ECO 3 EXPERT Ultrasound Diagnostic System
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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.
RF: (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),
RF = 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 RF 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).
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