Guidelines on Exposure to Electromagnetic Fields from Magnetic Resonance Clinical Systems - Safety Code 26

4. Health Effects of MR Fields

General reviews of health effects of MR fields have been published
^{( 4,5,6,7,24,25,30)}. In addition, comprehensive reviews of the biological effects of static magnetic fields⁽²⁸⁾, time-varying magnetic fields⁽²⁹⁾ and RF fields⁽¹¹⁾ are also available. Therefore, only a brief outline of biological effects of each of the three types of fields is given. A relatively limited number of studies have been reported on effects of MR exposures (all three fields), and these are reviewed here.

4.1 Static Magnetic Field

Static magnetic fields can interact with biological systems by exerting forces on molecules and cells having diamagnetic susceptibility. They can also affect enzyme kinetics and act on moving charges (including moving fluids). Molecules and some cellular structures such as retinal rods, DNA, and sickle cells are magnetically anisotropic, and therefore a force acts upon them in a static magnetic field, which tends to orient them with the field. Fields of the order of about 0.3 to 2 T have been reported to be effective in causing orientation in samples studies in vitro^(4,6,7,30). Enzymatic reactions can be affected by strong magnetic fields of the order of 20 T⁽⁶⁾.

A static magnetic field exerts a force on a moving charge in the field. The force is directed perpendicularly to the direction of the field and the direction of the motion. Through this mechanism magnetic fields can distort current loops for nerve conduction (propagation of the action potential) and can cause a decrease in the conduction potential and a decrease in the conduction velocity. Strong fields above 24 T are required for this effect^(6,30).

Another type of interaction involves moving conducting fluids such as blood flow and periodic movement of certain body parts e.g. chest and heart contractions. Motion of a conductor in a magnetic field results in induction of a potential across the conductor, or in the case of a human being across a blood vessel. The induced voltage depends on the magnetic flux density, the vessel diameter, blood flow rate and the orientation of the blood vessels with respect to the direction of the field. These potentials are detectable in ECG; however they are physiologically insignificant until a threshold for the depolarization of cardiac muscle fibers is reached. An approximate "worst case" calculation indicates that 2.5 T induces flow potentials of the order of 40 mV, which is the depolarization threshold for individual cardiac muscle fibers⁽²⁴⁾. However, the calculated potential refers to the cross-section of the aorta, and much lower potentials are induced across individual cells⁽²⁴⁾. The
potentials induced by movement of cross-sections such as the thorax are much lower than those calculated for the blood flow⁽⁶⁾.

The available scientific data on biological effects of static magnetic fields is rather limited and inconsistent^{(4,,5,67,,24,25,28,30)}. On the basis of a number of carefully performed studies, the following important biological processes appear not to be affected by static magnetic fields up to approximately 2 T⁽³⁰⁾:

1. cell growth and morphology,
2. DNA structure and gene expression,
3. reproduction and development,
4. bioelectric properties of isolated neurons,
5. animal behaviour,
6. visual response to photic stimulation,
7. cardiovascular dynamics,
8. hematological indices,
9. immune response,
10. physiological regulation and circadian rhythms. However, the scientific data base, at present, is not sufficient to assess the risk of exposure to higher static fields⁽³⁰⁾.

There have been very few human studies. Some evidence has emerged which indicates that occupational exposures of humans to up to 2 T for durations of a few hours do not seem to cause any adverse effects. Exposures to up to 0.5 T for prolonged periods of time did not result in any deleterious effects. These conclusions are drawn from a study of workers in nuclear physics laboratories^(5,25). Exposure limits of 0.01 to 0.03 T for 8h/day have been recommended for workers in nuclear physics laboratories in various countries⁽²⁷⁾, and can therefore serve as a reference level for MR operators. The guidelines permit, however, higher exposures for short periods of time.

4.2 Time-Varying Magnetic Field

Time-varying magnetic fields interact with biological systems primarily through induction of internal electric currents so called "eddy currents". The magnitude of the current depends on the time rate of change of the magnetic flux density and on the radius of the current loop. The current loops are in planes perpendicular to the direction of the magnetic field. The threshold current densities for known biological effects have been established^{(,5,6,7,24,25,30)}. The effects include fibrillation, electroshock, induction of visual phosphenes, and initiation of impulses in nerve and muscle cells. The thresholds ar e functions of the rate of change of the magnetic flux density and the time duration of the applied time-varying field.

