Additional information regarding the concept of ALARA and the possible bioeffects of Ultrasound is
available in a document from the AIUM (American Institute of Ultrasound Medicine) title “Medical
Ultrasound Safety”.
10.4
)zf10/3.0( ××－
10.5
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:
wateratten
c
10II ×=
Where is the attenuated intensity, 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.
atten
I
water
I
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.
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% ( >1)
aprt
A
33.4% ( ≤1)
aprt
A
Total Uncertainty for TIB 32.9% 5.1% 32.9% or 5.1%
Total Uncertainty for TIC 32.9% 5.1% 33.4%
10.6 Parameters Affecting Acoustic Power
Acoustic Power is affected by transmission conditions (focus, drive frequency, voltage applied to
piezoelectric elements, etc.), scan conditions, and settings of the control panel, and preset menu.
10-2 Acoustic Output
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