Currently, there is substantial interest in assessing the exposure of pregnant women and in utero foetuses to electromagnetic fields, which in turns creates a need for precise data describing the dielectric properties of pregnancy-related and foetal tissues.
To address the gap in available information, Azadeh Peyman from the UK's Health Protection Agency and various collaborators have measured the dielectric properties of human placenta, umbilical cord and amniotic fluid (Phys. Med. Biol. 56 N93) and, most recently, the dielectric properties of rat foetuses as a function of gestation (Phys. Med. Biol. 57 2103)
"Tabulated numerical dielectric data for rat foetal tissues and pregnancy-related tissues are now available at a wide range of medical and telecommunication frequencies," Peyman told medicalphysicsweb. "All of this data can be used in numerical simulations describing the exposure of pregnant women to electromagnetic fields. The existence of this data eliminates the need for rough estimations or using substitutes."
Embryo and foetal study
In their current study, Peyman and collaborator Camelia Gabriel from UK-based consultancy MCL-P measured the relative permittivity and conductivity of rat foetuses in the frequency range of 40 MHz to 20 GHz. This range covers the main frequencies of interest for dosimetric studies assessing the exposure of humans to electromagnetic fields from medical applications, such as MRI, to telecoms applications such as wireless local-area networks.
Twelve healthy Sprague Dawley female rats were time-mated for this study. The typical gestation period for rats is approximately 21 days, and animals were sacrificed on days 12, 14, 18, 19 and 20 of their pregnancies. Peyman and Gabriel note that all measurements were made on homogenized tissue between 2 and 4 hours after sacrifice and that the tissue temperature was carefully maintained at 36.5±0.5° throughout.
The dielectric properties of the tissue were measured using an open-ended co-axial probe and a computer-controlled network analyser. "The sample is incorporated into a transmission line assembly and the scattering parameters (i.e. transmission/reflection coefficients) are measured using a vector network analyser," explained Peyman. "A mathematical model is then used to relate the scattering parameters to the dielectric properties of the sample."
According to Peyman, this approach requires minimal handling of the samples and has an accuracy of between 1 and 2%.
Accurate dielectric data
Peyman and Gabriel reported a decrease in both the permittivity and conductivity of the homogenized foetuses as they developed from day 18 to day 20. "Although the magnitude of the decrease is not very large, it reflects that the total water content of the foetus decreases with age," state the authors. "This increases our understanding of the variation of dielectric properties of foetal tissues as a function of gestation."
The researchers were also able to confirm that the dielectric properties of rat amniotic fluid and placenta at body temperature were very close to those obtained in their previous study of human samples at microwave frequencies.
"The data obtained in this current study as well as our previous work on pregnancy-related tissues are being used to modify the existing pregnant female model and calculate specific absorption rate values under different exposure scenarios," said Peyman. "We will continue our research in the field of dielectric spectroscopy of biological tissues to fill the gaps in the literature. One of the main gaps is the availability of accurate dielectric data at frequencies below 1 MHz due to unresolved practical challenges."
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Dielectric properties of human placenta, umbilical cord and amniotic fluid
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