TwinTree Insert

18-04 Physiological Hazards

he physiological risks of magnetic and radiofrequency fields have been in­ten­si­ve­ly examined for a long time. There is, for instance, danger of ra­dio­fre­quen­cy heating, of shear forces between brain tissues, nerve stimulation by gra­dient fields, as well as genotoxic effects. There are reports about sensations of ver­ti­go, nau­sea, and metallic taste at 3 and 7 Tesla.

Yet, there are hardly any long-term studies following persons who were exposed to such fields. To get an impression of the complexity of the topic we shall discuss some possible physiological hazards in more detail on the following pages. A number of topics require further evaluation, in particular concerning ultrahigh mag­netic fields.

18-04-01 Physiological Hazards: Static Magnetic Fields

In every MR examination, a large static magnetic field is applied. Field strengths for cli­ni­cal equip­ment can vary between 0.2 and 3.0 T (even 7 Tesla in some regions); to date, experimental imaging units have a field strength of up to 17.5 T (Table 18-01).

Table 18-01:
Definition of field strength. Ultralow-field machines operate at a field strength below 0.1 T, low field bet­ween 0.1 and 0.5 T, medium field between 0.5 and 1 T, high field be­tween 1 and 2 T, and ultrahigh field machines above 2 T (Definitions set by EMRF in 1989).

There are a number of biophysical effects whereby static magnetic fields might in­flu­en­ce bio­lo­gi­cal processes or an organism's behavior. Hoff and collaborators stress that forces on metallic implants and radiofrequency power deposition and heating are safety considerations at 7 T. Patient bioeffects such as vertigo, dizziness, false feel­ings of motion, nausea, nystagmus, magnetophosphenes, and electrogustatory ef­fects are more common and potentially more pronounced at 7 T than at lower field strengths [⇒ Hoff 2019].

spaceholder redVestibular system. In 1988 a group at the General Electric Corporate Research and Development Center described in an abstract sensations of vertigo, nausea, and me­tal­lic taste in a group of volunteers. There was statistically significant evidence for field-dependent effects which were greater at 4 T than at 1.5 T. In addition, they found magnetic phosphenes caused by motion of the eyes within the static field. The results were published in a full paper in 1992 and considered proof that there is a sufficiently wide margin of safety for the exposure of patients to the sta­tic fields of conventional magnetic resonance equipment operated at 1.5 to 2 T and below [⇒ Schenck 1992].

More than twenty years later, scientific articles and two PhD theses from the Ne­ther­lands threw new light on hazards of ultrahigh field magnetic resonance equip­ment operating at fields higher than 2 Tesla, describing reversible de­cline in cog­ni­ti­ve func­tion as well as symptoms of nystagmus, vertigo, postural instability, nausea, and metallic taste in employees working with MRI at fields of 3 T and, to a higher de­gree, at 7 T [⇒ Roberts 2011, ⇒ Schaap 2015, ⇒ van Nierop 2015].

One third of the severely ill patients en­rolled in a comparative study at 7 Tesla com­plain­ed about vertigo and nausea caused by the equipment [⇒ Springer 2016].

These symptoms, with the exception of the observed change of taste, hint to an ef­­fect of the magnetic field on the vestibular system that is responsible for the sense of balance, spatial orientation, and posture which was substantiated by Houpt and colleagues. They observed that rats did not enter a 14.1 T magnet. After a first climb into 14.1 T, most rats refused to re-enter the magnet or climb past the 2 T field line. De­tection and avoidance required the vestibu­lar apparatus of the inner ear, because after surgical removal of the labyrinth rats read­ily traversed the mag­net [⇒ Houpt 2007].

It is not advisable to pre­scribe his­tamine-blockers such as di­phen­hydra­mine to pre­­ven­­ti­­ve­ly mi­ti­­gate the strength of ver­tigo and nausea at ultrahigh static mag­netic fields, although this pro­ce­dure has been pro­pos­ed to "pave the way to even higher field strength" [⇒ Thormann 2013].

Such patients should rather be referred to lower field machines.

