o_1977r8vv9vk1ts2ms0kd8pksa.pdf

100 D. Maulik
on average 0.23 8C above the temperature of the uterine
wall [32] and fetal core temperature is 0.75 8C
above its skin temperature [33].
Miller and Ziskin [23] suggested that the probability
of any measurable bioeffects from diagnostic insonation
is minimal or nonexistent if the maximum
temperature rise remains 28C or less in an afebrile
subject. Temperature elevations not exceeding 398C
are most unlikely to induce any fetal abnormalities.
However, at higher temperatures, the duration of ultrasound
exposure becomes a significant factor. Indeed,
as shown in Fig. 8.2, these authors have defined
a boundary line based on the temperature rise and
exposure duration; below this line, risks of thermal
bioeffects are virtually nonexistent. The authors recommended
that an ultrasound examination need not
be restricted if the combination of temperature elevation
and exposure duration remains below this
boundary line.
In obstetrical ultrasonography, fetal developmental
issues require additional considerations. Fetal vulnerability
in the worst case scenario demands prudent
practice while not depriving the patient of the benefits
of diagnostic ultrasonography. This issue has been discussed
extensively by the various international societies
who have issued official statements on thermal effects
and the safe use of diagnostic insonation.
The WFSUMB endorsed the following recommendations
regarding Doppler in 1997:
ªIt has been demonstrated in experiments with unperfused
tissue that some Doppler diagnostic
equipment has the potential to produce biologically
significant temperature rises, specifically at
bone/soft tissue interfaces. The effects of elevated
temperatures may be minimised by keeping the
time for which the beam passes through any one
point in tissue as short as possible. Where output
power can be controlled, the lowest available
power level consistent with obtaining the desired
diagnostic information should be used. Although
the data on humans are sparse, it is clear from animal
studies that exposures resulting in temperatures
less than 38.5 8C can be used without reservation
on thermal grounds. This includes obstetric
applicationsº [10].
The 1997 AIUM position on temperature elevation
and exposure duration is as follows [7]:
n ªFor exposure duration up to 50 hours, there have
been no significant biological effects observed due
to temperature increases less than or equal to 28C
above normal.
n For temperature increases greater than 28C above
normal, there have been no significant biological
effects observed due to temperature increases less
than or equal to 6±(log 10 t/0.6), where t is the exposure
duration ranging from 1 to 250 min. For
example, for temperature increases of 4 8C and
6 8C, the corresponding limits for the exposure
duration t are 16 min and 1 min respectively.
n In general, adult tissues are more tolerant of temperature
increases than fetal and neonatal tissues.
Fig. 8.2. Thermal bioeffects. A plot of thermally produced
biological effects that have been reported in the literature
in which the temperature elevation and exposure durations
are provided. Each data point represents either the lowest
temperature reported for any duration or the shortest
duration for any temperature reported for a given effect.
The solid lines link multiple data points relating to the
same bioeffect. The dashed line represents a lower boundary
(t43 = 1) for observed, thermally induced biological effects.
(With permission from [23])