Acoustic Output 15-5
Regions (except
eyes)
≤720 ≤ 190 ≤ 1.9
15.7.3 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
than0.3 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.
Similarly, the Mechanical Index was derived to indicate the relative possibility of mechanical (cavitation)
effects. The MI is based on the derated peak-rarefactional pressure and the center frequency of the
ultrasound wave. The actual peak-rarefactional pressure is affected by the actual attenuation caused by
tissue in the path between the probe and the focal point. Again, all solid tissues within the body have
higher attenuation than the prescribed 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 operator in implementing
ALARA at the time of the patient examination.
15.8 Measurement Uncertainty
The total estimated measurement uncertainty (where the total uncertainty includes the uncertainties in
hydrophone response, measurement, calculation, and positioning) are:
Acoustic Quantities Total Uncertainties (Standard)
Power 26.48% for non-scan modes; 6.03% for scan modes.
Frequency 0.22%
Pressure 13.01%
I
ta
26.48% for non-scan modes; 26.95% for scan modes.
I
pa
26.5%
Mechanical Index 13.01%
Non-scan Modes Scan-Modes
Total Uncertainty for TIS 26.48% 6.03%
Total Uncertainty for TIB 26.48% or 18.72% 6.03%
Total Uncertainty for TIC 26.48% 6.03%
To Next Page
Loading...