Some additional results are contradicto­ry and can­not be ex­plain­ed by bio­phy­sical or bio­che­mi­cal mecha­nisms. The effects ob­serv­ed may be at­tri­but­ed to other causes not con­si­der­ed by the re­sear­chers in the setup of the ex­perimental pro­to­col.

spaceholder redVolume forces — shear forces. Volume forces are dependent on tissue sus­cep­ti­bi­li­ty and the product of field strength and spatial field gradient. Their threshold for human tolerance is still unknown.

There is also limited knowledge about the susceptibility differences between iron con­tain­ing tis­sues in the cerebral cortex and surrounding tis­sues and possible shear­ing issues at ultrahigh magnetic fields, such as subtle variations in the magnetic pro­per­ties of brain tissue, possibly reflecting varying iron and myelin content [⇒ Fukunaga 2010].

spaceholder redNerve conductivity. As early as 1893, the first results of experiments about a pos­­sib­le influence of static magnetic fields upon nerve tissue were obtained [⇒ D’Arsonval 1893]. These and all later experi­ments showed negative results. There are apparently no effects on the conduction of impulses in the nerve fiber up to a field strength of 0.1 T generated by either chang­ing the electrical resistance or the po­ten­tial of the excitation [⇒ Abashin 1975, ⇒ ACR 2007].

The minimum magnetic field required to produce observable effects seems to be quite large. Theo­re­ti­cal considerations ar­gue that fields of 24 T are required to pro­­duce a 10% re­­duc­­tion of nerve impulse conduction velocity [⇒ Wikswo 1980].

spaceholder redChanges in enzyme kinetics. Up to 45 Tesla, no important effects on enzyme sys­­tems have been observed.

spaceholder redOrientation changes of macromolecules, liv­ing cell subcellular components, and magneto-biomaterials in the brain. A re-ori­entation caused by dia­mag­ne­tic an­iso­tro­py is seen in highly ordered biological struc­tures, such as sickle cells and re­ti­nal rods in magnetic fields of 0.35 and 1.0 Tesla, re­spectively. While it is not pos­si­ble to orient the individual constituent molecules with such fields, these structures can be oriented as a whole by summing the anisotropy over a large number of mu­tu­al­ly oriented mole­cules. These results are reproducible [⇒ Hong 1995].

It is still unknown what happens to mag­neto-biomaterials in the human brain at high and ultrahigh fields; it is also still un­known what their function is — whether they are for instance bioreceptors or biosensors [⇒ Kirschvink 1992, ⇒ Schult­heiss-Gras­si 1999].

Biogenic magnetite in the human brain was detected as a minimum of 5 million single-domain crystals per gram for most tissues in the brain and greater than 100 million crystals per gram for pia and dura. Magnetic property data indicate the crystals are in clumps of between 50 and 100 parti­cles. Apparently, such na­no­par­tic­les can also be incorporated into the brain by breathing in polluted air.

spaceholder redMagnetohydrodynamic effects. In a model of the human vasculature it was shown that changes in hydrostatic pressure in the presence of a large static mag­ne­tic field (10 Tesla) were less than 0.2% [⇒ Keltner 1990].

These changes are claimed to be caused by interaction of in­­du­ced elec­tri­cal po­ten­tials and currents within a solution, e.g. blood, and an elec­tri­cal volume force caus­ing a retardation in the direction opposite to the fluid flow. This decrease in flow velocity must be compensated for by an elevation in pressure. At and below 1.5 T no significant changes are ex­pect­ed [⇒ Budinger 1986, ⇒ Tenforde 1983].

spaceholder redMembrane transportation and blood sedi­mentation. Other potential ha­zards from static fields include, for instance, mem­brane transportation and blood se­di­men­ta­­tion induced by the field. As Mansfield pointed out, static magnetic field gra­dients of 0.01 T/cm make no significant dif­fe­ren­ce in the membrane transport pro­­ces­ses. The influence of a static magnetic field upon erythrocytes is not sufficient to provoke sedimentation, as long as there is a normal blood circulation [⇒ Mans­field 1981].

spaceholder redCardiac changes. A field-strength-dependent increase in the amplitude of the ECG in rats has been observed during exposure to homogeneous stationary magnetic fields. The minimum level at which augmentation could be observed was 0.3 T; at 2.0 T, the increase was by an average of 400%. The augmentation in T-wave am­pli­tu­de oc­cur­red instantaneously and was immediately reversible after exposure to the mag­ne­tic field ceased (Figure 18-10). There have been no abnormalities in the ECG in later follow-up. The authors suggest that augmentation of the signal amplitude in the T-wave segment may result from a superimposed electrical potential [⇒ Gaffey 1981].

