Table of Contents
Advertisement
20.7 Reminder
As a reminder, Always Practice ALARA. The display gives information in real time
and therefore shows the current acoustic exposure rates to the tissue or structure being
insonated.
For inquiries and information, contact your Toshiba representative. Additionally, as
further information becomes available, we will do our utmost to keep you informed.
20.8 Ultrasonic Output Power and Acoustic Output
(1) 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:
Where I
atten
(at distance z), f
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.
(2) Acoustic Output Limits
The derating (or attenuated) approach was incorporated into the FDA Acoustic
Output Limits, as listed below. The acoustic output level is designed to fall below
these limits.
FDA Maximum Acoustic Output Limits for Track 3 (Attenuated Values)
Application
All regions
(except eyes)
In addition, the TI (TIS, TIB and TIC) is designed not to exceed 6.0.
I
atten
is the attenuated intensity, I
is the center frequency of the ultrasound wave (as measured in
c
Ispta.3
(mW/cm
No. 2B730-973EN*D
20-7
×
×
=
(
. 0
/ 3
10
f
z
)
I
x
10
c
water
is the intensity measured in a water tank
water
Isppa.3
2
2
)
(W/cm
)
720
<190
MI
or
<1.9
Advertisement
Table of Contents
