165
Static, Low-Frequency, and Pulsed Magnetic Fields
larger for chord tones than for single tones and was significantly larger in the musicians
than controls. These results suggest that the P2m response is susceptible to modification
by musical training during a period of neural maturation, with a short refractory period
of neural activity for the auditory input of composite tones. The P2m activity may be
specialized to the processing of multifrequency sounds, such as musical timbre consisting of abundant harmonics.
With more recent and advanced applications, Chen, Xiang et al. (2010) investigated
noninvasive biomarkers for visuocortical development in healthy children using the
MEG system. The latency of M75 and M100 decreased with age. The amplitude ratio of
M100 to M75 increased significantly with age. The differences of MEG source images
between the left and right occipital cortices for M75 and M145 increased significantly
with age. The authors concluded that the latency of M75 and M100 and the amplitude
ratio of M100 to M75 are robust biomarkers for the development of visual function in
children and speculated that the results form a foundation for quantitative identification
of developmental delay and/or abnormalities of visual function in children with brain
disorders.
3.5.3 Magnetic Resonance Imaging
Magnetic resonance imaging is a method to obtain spatial distribution of nuclear magnetic resonance (NMR) signals using gradient magnetic fields and the Fourier transform
(FT). The basic principle of MRI was proposed by Lauterbur (1973). The principles of
MRI are described in detail by Callaghan (1993). Briefly, an MRI system consists of a
magnet, gradient coils, and RF coils. The MRI utilizes fusion techniques of spatially uniform SMF, spatially gradient magnetic fields, and RF pulse fields. The MRI has become
a standard technique for the routine diagnosis of many disease processes, replacing and
sometimes surpassing computed tomography (CT). The CT scans expose the body to
high doses of X-rays; in contrast to CT, MRI is noninvasive, uses nonionizing radiation,
and has a high soft-tissue resolution and discrimination in any imaging plane.
A guideline for exposure of the human body to SMFs set by ICNIRP (2009) suggests
2 T as the ceiling value for body parts, except for arms and legs, in occupational exposure. In the application of clinical MRI, the current exposure level is confirmed to be
less than or equal to 2 T. In SMFs at this strength it is not feasible to obtain resonance
images, except for hydrogen atoms. Tomasi and Wang (2007) estimated the magnetic
force in the human head during MRI at 4 T and confirmed that the induced gradient
fields increase the magnetic force on tissues. However, even for tissue components with
large magnetic susceptibility such as iron-containing proteins, this force is negligible
compared with the gravitational force. Therefore, the authors concluded that exposure
to uniform SMF at 4 T does not have a significant risk for tissues in the head.
Glover and Bowtell (2008) showed that current densities above regulatory limits may
be generated even for moderate natural movements near a clinical 3-T MRI scanner
although the subject did not report any perception of physiological effects. The electric
fields induced due to natural movements near the scanner are much higher than those
induced by time-varying fringe fields from the gradient coils in a subject standing still
next to the scanner while the scanner is operating.
Static, Low-Frequency, and Pulsed Magnetic Fields
larger for chord tones than for single tones and was significantly larger in the musicians
than controls. These results suggest that the P2m response is susceptible to modification
by musical training during a period of neural maturation, with a short refractory period
of neural activity for the auditory input of composite tones. The P2m activity may be
specialized to the processing of multifrequency sounds, such as musical timbre consisting of abundant harmonics.
With more recent and advanced applications, Chen, Xiang et al. (2010) investigated
noninvasive biomarkers for visuocortical development in healthy children using the
MEG system. The latency of M75 and M100 decreased with age. The amplitude ratio of
M100 to M75 increased significantly with age. The differences of MEG source images
between the left and right occipital cortices for M75 and M145 increased significantly
with age. The authors concluded that the latency of M75 and M100 and the amplitude
ratio of M100 to M75 are robust biomarkers for the development of visual function in
children and speculated that the results form a foundation for quantitative identification
of developmental delay and/or abnormalities of visual function in children with brain
disorders.
3.5.3 Magnetic Resonance Imaging
Magnetic resonance imaging is a method to obtain spatial distribution of nuclear magnetic resonance (NMR) signals using gradient magnetic fields and the Fourier transform
(FT). The basic principle of MRI was proposed by Lauterbur (1973). The principles of
MRI are described in detail by Callaghan (1993). Briefly, an MRI system consists of a
magnet, gradient coils, and RF coils. The MRI utilizes fusion techniques of spatially uniform SMF, spatially gradient magnetic fields, and RF pulse fields. The MRI has become
a standard technique for the routine diagnosis of many disease processes, replacing and
sometimes surpassing computed tomography (CT). The CT scans expose the body to
high doses of X-rays; in contrast to CT, MRI is noninvasive, uses nonionizing radiation,
and has a high soft-tissue resolution and discrimination in any imaging plane.
A guideline for exposure of the human body to SMFs set by ICNIRP (2009) suggests
2 T as the ceiling value for body parts, except for arms and legs, in occupational exposure. In the application of clinical MRI, the current exposure level is confirmed to be
less than or equal to 2 T. In SMFs at this strength it is not feasible to obtain resonance
images, except for hydrogen atoms. Tomasi and Wang (2007) estimated the magnetic
force in the human head during MRI at 4 T and confirmed that the induced gradient
fields increase the magnetic force on tissues. However, even for tissue components with
large magnetic susceptibility such as iron-containing proteins, this force is negligible
compared with the gravitational force. Therefore, the authors concluded that exposure
to uniform SMF at 4 T does not have a significant risk for tissues in the head.
Glover and Bowtell (2008) showed that current densities above regulatory limits may
be generated even for moderate natural movements near a clinical 3-T MRI scanner
although the subject did not report any perception of physiological effects. The electric
fields induced due to natural movements near the scanner are much higher than those
induced by time-varying fringe fields from the gradient coils in a subject standing still
next to the scanner while the scanner is operating.