Q9 Digital Color Doppler Ultrasound System
12
and MI
≤
0.23. Track-3 gives the user the freedom to increase the output acoustic power for a specific exam, and still
limit output acoustic power within the global maximum de-rated Ispta
≤
720 mW/cm
2
under an Output Display
Standard.
For any diagnostic ultrasonic systems, Track-3 provides an Output Indices Display Standard. The diagnostic
ultrasound systems and its operation manual contain the information regarding an ALARA (As Low As
Reasonably Achievable) education program for the clinical end-user and the acoustic output indices, MI and TI. The
MI describes the likelihood of cavitations, and the TI offers the predicted maximum temperature rise in
tissue as a result of the diagnostic examination. In general, a temperature increase of 2.5°C must be
present consistently at one spot for 2 hours to cause fetal abnormalities. Avoiding a local temperature rise
above 1°C should ensure that no thermally induced biologic effect occurs. When referring to the TI for
potential thermal effect, a TI equal to 1 does not mean the temperature will rise 1 degree C. It only means an
increased potential for thermal effects can be expected as the TI increases. A high index does not mean that bioeffects
are occurring, but only that the potential exists and there is no consideration in the TI for the scan duration, so
minimizing the overall scan time will reduce the potential for effects. These operator control and display features shift
the safety responsibility from the manufacturer to the user. So it is very important to have the Ultrasound systems
display the acoustic output indices correctly and the education of the user to interpret the value appropriately.
R
F
: (De-rating factor)
In Situ intensity and pressure cannot currently be measured. Therefore, the acoustic power measurement is normally
done in the water tank, and when soft tissue replaces water along the ultrasound path, a decrease in intensity is
expected. The fractional reduction in intensity caused by attenuation is denoted by the de-rating factor (RF),
R
F
= 10
(-0.1 a f z)
Where a is the attenuation coefficient in dB cm-1 MHz-1, f is the transducer center frequency, and z is the distance
along the beam axis between the source and the point of interest.
De-rating factor R
F
for the various distances and frequencies with attenuation coefficient 0.3dB cm-1 MHz-1 in
homogeneous soft tissue is listed in the following table. An example is if the user uses 7.5MHz frequency, the power will
be attenuated by .0750 at 5cm, or 0.3x7.5x5=-11.25dB. The De- rated Intensity is also referred to as ‘.3’ at the end (e.g.
Ispta.3).
Distance
Frequency (MHz)
(cm)
1
3
5
7.5
1
0.9332
0.8128
0.7080
0.5957
2
0.8710
0.6607
0.5012
0.3548
3
0.8128
0.5370
0.3548
0.2113
4
0.7586
0.4365
0.2512
0.1259
5
0.7080
0.3548
0.1778
0.0750
6
0.6607
0.2884
0.1259
0.0447
7
0.6166
0.2344
0.0891
0.0266