Current densities induced in a human body and its parts should be calculated by assuming a "worst case" radius loops, i.e. the largest realistic loop under practical exposure conditions.

Approximate threshold current densities in living tissues are as follows⁽³⁰⁾:

1. 1 A/m2 for cardiac fibrillation,
2. 10 mA/m2 for reversible visual effects (magnetophosphenes),
3. 10-100 mA/m2 applied chronically for irreversible alterations in the biochemistry and physiology of cells and tissues (e.g. current densities used in bone healing). Fields that induce current densities less than approximately 1-10 mA/m2, which is the range of endogenous current densities (EEG, EKG) appear to cause few, if any, biological effects for non-chronic exposures⁽³⁰⁾.

Evaluation of the above thresholds and biological effects observed has led to the conclusion that human exposure to 3 T/s is of minimal, if any, health hazard⁽²⁷⁾, and was adopted in the U.S. recommendations⁽¹⁴⁾. The U.K. recommendation⁽¹⁾ of 20 T/s was based on an estimate that this rate of change of the magnetic flux induces a maximum of 0.3 A/m2 in any part of the body, which is a factor of approximately 3 below the threshold for cardiac fibrillation. The F.R.G. recommendation⁽²²⁾ of 30 mA/m2 corresponds approximately to 3 T/s⁽³⁰⁾. The higher limits for short-duration pulses (less than 10 ms) in the U.K. and the F.R.G. recommendations are based on the relationship between the duration of the electric current flow and human response⁽¹⁾.

Recently, a study was performed to assess effects of pulsed magnetic fields on foetal development in mice⁽¹⁷⁾. Exposures ranged from 3.5 - 12 kT/s with pulse periods 0.33 - 0.56 ms. Exposures were of short durations during various stages of gestation. Some exposure conditions resulted in stimulation of superficial skeletal muscle. No adverse effects were observed on pregnancy, litter size and growth of off-springs of exposed mice as compared to controls⁽¹⁷⁾.

4.3 Radiofrequency Field

Detrimental health effects from exposure to radiofrequency (RF) fields are associated with high rates of energy deposition. Because the interactions of RF fields depend on the field frequency, type of field (electric, magnetic, far-field, near-field) and the body size and shape, a parameter called the specific absorption rate, (SAR) has been used to quantify the effects. The SAR is the dose rate, defined as the rate at which RF energy is imparted into a unit mass of the exposed biological body. The unit of the SAR is the watt per kilogram (W/kg). The SAR is usually spatially nonuniform within the human body. In
the case of MR systems, the spatial distribution depends on the design of the transmitter coils, the frequency, and the shape, size and tissue type of the imaged object.

Exposure to RF fields at sufficient SARs results in local or whole-body temperature increases and RF fields⁽¹¹⁾. It has been estimated that a whole body average SAR between 1 and 4 W/kg for short durations (approximately 1 h) produces significant increases in human body temperatures, (about 0.5oC at SAR = 1.4 W/kg) at ambient temperatures of 25 to 30oC and RF fields⁽¹¹⁾. Higher increases in whole-body temperature can be expected in people having impaired thermoregulatory capability. Furthermore, local temperature increases in locations of high SARs may be much greater and RF fields⁽¹¹⁾.

Effects of RF fields on various systems have been investigated and threshold limits in terms of the SAR and exposure duration have been established for several effects and RF fields⁽¹¹⁾. Many of the effects can be explained on the basis of general or localized heating. However, some of the effects are due to other non-thermal mechanisms.
Several potentially significant effects have been documented at whole-body average SARs of 1 to 3 W/kg for prolonged exposures. These include: behavioral response alterations, promotion of cancer development in mice, a decrease in the number of Purkinje cells in the brain of rats, changes in endocrine gland function and blood chemistry, and reversible changes in hematologic and immunologic systems and RF fields⁽¹¹⁾. Furthermore, such non-thermal effects as changes in cellular energy metabolism in the rat brain and changes in calcium-ion efflux have been reported. The latter are for RF fields modulated at extremely low frequencies (i.e. frequencies between 1 and 300 Hz). RF fields resulting in higher SARs between 4 and 8 W/kg have been shown to result in such detrimental effects in experimental animals as behavioral disruption, temporary sterility, and bradycardia and RF fields⁽¹¹⁾.

Human data are very limited and not useful for the development of quantitative recommendations on safe exposure limits.