Philips EPIQ 7 User Manual page 55

Biological Safety
The thermal index further consists of the following indices: soft tissue (TIS), bone (TIB), and
cranial bone (TIC). Only one of these is displayed at any time. Each transducer application has a
default selection that is appropriate for that combination. The TIB, TIS, or TIC is continuously
displayed over the range of 0.0 to maximum output, based on the transducer and application,
in increments of 0.1. For the location of the output display, see
The application-specific nature of the default setting is also an important factor of index
behavior. A default setting is a system control state that is preset by the manufacturer or the
operator. The system has default index settings for the transducer application. The default
settings are invoked automatically by the ultrasound system when power is turned on, when
new patient data is entered into the system database, 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.
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Safety
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