Artikkeli vuodelta 2011
Nämäkin ultraäänilaitteet ovat kehittyneet sen jälkeen edelleen.
Int J Hyperthermia. 2011;27(4):374-87. doi: 10.3109/02656736.2011.553769.
Thermal thresholds for teratogenicity, reproduction, and development.
Abstract
The
human embryo and foetus may be especially vulnerable to chemical and
physical insults during defined stages of development. In particular,
the scheduled processes of cell proliferation, cell migration, cell
differentiation, and apoptosis that occur at different times for
different organ structures can be susceptible to elevated temperatures.
With limited ability to regulate temperature on its own, the developing
embryo and foetus is entirely dependent upon the mother's
thermoregulatory capacity. As a general rule, maternal core body
temperature increases of ∼2°C above normal for extended periods of time,
2-2.5°C above normal for 0.5-1 h, or ≥4°C above normal for 15 min have
resulted in developmental abnormalities in animal models. Significant
differences in thermoregulation and thermoneutral ambient temperatures
make direct extrapolation of animal data to humans challenging, and the
above temperatures may or may not be reasonable threshold predictions
for adverse developmental effects in humans. Corresponding specific
absorption rate (SAR) values that would be necessary to cause such
temperature elevations in a healthy adult female would be in the range
of ≥15 W/kg (whole body average or WBA), with ∼4 W/kg required to
increase core temperature 1°C. However, smaller levels of thermal stress
in the mother that are asymptomatic might theoretically result in
increased shunting of blood volume to the periphery as a heat
dissipation mechanism. This could conceivably result in altered
placental and umbilical blood perfusion and reduce heat exchange with
the foetus. It is difficult to predict the magnitude and threshold for
such an effect, as many factors are involved in the thermoregulatory
response. However, a very conservative estimate of 1.5 W/kg WBA (1/10th
the threshold to protect against measurable temperature increases) would
seem sufficient to protect against any significant reduction in blood
flow to the embryo or foetus in the pregnant mother. This is more than
three times above the current WBA limit for occupational exposure
(0.4 W/kg) as outlined in both IEEE C95.1-2005 and ICNIRP-1998
international safety standards for radiofrequency (RF) exposures. With
regard to local RF exposure directly to the embryo or foetus,
significant absorption by the mother as well as heat dissipation due to
conductive and convective exchange would offer significant protection.
However, a theoretical 1-W/kg exposure averaged over the entire 28-day
embryo, or averaged over a 1-g volume in the foetus, should not elevate
temperature more than 0.2°C. Because of safety standards, exposures to
the foetus this great would not be attainable with the usual RF sources.
Foetal exposures to ultrasound are limited by the US Food and Drug
Administration (FDA) to a maximum spatial peak temporal average
intensity of 720 mW/cm(2). Routine ultrasound scanning typically occurs
at lower values and temperature elevations are negligible. However, some
higher power Doppler ultrasound devices under some conditions are
capable of raising foetal temperature several degrees and their use in
examinations of the foetus should be minimised.
- PMID:
- 21591900
- DOI:
- 10.3109/02656736.2011.553769
- [Indexed for MEDLINE]
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