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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. 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:
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•
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Mechanical Index (MI) Display
Thermal Index (TI) Displays
There are three TIs which used for different combinations of soft tissue and bone in the area to be
examined. The TI is intending to keep us making aware of condition that cause increased temperature
elevations, no matter at surface, within the tissue, or at the point where the ultrasound is focusing on
bone.
Thermal index (TI)
Soft Tissue
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 un-scanned modes of operation affect the thermal
index. For scanned modes, heating tends to be near the surface; for un-
scanned 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.
The scientific evidence suggests the mechanical bioeffects are threshold
phenomena that does occur when a certain level of output is exceeded. The
threshold level varies depending on the tissue. The potential for mechanical
bioeffects varies with peak rarefactional pressure and ultrasound frequency.
The higher MI value reading, the greater the potential. There is no specific
MI value, which means that a mechanical effect is occurring in fact. The MI
should be used as a guide for implementing the ALARA principle.
Scanned Mode
TIS at Surface
Un-scanned Mode
TIS
Small Aperture
Large Aperture
P32
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