TELEMED ClarUs User Guide, REV 4.0 2013.12.04
37
If the
Thermal Index (TI)
does not exceed 1, currently available evidence
indicates that the risk of an injury due to
ultrasonic heating
is negligible for
the vast majority of conditions of the diagnostic ultrasound examination.
During the first trimester, and in the case of trans-abdominal fetal
examinations through a bladder path greater than 5 cm in length, current
evidence indicates that it is possible that the maximum temperature elevation
which could be obtained is as much as 2-3 times that of the displayed
Soft
Tissue Thermal Index (TIS)
. More caution may be warranted in these
situations, particularly if the
TIS
exceeds 1.
The
Soft Tissue Thermal Index (TIS)
is the appropriate indicator of the
potential for
ultrasonic heating
for examinations in which the ultrasound
beam travels a path which is principally made up of homogeneous soft tissue
or a soft tissue/fluid path, as during a first trimester fetal examination or an
abdominal examination.
If bone, including 2
nd
or 3
rd
trimester fetal bone, is within the ultrasound beam
the
Bone Thermal Index (TIB)
is often the appropriate indicator, except as
noted in the next conclusion.
If bone is in contact with the transducer the
Cranial Thermal Index (TIC)
is
the appropriate indicator. If bone is within approximately 1 cm of the
transducer and this is closer than the nearest focal zone, the
Cranial Thermal
Index (TIC)
is the appropriate indicator. More caution may be warranted in
these cases because of the potential for transducer self-heating and heating of
the transducer may add significantly to any
ultrasonic heating
which may
occur.
Generally, more caution may be warranted for transvaginal, transesophegeal
and transrectal examinations because heating of the transducer may
potentially produce additional heat to adjacent tissue.
This conclusion and the following one provide guidance to the user if the
temperature elevation in the fetus can possibly
exceed 1 °C as a result of a
diagnostic ultrasound exposure. If the exposure produces a maximum
in situ
temperature of no more than 38.5 °C (1.5 °C above normal physiological
levels) then it may be used clinically without reservation on thermal grounds.
To be considered potentially hazardous on thermal grounds, it appears that a
diagnostic ultrasound exposure must elevate embryonic and fetal
in situ
temperatures to the following temperatures for approximately the
corresponding durations:
39 °C, (2 degrees above normal), 60 minutes;
40 °C, (3 degrees above normal), 15 minutes;
41 °C, (4 degrees above normal), 4 minutes;
42 °C, (5 degrees above normal), 1 minute;
43 °C, (6 degrees above normal), 0.25 minutes.
Mechanical Effects
At exposures that do not exceed the output limits recommended in the section
entitled
Thermal effects
, there is no demonstrated risk of clinically significant
damage in humans from the mechanical effects of ultrasound exposure during
a diagnostic examination. However, capillary hemorrhaging has been
observed in lungs and in the intestine of mammals at diagnostically relevant
exposures. This effect has also been observed in other soft tissues if gas
contrast agents are used. For the most part, thresholds are just as likely to be
