Doc #: 60-00118 Rev. A
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The acoustic output should be adjusted in accordance with the ALARA principle by starting the
exam at a low power setting and then increasing the power level as necessary until the desired
diagnostic information is obtained.
For the eye, foramen magnum, burr holes and fontanelles, a precaution is displayed to the user
during entry into any exam screen which states to use 10% or lower power levels.
When selecting any of the ophthalmic vessels, including siphons (ROPTH, LOPTH, LSIPHN,
RSIPHN) the Lucid
™ M1 System will automatically set the power setting to 10%.
Managing Ultrasound Exposure
As previously stated, ultrasound energy may cause tissue damage due to mechanical or thermal
mechanisms. To assist users, thermal and mechanical indexes are modeled to represent relative
measure of potential tissue damage. For tissue damage due to heating, the thermal index is
displayed to the user.
The following describe indices that are used to describe hazards (which are potential sources of
harm):
MI
– Mechanical Index, the displayed parameter representing potential cavitation bioeffects
TI
– Thermal Index, the ratio of the total acoustic power to the acoustic power required to raise
tissue temperature by 1
C. In the calculation of all thermal indices in the output display standard,
the average ultrasonic attenuation is assumed to be 0.3 dB/cm-MHz along the beam axis in the
body.
TIB
– Bone Thermal Index, thermal index in which ultrasound beam passes through soft tissue
and a focal region is in the immediate vicinity of bone.
TIC
– Cranial Thermal Index, thermal index for applications in which the ultrasound beam passes
through bone near the beam entrance into the body, such as adult cranial applications.
TIS
– Soft Tissue Thermal Index, thermal index related to soft tissues.
The Lucid
™ M1 System displays the Cranial Thermal Index (TIC) in the exam screen and
provides assistance in setting the power to conform to the ALARA principle. The calculation of
the TIC is TI = W
o
/40d
eq
which
is found in Table 2-2 of the NEMA UD 3-2004 (R2009) Revision 2.
As described above, the TIC is the calculated thermal index when the ultrasound beam passes
through bone near the beam entrance into the body. The higher the power setting, the higher the
TIC value.
Likewise, a reduction in power setting leads to a corresponding lower TIC value.
Hence by using the TIC value as guidance, the user can adhere to the ALARA principles which
should lead to a reduction in potential tissue heating. Although the MI, TIB and TIS indexes are
described above for completeness, per the NEMA UD 3-2004 (R2009) Revision 2 standard they
are not required to be displayed because their maximum values when using the Lucid
™ M1
System are less than one or because the Lucid
™ M1 is for adult cephalic usage. Indices with a
maximum value of less than one are considered be below values for which there is a potential
safety concern.
