ultrasound system when power is turned on, when new patient data is entered
into the system data base, or when an application change occurs.
The decision as to which of the three thermal indices to display should be based
on the following criteria:
• Appropriate index for the application: TIS is used for imaging soft tissue,
TIB for a focus at or near bone, and TIC for imaging through bone near the
surface, as in a cranial exam.
• Mitigating factors that might create artificially high or low thermal index
readings: location of fluid or bone, or blood flow. For example, is there a
highly attenuating tissue path so that the actual potential for local zone
heating is less than the thermal index displays?
• Scanned modes versus unscanned modes of operation affect the thermal
index. For scanned modes, heating tends to be near the surface; for
unscanned modes, the potential for heating tends to be deeper in the focal
zone.
• Always limit ultrasound exposure time. Do not rush the exam. Ensure that
the indices are kept to a minimum and that exposure time is limited without
compromising diagnostic sensitivity.
Mechanical Index (MI) Display
Mechanical bioeffects are threshold phenomena that occur when a certain level
of output is exceeded. The threshold level varies, however, with the type of
tissue. The potential for mechanical bioeffects varies with peak rarefactional
pressure and ultrasound frequency. The MI accounts for these two factors. The
higher the MI value, the greater the likelihood of mechanical bioeffects occurring.
There is no specific MI value that means that a mechanical effect is actually
occurring. The MI should be used as a guide for implementing the ALARA
principle.
Thermal Index (TI) Displays
The TI informs the user about the conditions that exist that might lead to an
increase in temperature at the surface of the body, within the body tissue, or at
the point of focus of the ultrasound beam on bone. That is, the TI informs the
user of the potential for temperature rise in body tissue. It is an estimate of
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iU22 User Manual
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Safety
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