Figure 18-10:
Flowing blood can behave as a moving con­duc­tor in a magnetic field. The field can induce a vol­tage that will be highest during the part of the cardiac cycle with the fastest blood velocity. This coincides with the T-wave of the ECG and enhances the T-wa­ve, po­tentially mimicking pathology.

At field strengths of between 7 and 10 T, no arrhythmia could be proven [⇒ Batto­cletti 1981]. According to the national radiation protection agencies, it is un­­li­­ke­­ly that cardiac fibrillation occurs as a result of induced flow potential in the major blood vessels or heart chambers at this level of field intensity.

No cir­cu­la­to­ry alterations coincide with the ECG changes. No biological risks are be­lieved to be associated with them.

spaceholder redGenetic effects. There have been several reports that static magnetic fields may provoke genetic mutations, changes in growth rate and leukocyte count and other effects; however, some results of these experiments could be reproduced, others could not [⇒ Schwartz 1982, ⇒ Vijayalaxmi 2015].

Nevertheless, some authors claim it be un­li­ke­ly that mutagenic effects are in­tro­­duced by fields lower than 1.0 T [⇒ Mansfield 1981], in addito­on, there is no con­vinc­ing evidence for a genotoxic effect from MRI up to 7 T [⇒ Budinger 2016], al­though, for instance, Takashima and collabo­rators described genotoxic effects in DNA-repair defective mutants of drosophila melanogaster after 24-hour ex­posure to sta­tic magnetic fields of 2, 5, and 14 T [⇒ Takashima 2004].

18-04-02 Physiological Hazards: Varying Magnetic Fields

Varying magnetic fields are necessary for the localization of nuclei with magnetic pro­per­ties within the sample.

spaceholder redMag­ne­tic phos­phe­nes are a well described effect of varying magnetic fields. They were first observed in the late 19th century [⇒ D’Arsonval 1896].

Phosphenes are stimulations of the optic nerve or the retina, producing a flashing sen­sa­tion in the eyes. They seem not to cause any damage in the eye or the nerve. They are attributed to magnetic-field variations, but difficult to create in com­mon cli­ni­cal systems and may start oc­cur­ring in a threshold field change of between 2 and 5 T/s. Motion-induced magnetic phosphenes were easily visible at 4 Tesla [⇒ Schenck 1992].

spaceholder redElectrogustatory effects are claimed not to be connected to the presence of me­tal­lic tooth-fil­lings. An exact threshold could not be determined. It seems to be set off by the motion of the head, depending on rate and direction.

However, one experimental publication hints that in an ex vivo setting, mercury may be released from amalgam fillings after exposure to 7.0 Tesla but not at 1.5 Tesla [⇒ Yilmaz 2018].

spaceholder redPeripheral nerve stimulation (PNS). The mean threshold levels for various sti­mu­la­tions are 3600 T/s for the heart, 900 T/s for the respiratory system, and 60 T/s for the peripheral nerves. They increase with field [⇒ Budinger 1991].

Guidelines in the United States limit switching rates at a factor of three below the mean threshold for peripheral nerve stimulation.

Varying magnetic fields are also used to stimulate bone heal­ing in non-unions and pseud­ar­thro­ses. The reasons why pulsed magnetic fields support bone heal­ing are not completely understood [⇒ Bassett 1982].

Rapid echo-planar imaging and high-performance gradient systems create fast-switching magnetic fields inducing currents that stimulate muscle and nerve tissues (cf. EPI).

