Chapter 2. Safety 2 – 25
Always limit ultrasound exposure time. Do not rush the scanning. Ensure that the indices are
kept to a minimum, and that expos ure 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 biological effects varies with peak pressure and ultrasound frequency. The MI
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 of potential temperature increase occurring on 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, and mode of
operation. 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, such as the skeletal
structure of a 2-3 month old fetus. The cranial bone thermal index (TIc) informs the user
about the potential heating of bone at or near the surface, for example, the 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 the TI to display at Setup > Imaging > Display.
■ Mechanical and Thermal Indices Display Precision and Accuracy
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 manual's Acoustic
Output Tables. These accuracy estimates are based on the variability ranges of probes and
systems, inherent acoustic output modeling errors, and the measurement variability, 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 of 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 to
calculate 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 the measurement and calculation process. For example, the
acoustic output values measured underwater are derated using a conservative, industry
standard, attenuation coefficient of 0.3dB/cm-MHz.
Conservative values for tissue characteristics were selected for use in the TI models.
Conservative values for tissue or bone absorption rates, blood perfusion rates, blood heat
capacity, and tissue thermal conductivity were selected.
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