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accounts for these two factors. The higher the MI value, the greater the likelihood of mechanical
bioeffects occurring. However, there is no specific MI value that means that a mechanical
bioeffect will actually occur. The MI should be used as a guide for implementing the ALARA
principle.
Thermal Index (TI) Display
The TI informs the user about the potential for temperature increase occurring at the body surface,
within body tissue, or at the point of focus of the ultrasound beam on bone. The TI is an estimate
of the temperature increase in specific body tissues. The actual amount of any temperature rise is
influenced by factors such as tissue type, vascularity, mode of operation, etc. The TI should be
used as a guide for implementing the ALARA principle.
The bone thermal index (TIb) informs the user about potential heating at or near the focus, after
the ultrasound beam has passed through soft tissue or fluid, for example, at or near second or
third trimester fetal bone. The cranial bone thermal index (TIc) informs the user about the
potential heating of bone at or near the surface of, for example, cranial bone. The soft tissue
thermal index (TIs) informs the user about the potential for heating within soft homogeneous
tissue. TIc is displayed when you select a trans-cranial application.
You can select TI Display in Utility > Setup > Display > Display.
Precision and Accuracy of Mechanical and Thermal Indices Displays
The mechanical and thermal indices on the system are precise to 0.1 units.
The MI and TI display accuracy estimates for the system are given in the Acoustic Output Tables
manual. These accuracy estimates are based on the various range of probes and systems,
inherent acoustic output modeling errors, and measurement variation, as described below.
The displayed values should be interpreted as relative information to help the system operator
achieve the ALARA principle through prudent use of the system. The values should not be
interpreted as actual physical values from investigated tissue or organs. The initial data that is
used to support the output display is derived from laboratory measurements based on the AIUM
measurement standard. The measurements are then put into algorithms for calculating the
displayed output values.
Many of the assumptions used in the process of measurement and calculation are conservative in
nature. Over-estimation of actual in situ exposure, for the vast majority of tissue paths, is built into
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