18-04-03 Physiological Hazards: Varying Magnetic Fields

Radiofrequency pulses are used in MR imaging for the excitation of the nuclei.

spaceholder redEx­tre­me­ly low-frequency (ELF) fields. Their influence has been blamed for nu­me­rous reactions, occurrences and diseases in animals and humans, for instance can­cers, Alzheimer's disease, or even causing a decrease in milk production in cows.

The most likely best known publications among articles about this topic are those as­so­ci­a­ting an increase in the incidence of leu­ke­mia with the location of buildings close to high-current power lines with ELF electromagnetic radiation of 50-60 Hz, and industrial exposure to electric and magnetic fields. In 1979, Wertheimer and Lee­per reported an association between childhood cancer and elec­tri­cal cur­rent con­fi­gu­ra­tion of houses in Denver, Colorado [⇒ Wertheimer 1979]. This publication pro­­vok­­ed a torrent of questions and research programs [⇒ Milham 1982, ⇒ Willett 2003].

To date, there is neither a final confirmation of a connection nor is there a cor­ro­bo­ra­tion of the contrary. Anyway, a transposition of such effects to MRI seems rather un­li­ke­ly, if they exist at all.

Because of the nearly unlimited number of variables it is nearly impossible to col­lect un­bi­as­ed statistics in huge populations; for instance, the death toll caused by air pollution is orders of magnitude higher than the claimed toll by leukemia caused by ELF.

spaceholder redHeat deposition. RF fields may interact with both tissues and foreign bodies, such as metallic im­plants, in the pa­tient. The main result of this type of interaction is heat.

The higher the frequency (and thus magnetic field), the larger will be the amount of heat developed; and the more ionic the biochemical environment in the tissue, the more energy that will be deposited as heat [⇒ Led 1978, ⇒ NRPB 1981, 1992].

This effect is well-known for homogeneous model systems, but the com­plex struc­ture of various human tissues makes detailed theoretical calculations very difficult, if not impossible.

RF power deposition and thus heating are increased by changing MR parameters such as decreasing the RF repetition time, adjusting flip angles, and chang­ing mat­rix size [⇒ Bottomley 1981, ⇒ Mollerus 2010].

In several in vitro and in vivo low and medium field experiments, no life threa­­te­n­­ing increase in tem­pe­ra­ture could be shown. Even in high magnetic fields, no local tem­pe­ra­tu­re increase greater than 1° C occurred [⇒ Budinger 1986, ⇒ Liboff 1965].

The highest skin temperature increase described in humans reached 2.1° C [⇒ Shellock 1994], however in the uterus of pregnant animals at ultrahigh field (3 Tes­la) 2.5° C were measured [⇒ Cannie 2016].

spaceholder redEddy currents may heat up implants and thus may cause local heating. In vitro worst-case experiments performed with a large and very thin thermally in­su­la­ted alu­mi­num sheet at 1.5 T after 15 minutes of exposure showed a tem­pe­ra­tu­re rise of only 0.08°C.

spaceholder redHot spots may occur in the exposed tissue. At present, it seems unlikely that such hot spots in the body exist, but to avoid or at least minimize effects of such theo­re­ti­cal com­pli­ca­tions, the frequency and the power of the RF irradiation should be kept at the lowest possible level.

spaceholder redThe Specific Absorption Rate (SAR) helps to estimate RF heating effects. It might be a poor indicator of magnetic resonance-related implant heating [⇒ Nitz 2005]. However, SAR is regulated and MR operators are required to follow these regulations.

SAR increases with field strength, radiofrequency power and duty cycle, as well as trans­­mit­­ter coil type and body size. In high and ultrahigh fields, some pulse se­quen­ces or procedures may create a higher SAR than recommended by the agencies.

At low fields, the maximum SAR is at the surface; this changes if the field strength is increased to high and ultrahigh fields. With the exception of the eyes, the human head, for instance, has good heat removal mechanisms for its surface, but not for the brain. As Hoult pointed out in a review paper [⇒ Hoult 2000], caution is called for when imaging at ultrahigh fields.