15-4 Acoustic Output
Controls that indirectly control output
Controls that are receiver controls
Direct controls
It is possible to control, if necessary, the acoustic output with the “A.power” item on the touch
screen or the corresponding knob at the bottom of touch screen. In this case, the maximum value
of the acoustic output never exceeds an MI of 1.9 and an I
SPTA.3
of 720 mW/cm
2
in any mode of
operation.
Indirect controls
The controls that indirectly affect output are the many imaging parameters. These are operating
modes, frequency, focal point positions, overall depth, and PRF.
The operating mode determines whether the ultrasound beam is scanning or non-scanning.
Thermal bioeffect is closely connected to M mode, Doppler and Color mode. Acoustic attenuation
of tissue is directly related to probe frequency. The focal point is related to active aperture of probe
and beam width. For the higher PRF (pulse repetition frequency), the more output pulses occur
over a period of time.
Receiver controls
The receiver controls (for example, gain, dynamic range, and image post-processing, etc.) won’t
affect output. These controls should be used, when possible, to improve the image quality before
using controls that directly or indirectly affect output.
15.7 Acoustic Output
15.7.1 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 probe. 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
probe. 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 probe and the fetus,
and the attenuation of fluid is very small. Therefore the attenuation coefficient was lowered to account
for this case.
15.7.2 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 probe in any operating mode is expected to fall below these limits.
FDA Maximum Acoustic Output Limits for Track 3 (Attenuated Values)
Application I
spta.3
(mW/cm
2
) I
sppa.3
(W/cm
2
) or MI
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