for the safety of patients and utilize ultrasound deliberately.
Deliberate use of ultrasound means that output power of ultrasound must be selected based
on ALARA.
It was verified that the MI and TI values didn’t exceed 1.0 in each exam mode.
14.4 Derated Ultrasonic Output Parameters
In order to determine the relevant Ultrasonic Output Parameters, a method is used which
allows for the comparison of ultrasound systems which operate at different frequencies and
are focused at different depths. This approach, called "derating" or "attenuating", adjusts the
acoustic output as measured in a water tank to account for the effect of ultrasound
propagation through tissue. By convention, a specific average intensity attenuation value is
used, which corresponds to a loss of 0.3 dB/cm/MHz. That is, the intensity of ultrasound will
be reduced by 0.3 dB/MHz for every centimeter of travel from the transducer. This can be
expressed by the following equation:
)10/3.0－(
10
zf
wateratten
c
II
××
×=
Where I
atten
is the attenuated intensity, I
atten
is the intensity measured in a water tank (at
distance z), fc is the center frequency of the ultrasound wave (as measured in water), and z is
the distance from the transducer. The equation for attenuating pressure values is similar
except that the attenuation coefficient is 0.15 dB/cm/MHz, or one-half the intensity coefficient.
The intensity coefficient is double the pressure coefficient because intensity is proportional to
the square of pressure.
Although the attenuation coefficient chosen, 0.3 dB/cm/MHz, is significantly lower than any
specific solid tissue in the body, this value was chosen to account for fetal examinations. In
early trimester ultrasound fetal examinations, there may be a significant fluid path between
the transducer and the fetus, and the attenuation of fluid is very small. Therefore the
attenuation coefficient was lowered to account for this case.
It was verified that the MI and TI values didn’t exceed 1.0 in each exam mode.
14.5 Measurement Uncertainty
Total Uncertainty for Mechanical Index 16.4 %
Non-scan Mode Scan-Mode Combined-Mode
Total Uncertainty for TIS 32.9% 5.1% 32.9% or 5.1% (A
aprt
>1)
33.4% (A
aprt
≤1)
Total Uncertainty for TIB 32.9% 5.1% 32.9% or 5.1%
Total Uncertainty for TIC 32.9% 5.1% 33.4%
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