6.7 Biological Effects of Low Frequency EM Fields
201
6.6.2 Nuclear Magnetic Resonance and Imaging
Just as in the case of Electron Spin Resonance (ESR), isolated protons will flip their
spins by resonantly absorbing a photon at frequency
f p = eg p B/(2πm p c) = (42.5781 MHz/T) B.
For B of 1 T, f p = 42.5781 MHz, which is in the radio frequency band of the
electromagnetic spectrum. Typical measurements use a sweeping frequency in the
range 300–1000 MHz, with a fixed magnetic field with a strength about 1 T.
As water is plentiful in soft biological tissue, protons in water molecules will
dominate the ‘nuclear magnetic resonance’ (NMR) of soft tissue.
To generate a ‘Magnetic Resonance Image’ (MRI), the tissue is placed in a strong
magnetic field with a gradient, i.e. a field which increases in a particular direction.
A broad-band radio frequency pulse is passed through the tissue. The absorption of
this pulse is monitored over each segment of tissue perpendicular to the direction of
the gradient of the magnetic field and at various angles in this plane. After scanning
all segments, the absorption data is computer analyzed to find what the densities of
absorbing protons are for particular magnetic environments.
MRI is relatively safe for live tissue, 15 not least because there is no ionizing
radiation involved. There are quite large magnetic fields, so any ferromagnetic
materials (such as might be in implants or pacemakers) are prohibited. The radio
waves used to stimulate the protons must also be limited to prevent overheating.
Specific Absorption Rates have been established for this purpose, determined by the
temperature rise in a given mass of tissue. (See the Table at the end of Sect. 6.7.2.)
6.7 Biological Effects of Low Frequency EM Fields
Electromagnetic waves in the radio and microwave range are considered low
frequency (0–300 GHz). The photons of these waves have insufficient energy per
photon to ionize atoms or to break molecular bonds. Biological tissue can be
affected because the electron orbits are weakly distorted by the passing wave. This
distortion produces a weak electric dipole moment, and the atoms and molecules
respond by movement in synchrony with the wave frequency, but with some phase
delay. If the atoms or molecules are only weakly held by intramolecular forces, their
resulting vibrational motion can be dissipated into heat, causing some absorption of
energy from the passing wave. Ions and quasi-free electrons will also easily respond
to passing electromagnetic waves.
15 The word ‘nuclear’ has been removed from the title of this procedure by doctors and equipment
manufacturers due to its negative connotation in some patients. In fact, the nuclei involved undergo
no nuclear transitions, so there is no nuclear radiation generated.
201
6.6.2 Nuclear Magnetic Resonance and Imaging
Just as in the case of Electron Spin Resonance (ESR), isolated protons will flip their
spins by resonantly absorbing a photon at frequency
f p = eg p B/(2πm p c) = (42.5781 MHz/T) B.
For B of 1 T, f p = 42.5781 MHz, which is in the radio frequency band of the
electromagnetic spectrum. Typical measurements use a sweeping frequency in the
range 300–1000 MHz, with a fixed magnetic field with a strength about 1 T.
As water is plentiful in soft biological tissue, protons in water molecules will
dominate the ‘nuclear magnetic resonance’ (NMR) of soft tissue.
To generate a ‘Magnetic Resonance Image’ (MRI), the tissue is placed in a strong
magnetic field with a gradient, i.e. a field which increases in a particular direction.
A broad-band radio frequency pulse is passed through the tissue. The absorption of
this pulse is monitored over each segment of tissue perpendicular to the direction of
the gradient of the magnetic field and at various angles in this plane. After scanning
all segments, the absorption data is computer analyzed to find what the densities of
absorbing protons are for particular magnetic environments.
MRI is relatively safe for live tissue, 15 not least because there is no ionizing
radiation involved. There are quite large magnetic fields, so any ferromagnetic
materials (such as might be in implants or pacemakers) are prohibited. The radio
waves used to stimulate the protons must also be limited to prevent overheating.
Specific Absorption Rates have been established for this purpose, determined by the
temperature rise in a given mass of tissue. (See the Table at the end of Sect. 6.7.2.)
6.7 Biological Effects of Low Frequency EM Fields
Electromagnetic waves in the radio and microwave range are considered low
frequency (0–300 GHz). The photons of these waves have insufficient energy per
photon to ionize atoms or to break molecular bonds. Biological tissue can be
affected because the electron orbits are weakly distorted by the passing wave. This
distortion produces a weak electric dipole moment, and the atoms and molecules
respond by movement in synchrony with the wave frequency, but with some phase
delay. If the atoms or molecules are only weakly held by intramolecular forces, their
resulting vibrational motion can be dissipated into heat, causing some absorption of
energy from the passing wave. Ions and quasi-free electrons will also easily respond
to passing electromagnetic waves.
15 The word ‘nuclear’ has been removed from the title of this procedure by doctors and equipment
manufacturers due to its negative connotation in some patients. In fact, the nuclei involved undergo
no nuclear transitions, so there is no nuclear radiation generated.