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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 for them exists, and there is no consideration in the TI for the scan duration, so minimising
the overall scan time will reduce the potential effects. These operating and display features shift the responsibility for
safety from the manufacturer to the user. It is thus very important to make sure that the ultrasound systems display
the acoustic output indices correctly and that users are trained so as to be able to interpret the values appropriately.
RF: (Derating factor)
In-situ intensity and pressure cannot be measured concurrently. Therefore, the acoustic power measurement is
normally performed in the water tank, and when the 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 derating factor
(RF).
RF= 10 (-0.1 a f z)
Where a is the attenuation coefficient in dB cm-1 MHz-1, f is the transducer centre frequency, and z is the distance
along the beam axis between the source and the point of interest.
The derating factor RF for the various distances and frequencies with attenuation coefficient 0.3 dB cm-1 MHz-1 in ho-
mogeneous soft tissue is listed in the following table. For example, if the user uses a frequency of 7.5MHz, the power will
be attenuated by 0.0750 at 5 cm, or 0.3x7.5x5=-11.25 dB. The derating intensity is also referred to using a '.3' at the end
(e.g. ISPTA.3).