Acoustic Output 14-5
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
water
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.
14.7.2 Limits of Acoustic Output
In accordance with the FDA Track 3 requirements, the derating (or attenuated) approach
was incorporated into the FDA Acoustic Output Limits, as listed below. The maximum
acoustic output level from any transducer in any operating mode is expected to fall below
these limits.
FDA Maximum Acoustic Output Limits for Track 3 (Attenuated Values)
14.7.3 Differences between Actual and Displayed MI
and TI
In operation, the system will display to the operator the Acoustic Output Parameters Thermal
Index, TI, or Mechanical Index, MI (or sometimes both parameters simultaneously). These
parameters were developed as general indicators of risk from either thermal or mechanical
action of the ultrasound wave. They serve to indicate to the operator whether a particular
setting of the system increases or decreases the possibility of Thermal or Mechanical effect.
More specifically, they were designed to assist in the implementation of the ALARA principle.
As an operator changes a given system control, the potential effect of the change in output
will be indicated. However, the Thermal Index is not the same as temperature rise in the
body, for several reasons. First of all, in order to provide a single display index to you, a
number of simplifying assumptions had to be made. The biggest assumption was the use of
the attenuating formula described above, which is much lower than the actual value for most
tissues within the body. Scanning through muscle or organ tissue, for example, will produce
much higher attenuation than 0.3 dB/cm/MHz. There were also significant simplifications
made for the thermal properties of tissue. Therefore, scanning through highly perfused tissue,
such as the heart or vasculature, will produce significantly less thermal effect than that
suggested by the Thermal Index.
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