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a Chapter 8 Biological Safety of Diagnostic Sonography 99
elevation does not exceed 1.5 8C above the normal
core temperature. Most evidence shows that developmental
injury requires a temperature rise of at least
1.5 8C. Above this threshold, teratogenic effects are
determined by the magnitude of the temperature rise
and its duration. For temperature increases of 4 8C
and 6 8C above normal, the respective limits for the
duration are 16 min and 1 min. The lowest temperature
value for consistent teratogenesis in mammals
has been reported to be 41.5 8C [24].
Acoustic Heating of Fetal Bones
Mineralized bone has the highest coefficient, 10 dB/
cm ´ MHz, and therefore the highest likelihood of
thermal effect. Mineralization and ossification of fetal
bones begins in the 12th week of pregnancy and progresses
with advancing gestation. Drewniak and associates
[25] studied the effect of progressive ossification
with advancing gestation on ultrasound-induced
heat generation in human fetal femur ex utero. By 15
weeks of pregnancy, the rate of temperature rise in
the bone was 30 times greater than that in the soft
tissue.
Acoustic Heating of Fetal Brain
The average absorption coefficient for neural tissue is
0.2 dB/cm ´ MHz. Despite its low absorption coefficient,
the fetal central nervous system being enclosed
in the skull and the spinal canal vertebrae is vulnerable
to bone-related heating. Similar risks may also
exist for other structures lying close to bone, such as
the pituitary gland or the hypothalamus. However,
there is conflicting evidence on the actual risk of
brain heating from insonation.
Bosward et al. [26] noted in fresh and formalinfixed
fetal guinea-pig brains a mean temperature elevation
of 5.2 8C following a 2-min insonation with
I SPTA of 2.9 W/cm 2 with a stationary beam in a tank
containing water at 38 8C. The greatest temperature
rise in brain tissue occurred close to the bone and
correlated with both gestational age and progression
in bone development. Barnett [27] has recently reviewed
the experimental evidence regarding brain
temperature elevation and concluded that insonationinduced
intracranial heating increases with gestational
age concomitant with progressive fetal bone
development. Pulsed spectral Doppler ultrasound can
produce a biologically significant temperature rise in
the fetal brain with approximately 75% of the maximum
heating occurring within 30 s. Brain blood flow
does not significantly impact heating induced by exposure
to a narrow focused ultrasound beam. An insonation-induced
temperature rise of 4 8C sustained
for 5 min leads to irreversible injury to the fetal
brain, whereas a temperature increase of up to 1.5 8C
for 120 s does not elicit measurable electrophysiological
responses in the fetal brain.
In contrast to the above findings, others have
failed to note any significant temperature elevation in
animal models. Stone and associates [28] conducted
investigations on the heating effects of pulsed Doppler
ultrasound on fetal brain tissue in dead and live
animals. Whereas tissue heating was observed at the
skull bone-to-brain interface in the dead lamb brain,
minimal or no temperature elevation was observed in
live lambs. Tarantal and colleagues [24] investigated
in vivo temperature elevations in gravid primates
(macaques) measured intracranially or at the musclebone
interface consequent to imaging and pulsed
Doppler ultrasonic exposure. This study is one of the
very few reports on pulsed Doppler exposure. Utilizing
a commercial ultrasound instrument and with
varying duration of insonation involving the imaging
and the pulsed Doppler mode, the highest temperature
elevation observed was 0.68C. Both the reports
suggest that the presence of tissue perfusion in a living
animal may play a protective role by dissipating
the heat.
A rare report involving direct thermocouple recording
of intracranial temperature in a neurosurgical
patient during color transcranial Doppler ultrasound
for 30 min failed to demonstrate any temperature elevation
either in the brain parenchyma or at the bone/
soft tissue interface [29]. A 2.5-MHz transducer was
used with the Doppler mode power settings at SPTA
of 2,132 mW/cm 2 and a maximum power of
149.3 mW. The ipsilateral tympanic temperature rose
by 0.06 8C, indicating an overall increase in brain
temperature.
Clinical Significance
of the Ultrasound Thermal Effects
Although ultrasound exposure carries the potential
for tissue heating, there is no clinical evidence of
thermal injury to the human fetus from diagnostic
insonation. However, as theoretical risks exist, one
must exercise caution especially in using spectral
pulsed Doppler ultrasound. The ability of the human
body to tolerate limited temperature elevations without
any harm is well known. In healthy humans, variations
in the body temperature occur under different
physiological circumstances such as during physical
exercise. However, febrile illnesses may increase the
risk enough for extra caution. The fetus is dependent
on the mother for heat dissipation mostly through
the placental circulation and to some extent across
the fetal skin and amniotic fluid to maternal tissues
and circulation [30]. The fetus is warmer than the
mother [31]. Fetal skin temperature has been shown