Konica Minolta SONIMAGE HS2 - 9.8 Acoustic Output Parameters and Effects

281
Chapter 9
z
Derated Ultrasonic Output Parameters
In order to determine the relevant Ultrasonic Output Parameters, a method is used which allows for the compari-
son of diagnostic ultrasound systems which operate at di󰀨erent frequencies and focus at di󰀨erent depths.
This approach, called "derating" or "attenuating", adjusts the acoustic output as measured in a water tank to ac-
count for the e󰀨ect of ultrasound propagation through tissue. By convention, a specic average intensity attenua-
tion 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 from the transducer. This
can be expressed by the following equation:
I
atten
=
I
water
× 10
(-0.3/10 × fc × z)
Where I
atten
is the attenuated intensity, I
water
is the intensity measured in a water tank (at distance z), f
c
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 coe󰀩cient is 0.15 dB/cm/MHz,
or one-half the intensity coe󰀩cient. The intensity coe󰀩cient is double the pressure coe󰀩cient because intensity is
proportional to the square of pressure.
Although the attenuation coe󰀩cient chosen, 0.3 dB/cm/MHz, is signicantly lower than any specic solid tissue
in the body, this value was chosen to account for fetal examinations. In early trimester ultrasound fetal examina-
tions, there may be a signicant uid path between the transducer and the fetus, and the attenuation of uid is
very small.
Therefore the attenuation coe󰀩cient was lowered to account for this case.
z
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 Acoustic Output Limits for Track 3 (Attenuated Values)
Application I
spta.3
(mW/cm
2
)
I
sppa.3
≤190 (W/cm
2
) or MI≤1.9
All regions (except eyes) 720
In addition, the TI (TIS, TIB and TIC) is designed not to exceed 6.0 for any mode.
z
Di󰀨erences between displayed and "actual" Thermal and Mechanical e󰀨ects
The system displays the Thermal Index (TI) and Mechanical Index (MI). 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 user whether a particular setting of the system increases or decreases the possibility of Thermal or Me-
chanical e󰀨ect.
More specically, they were designed to assist in the implementation of the ALARA principle.
Refer to "9.7 Caution".
They also indicate the changes in acoustic output when the user changes given settings of the system.
Refer to "9.6 Imaging Functions that Change Acoustic Output".
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 the user, 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 that 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 are also significant simplifications made for the thermal
properties of tissue.
Therefore, scanning through highly perfused tissue, such as the heart or vasculature, will produce signicantly
less thermal e󰀨ect than that suggested by the Thermal Index.
Similarly, the Mechanical Index was derived to indicate the relative possibility of mechanical (cavitation) e󰀨ects.
The MI is based on the derated peak rarefactional pressure and the center frequency of the ultrasound wave.
Refer to "9.2 MI/TI Display Description".
The actual peak rarefactional pressure is a󰀨ected by the actual attenuation caused by tissue in the path between
the transducer and the focal point. Again, all solid tissues within the body have higher attenuation than the pre-
scribed 0.3 dB/cm/MHz value, and therefore, the actual peak rarefactional pressure will be lower.
Further, the actual peak rarefactional pressure will change depending upon the region of the body being scanned.
For these reasons, the TI and MI displays should only be used to assist the user in implementing ALARA at the
time of the patient examination.
9.8 Acoustic Output