162
Electromagnetic Fields in Biological Systems
where E is the electric field (in volts per meter), σ is the electric conductivity (in siemens
per meter; approximately 0.2 S/m in average tissue conductivity [ICNIRP 1998; Reilly
1998]; approximately 0.1 S/m in brain tissue conductivity [Saunders and Jefferys 2002]),
and δ is the human body density (in kilogram per cubic meter; approximately l000 kg/m 3
in human body [Schult et al. 2010]). For instance, the whole-body average SAR limits of
0.4 W/kg for controlled environments and 0.08 W/kg for uncontrolled environments estimated by Gandhi (2002) are identical to those mentioned in the International Commission
on Non-Ionizing Radiation Protection (ICNIRP) 1998 standard. In contrast, heating
abnormal tissue with high-SAR power such as microwave cardiac ablation (MCA) is used
to treat heart tissue that allows abnormal electrical conduction by heating it to the point of
inactivation (Lin 1999; Rappaport 2004).
Lin (2007) commented that most exposure guidelines are promulgated on a 4-W/kg
SAR to prevent any whole-body exposure from raising the body temperature to 1°C above
the norm at 37°C. However, the Lin research group later suggeted that temperature distributions do not always correlate well with SAR distributions and the regulatory limits
on local temperature may not be exceeded as readily as those on local SAR (Wang et al.
2007). In modeling temperature increase during MRI of the human head in a head-sized
volume coil at up to 3.0-W/kg head-average SAR, the Lin research group concluded that
it may not be necessary to consider thermally induced changes in physiological response
(Wang et al. 2008). In addition, Barnes (2006) mentioned that there is a debate regarding whether effects other than the increase in temperature should be used to limit the
amount of radiated power.
In the case of EMFs over about 100 kHz, it was assumed that “Joule energy” could
contribute to the thermal effect through large induced currents. In contrast, at frequencies below 100 kHz, it was presumed that electric fields and currents are induced in the
body by time-varying external magnetic fields and then the induced E fields in the body
may lead to a variation of membrane potentials at the cellular level. Electric excitation
of the membrane might be the result of such changes in membrane potentials (ICNIRP
2004).
For TMS and rTMS, a high-intensity, fast magnetic pulse produces a cortical stimulus or excitation through the induction of locally confined eddy currents and induces
a rapid depolarization of the nerve cells; therefore, safety guidelines have been
reviewed and discussed for avoiding excitation of the central nervous system (e.g.,
IEEE 2002; Gandhi 2002; Medical Advisory Secretariat 2004; Reilly 2005; Barnes
2006; Blackman 2006, 2009; ARPANSA 2007; Lin 2007; WHO 2007; Wang et al. 2007,
2008; Wood 2008; Rossi et al. 2009; Casali et al. 2010; ICNIRP 2010; Sandrini, Umiltà,
and Rusconi 2011).
It has been assumed that the greatest stimulation efficiency occurs if the TMS coil’s
induced electric field (y axis) is parallel to cortical columns (Rábago et al. 2009).
Multiple target sites within the cortical area of activation were created from the coregistered anatomical and functional images. Electromyography (EMG) feedback from the
left FDI was used to assess each target site. The site chosen as the final target site was
the one that elicited the best MEP response at the lowest rMT. The TMS-induced MEP
response has been used in experiments regarding mirror neurons (Catmur, Walsh, and
Heyes 2007).
Electromagnetic Fields in Biological Systems
where E is the electric field (in volts per meter), σ is the electric conductivity (in siemens
per meter; approximately 0.2 S/m in average tissue conductivity [ICNIRP 1998; Reilly
1998]; approximately 0.1 S/m in brain tissue conductivity [Saunders and Jefferys 2002]),
and δ is the human body density (in kilogram per cubic meter; approximately l000 kg/m 3
in human body [Schult et al. 2010]). For instance, the whole-body average SAR limits of
0.4 W/kg for controlled environments and 0.08 W/kg for uncontrolled environments estimated by Gandhi (2002) are identical to those mentioned in the International Commission
on Non-Ionizing Radiation Protection (ICNIRP) 1998 standard. In contrast, heating
abnormal tissue with high-SAR power such as microwave cardiac ablation (MCA) is used
to treat heart tissue that allows abnormal electrical conduction by heating it to the point of
inactivation (Lin 1999; Rappaport 2004).
Lin (2007) commented that most exposure guidelines are promulgated on a 4-W/kg
SAR to prevent any whole-body exposure from raising the body temperature to 1°C above
the norm at 37°C. However, the Lin research group later suggeted that temperature distributions do not always correlate well with SAR distributions and the regulatory limits
on local temperature may not be exceeded as readily as those on local SAR (Wang et al.
2007). In modeling temperature increase during MRI of the human head in a head-sized
volume coil at up to 3.0-W/kg head-average SAR, the Lin research group concluded that
it may not be necessary to consider thermally induced changes in physiological response
(Wang et al. 2008). In addition, Barnes (2006) mentioned that there is a debate regarding whether effects other than the increase in temperature should be used to limit the
amount of radiated power.
In the case of EMFs over about 100 kHz, it was assumed that “Joule energy” could
contribute to the thermal effect through large induced currents. In contrast, at frequencies below 100 kHz, it was presumed that electric fields and currents are induced in the
body by time-varying external magnetic fields and then the induced E fields in the body
may lead to a variation of membrane potentials at the cellular level. Electric excitation
of the membrane might be the result of such changes in membrane potentials (ICNIRP
2004).
For TMS and rTMS, a high-intensity, fast magnetic pulse produces a cortical stimulus or excitation through the induction of locally confined eddy currents and induces
a rapid depolarization of the nerve cells; therefore, safety guidelines have been
reviewed and discussed for avoiding excitation of the central nervous system (e.g.,
IEEE 2002; Gandhi 2002; Medical Advisory Secretariat 2004; Reilly 2005; Barnes
2006; Blackman 2006, 2009; ARPANSA 2007; Lin 2007; WHO 2007; Wang et al. 2007,
2008; Wood 2008; Rossi et al. 2009; Casali et al. 2010; ICNIRP 2010; Sandrini, Umiltà,
and Rusconi 2011).
It has been assumed that the greatest stimulation efficiency occurs if the TMS coil’s
induced electric field (y axis) is parallel to cortical columns (Rábago et al. 2009).
Multiple target sites within the cortical area of activation were created from the coregistered anatomical and functional images. Electromyography (EMG) feedback from the
left FDI was used to assess each target site. The site chosen as the final target site was
the one that elicited the best MEP response at the lowest rMT. The TMS-induced MEP
response has been used in experiments regarding mirror neurons (Catmur, Walsh, and
Heyes 2007).