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Mobile Communication Fields in Biological Systems
and the environment cause variations in the peak temperature increase of about 10%
(Wang and Fujiwara 1999).
Regarding children dosimetry, the most widely accepted database of biological tissues’ dielectric properties (Gabriel, Gabriel, and Corthout 1996) lacks data for children. As a result, in EM dosimetry of children, the dielectric properties of biological
tissues for adults are so far being used. Nevertheless, age-dependent changes of the
tissues’ dielectric properties have been repeatedly reported (Conil et al. 2008; Gabriel
2005; Keshvari, Keshvari, and Lang 2006; Schmid and Uberbacher 2005). The dielectric properties, that is, permittivity and conductivity, have been demonstrated to
decrease with age due to the changes of water content and organic composition of tissues (Penman, Rezazadeh, and Gabriel 2001). Higher conductivity has been found for
the brain and skull of newborn rats compared to adult rats, at 900 MHz (16% and
43%, respectively), which suggests a possibility of SAR increase due to the higher tissue
conductivity. A lower increase of permittivity has also been observed (9.9% and 33%,
respectively). Furthermore, recently, a significant dependence of the dielectric properties of white matter and spinal cord on age was reported while no age-related variation
has been found for the gray matter (Penman et al. 2007). In the research of Wang,
Fujiwara, and Watanabe (2006), an empirical formula has been derived for the complex
permittivity of various tissues as a function of the total body water (TBW), according
to Lichtenecker’s exponential law. With the use of the aforementioned formula, the
dielectric properties in 7-year-old and 3-year-old child head models have been derived.
Finally, a systematic evaluation of the age-dependent changes of the dielectric properties of a large number of different tissues has recently been published (Peyman et al.
2009). The establishment of a database for children’s dielectric properties should be an
essential and urgent task.
5.2.3 Computational Methods
Analytical and numerical methods have been developed over the last 40 years to understand coupling of electromagnetic fields to biological bodies. Analytical methods are
restricted to very simple configurations, but they provide valuable insight into the physical mechanisms, yield typical parameters, and they are used for testing of numerical
methods.
Studies on complex-shaped inhomogeneous bodies have been based on the use of
boundary or volume techniques. In boundary techniques, the space is divided into linear, homogeneous, and isotropic domains, the boundaries of which are discretized. In
volume techniques, the space is discretized directly. Boundary techniques are more efficient for geometrically simple configurations with a low surface to volume ratio, while
volume techniques are preferable in modeling nonhomogeneous materials and more
complex geometries. The numerical methods used to evaluate the power absorption into
biological bodies mainly include the method of moments (MoM), the finite difference
time domain (FDTD) method, the finite integration technique (FIT) and the finite elements method (FEM) (Lin and Bernardi 2007). Hybrid methods derived from the combination of these methods and other methods for EM propagation characterization are
also used.
Mobile Communication Fields in Biological Systems
and the environment cause variations in the peak temperature increase of about 10%
(Wang and Fujiwara 1999).
Regarding children dosimetry, the most widely accepted database of biological tissues’ dielectric properties (Gabriel, Gabriel, and Corthout 1996) lacks data for children. As a result, in EM dosimetry of children, the dielectric properties of biological
tissues for adults are so far being used. Nevertheless, age-dependent changes of the
tissues’ dielectric properties have been repeatedly reported (Conil et al. 2008; Gabriel
2005; Keshvari, Keshvari, and Lang 2006; Schmid and Uberbacher 2005). The dielectric properties, that is, permittivity and conductivity, have been demonstrated to
decrease with age due to the changes of water content and organic composition of tissues (Penman, Rezazadeh, and Gabriel 2001). Higher conductivity has been found for
the brain and skull of newborn rats compared to adult rats, at 900 MHz (16% and
43%, respectively), which suggests a possibility of SAR increase due to the higher tissue
conductivity. A lower increase of permittivity has also been observed (9.9% and 33%,
respectively). Furthermore, recently, a significant dependence of the dielectric properties of white matter and spinal cord on age was reported while no age-related variation
has been found for the gray matter (Penman et al. 2007). In the research of Wang,
Fujiwara, and Watanabe (2006), an empirical formula has been derived for the complex
permittivity of various tissues as a function of the total body water (TBW), according
to Lichtenecker’s exponential law. With the use of the aforementioned formula, the
dielectric properties in 7-year-old and 3-year-old child head models have been derived.
Finally, a systematic evaluation of the age-dependent changes of the dielectric properties of a large number of different tissues has recently been published (Peyman et al.
2009). The establishment of a database for children’s dielectric properties should be an
essential and urgent task.
5.2.3 Computational Methods
Analytical and numerical methods have been developed over the last 40 years to understand coupling of electromagnetic fields to biological bodies. Analytical methods are
restricted to very simple configurations, but they provide valuable insight into the physical mechanisms, yield typical parameters, and they are used for testing of numerical
methods.
Studies on complex-shaped inhomogeneous bodies have been based on the use of
boundary or volume techniques. In boundary techniques, the space is divided into linear, homogeneous, and isotropic domains, the boundaries of which are discretized. In
volume techniques, the space is discretized directly. Boundary techniques are more efficient for geometrically simple configurations with a low surface to volume ratio, while
volume techniques are preferable in modeling nonhomogeneous materials and more
complex geometries. The numerical methods used to evaluate the power absorption into
biological bodies mainly include the method of moments (MoM), the finite difference
time domain (FDTD) method, the finite integration technique (FIT) and the finite elements method (FEM) (Lin and Bernardi 2007). Hybrid methods derived from the combination of these methods and other methods for EM propagation characterization are
also used.