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Electromagnetic Fields in Biological Systems
to treat chronic neuropathic pain and various neuropathic disorders including schizophrenia (Demirtas-Tatlidede et al. 2010) although double-blind investigations could not
find an effect (Mogg et al. 2007) or missed statistical evidence for an effect (Dlabac-de
Lange, Knegtering, and Aleman 2010). Other studies suggested some therapeutic effect
in treating depression by rTMS over the prefrontal cortex. However, a meta-analysis
concluded that there was only limited efficacy (Couturier 2005).
The rheobase for brain-cell stimulation (the minimum value required for even long
stimulating time) has been calculated to be 20 V/m (Reilly 1998). For durations of magnetic pulses longer than several milliseconds, it could be shown that cellular excitability
can be influenced by induced fields above a few volts per meter (Jefferys et al. 2003).
For reliable brain-cell stimulation, the induced electric field needs to exceed 100 V/m.
Amplitudes of applied magnetic pulses amount to 2–3 T, pulse durations are 50–600 μs,
and gradients extend up to 10,000 T/s during rise times up to 100 μs.
The depth until which tissue can be stimulated depends on the coil design and can
extend, for example, up to 8-cm deep. Although standard coils such as figure-8 coils or
butterfly coils are limited to superficial brain regions, deeper brain stimulation can be
achieved by sophisticated coil configurations (Roth et al. 2010).
6.2.2.2 Magnetic Defibrillation
Magnetic cardiac stimulation could overcome problems of conventional electric cardiac defibrillators associated with electrode contacting and placement. Compared to
nerve stimulation, magnetic cardiac stimulation requires considerably higher energies
(Pastore et al. 2010). However, compared to electric cardiac defibrillation, it offers the
advantage to be quickly applicable without any preparation of the patient (undressing
and preparing electrodes with contact gel) and avoids risks from high contact voltage of
up to 5 kV (IEC 60601-2-4).
Cardiac stimulation requires current densities above the fibrillation threshold, which
has been reported to be about 40–50 A/m 2 with pulse durations of about 1 ms (Irnich
1994). For magnetic cardiac defibrillation, coil design must allow in-depth stimulation
and provide sufficient high-pulse parameters to achieve reliable coverage of cardiac
muscle (Pastore et al. 2010).
6.2.3 Magnetic Therapy
Magnetic fields, in spite of being suspected to cause childhood leukemia if emitted from
power lines or transformer stations (IARC 2002), are frequently considered a last resort
of people with various health complaints. Aggressive marketing and fanciful therapeutic promises have made them a big business. The dispute whether magnet therapy is
beneficial or hogwash is ongoing (Flamm 2007; Pieber, Herceg, and Paternostro-Sluga
2010; Kröling et al. 2009). Magnetic therapy device manufacturers provide long lists of
indications and disorders from angina, depression, headache, and toothache to rheumatism. Authorities are reluctant in generally accepting the clinical efficiency of magnetic
therapy devices. The U.S. Food and Drug Administration (FDA) has restricted acceptance of magnetic therapy only to some indications of proven clinical evidence (FDA
2006); the German Federal Board of Physicians and Health Insurance has classified
Electromagnetic Fields in Biological Systems
to treat chronic neuropathic pain and various neuropathic disorders including schizophrenia (Demirtas-Tatlidede et al. 2010) although double-blind investigations could not
find an effect (Mogg et al. 2007) or missed statistical evidence for an effect (Dlabac-de
Lange, Knegtering, and Aleman 2010). Other studies suggested some therapeutic effect
in treating depression by rTMS over the prefrontal cortex. However, a meta-analysis
concluded that there was only limited efficacy (Couturier 2005).
The rheobase for brain-cell stimulation (the minimum value required for even long
stimulating time) has been calculated to be 20 V/m (Reilly 1998). For durations of magnetic pulses longer than several milliseconds, it could be shown that cellular excitability
can be influenced by induced fields above a few volts per meter (Jefferys et al. 2003).
For reliable brain-cell stimulation, the induced electric field needs to exceed 100 V/m.
Amplitudes of applied magnetic pulses amount to 2–3 T, pulse durations are 50–600 μs,
and gradients extend up to 10,000 T/s during rise times up to 100 μs.
The depth until which tissue can be stimulated depends on the coil design and can
extend, for example, up to 8-cm deep. Although standard coils such as figure-8 coils or
butterfly coils are limited to superficial brain regions, deeper brain stimulation can be
achieved by sophisticated coil configurations (Roth et al. 2010).
6.2.2.2 Magnetic Defibrillation
Magnetic cardiac stimulation could overcome problems of conventional electric cardiac defibrillators associated with electrode contacting and placement. Compared to
nerve stimulation, magnetic cardiac stimulation requires considerably higher energies
(Pastore et al. 2010). However, compared to electric cardiac defibrillation, it offers the
advantage to be quickly applicable without any preparation of the patient (undressing
and preparing electrodes with contact gel) and avoids risks from high contact voltage of
up to 5 kV (IEC 60601-2-4).
Cardiac stimulation requires current densities above the fibrillation threshold, which
has been reported to be about 40–50 A/m 2 with pulse durations of about 1 ms (Irnich
1994). For magnetic cardiac defibrillation, coil design must allow in-depth stimulation
and provide sufficient high-pulse parameters to achieve reliable coverage of cardiac
muscle (Pastore et al. 2010).
6.2.3 Magnetic Therapy
Magnetic fields, in spite of being suspected to cause childhood leukemia if emitted from
power lines or transformer stations (IARC 2002), are frequently considered a last resort
of people with various health complaints. Aggressive marketing and fanciful therapeutic promises have made them a big business. The dispute whether magnet therapy is
beneficial or hogwash is ongoing (Flamm 2007; Pieber, Herceg, and Paternostro-Sluga
2010; Kröling et al. 2009). Magnetic therapy device manufacturers provide long lists of
indications and disorders from angina, depression, headache, and toothache to rheumatism. Authorities are reluctant in generally accepting the clinical efficiency of magnetic
therapy devices. The U.S. Food and Drug Administration (FDA) has restricted acceptance of magnetic therapy only to some indications of proven clinical evidence (FDA
2006); the German Federal Board of Physicians and Health Insurance has classified