Chapter 12 - System Safety and Maintenance
Acoustic Output Indices
Terason t3000 / Echo Ultrasound System User Guide
269
MI: The Mechanical Index
MI is of possible clinical interest if the beam focus is close to the surface of lung tissue.
MI has the following characteristics:
•
Potential bioeffect:
Any possible mechanical or non-thermal mechanisms - although
the likelihood of adverse consequences from these causes is not well understood, such
risk may be highest in the presence of gas-saturated structures such as lung tissue.
•
Mode type:
Calculated for 2D only operation.
•
Tissue type and location:
Soft tissue at all locations in the scan field.
•
Acoustic parameter:
Maximum negative (rarefactional) ultrasound pressure at focus.
TIS: The Soft Tissue Thermal Index
TIS is of interest in the absence of bone, either at the tissue surface or near the beam focus.
Applications of clinical interest include general abdominal examinations, first-trimester
scanning before fetal bone has ossified, and cardiology. TIS has the following
characteristics.
•
Potential bioeffect:
Thermal heating of soft tissue due to absorption of ultrasound. The
TIS value is the ratio of the current transducer power to the reference level that would
cause a 1ºC temperature rise in soft tissue.
•
Mode type:
Relevant for all non-2D only modes, in both scanned and non-scanned
modes.
•
Tissue type and location:
In scanned modes, soft tissue at the surface is of concern. In
non-scanned modes, heating of soft tissue along the beam axis between the surface and
focus is considered.
•
Acoustic parameters:
For scanned modes, the associated intensity at the surface is
usually related to surface tissue heating. For unscanned modes, the maximum derated
power through a 1-cm
2
area anywhere along the beam axis is the basis for estimating
tissue heating: unscanned beams less than 1cm
2
in area at the surface are assumed to
contribute only to surface heating, and the calculated effects are combined with those of
scanned modes to estimate total soft-tissue heating at the surface. Unscanned beams
larger than 1 cm
2
at the surface are assumed to heat tissue only near the focus. Total
heating effects at the surface and focus are compiled separately, and the larger value is
reported as TIS.
TIB: The Bone Tissue Thermal Index
TIB is of concern when bone is near the beam focus. Applications of clinical interest
include second- and third-trimester Doppler (when the beam focus is near fetal bone), and
Neonatal Cephalic Doppler (when the beam focus is near cranial bone). TIB has the
following characteristics.
•
Potential bioeffect:
Thermal heating of bone (and indirect heating of adjacent soft
tissue) due to absorption of ultrasound at the bone surface. The TIB value indicates the
