MR safety update: Why we may not need a 20-Tesla MRI machine
resh and perhaps exciting research might be presented on stage at the upcoming ESMRMB's European Magnetic Resonance meeting in Vienna, but some problems concerning MR research and applications will be discussed in the back rooms. Among them is the field strength question.
When I started working with one of the whole-body MRI prototypes in Germany in the early 1980s, some 36 years ago, I sat down to find out more about possible side effects of MR examinations and wrote an overview of the risks and dangers . A year later I was sitting on the board of a commission of the German Federal Health Agency, dealing with the same topic and giving recommendations for Germany . The research results collected and used stretched over a whole century, beginning in the late 1800s. Much of the evidence was contradictory, while some got straight to the point and was reproducible.
Safety limits for magnetic fields and electromagnetic radiation were set. The same happened in other European countries and in North America. Exposure limits were put with wide safety margins to stay on the safe side. During the following decades these limits were slowly raised because no severe of lasting side effects upon the human organism were seen – except for projectile damage caused by negligence and auditory damage by the noise at high field.
Thomas F. Budinger of the Lawrence Berkeley National Laboratory in Berkeley was deeply involved in basic research of MRI risks since the 1980s. In 1998, he wrote in an article entitled “MR safety: past, present, and future from a historical perspective”:
“Contemporary experiments and theories on health effects demonstrate that currently MR imaging is practiced in a safe manner. Technological capabilities and medical science objectives, however, will lead to procedures that will challenge the thresholds of physiological effects. Thus progress in this field will require continual surveillance and better definitions of guidelines which at present are considered prudent but too restrictive.” 
Progress in this field will require continual surveillance
and better definitions of guidelines
Early days of 10T dreams
Thirty years ago, Budinger was contemplating the “Dekatesla Project” together with the late Paul C. Lauterbur and Gerald M. Pohost – a 10-Tesla whole-body machine that was never built, for numerous reasons. Now, at the age of 84, he proposes doubling the field strength.
Budinger and a number of noted co-authors, mostly between their mid-50s and mid-80s, are established scientists with a proven ethical background who know what they are doing. They published a review of research opportunities and possible biophysical and physiological effects of magnetic resonance equipment operating at 20 T . The list of authors reads like an excerpt from the Who’s Who of basic and applied biomedical MR research. The article is an example of a well written review paper.
There are always new prospects and possible “added values” to them. Today it is, for instance, sodium MRI and phosphorus MRS at 7 T, and more scientific schemes exist for 20 T.
However, the mere idea of ultra-high field MRI is debatable; there will be problems and risks beyond heat deposition and deafening noise, both to laboratory animals and humans. Although there are some new results, in general there is a paucity of data on the physiological impact of MRI at high and ultra-high fields.
In a 2007 article Thomas A. Houpt and his collaborators wrote: “Rats, for instance, find entry into a 14.1 T magnet aversive … After their 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 requires the vestibular apparatus of the inner ear, because labyrinthectomized rats readily traversed the magnet. The inner ear is a novel site for magnetic field transduction in mammals, but perturbation of the vestibular apparatus would be consistent with human reports of vertigo and nausea around high strength MRI machines.” 
Already 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 that 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 scanners operated at 1.5 to 2 T and below .
At 7 Tesla, one third of the severely ill patients enrolled in a clinical study complained about vertigo and nausea caused by the equipment . It seems not advisable to prescribe histamine-blockers such as diphenhydramine to preventively mitigate the strength of vertigo and nausea at ultra-high static magnetic fields, although this procedure has been proposed to "pave the way to even higher field strength"  – but rather to refer patients to side-effect free 1.5 Tesla machines.
Ignoring uncomfortable news
However, people tend to look the other way and ignore uncomfortable news. Machines operating at higher field strengths became available in the research and clinical market.
More than 20 years later, scientific publications and two PhD theses from the Netherlands throw new light on hazards of ultra-high field magnetic resonance equipment operating at fields higher than 2 T. These and other articles describe some 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, at a higher degree, at 7 T [9, 10, 11]. Even if these effects are not considered to be deleterious, one cannot expect that employees and patients accept getting sick and dizzy in or close to an ultra-high field MR machine.
There are a number of additional aspects that have to be taken into account, among them volume and shear forces on diamagnetic tissues. As Budinger and coauthors stress in their paper about the 20 T project these might become a main limiting factor in ultra-high field imaging:
“Shear forces between tissue and fat or tissue and bone might be sensed but not be uncomfortable. But the susceptibility differences between iron-loaded tissues and adjacent tissues such as the cerebral cortex and other tissues will need evaluation. The importance of these differences will need to be ascertained before human subject exposures to ultrahigh fields and high-field gradients.” 
It is also still unknown what happens to magneto-biomaterials in the human brain at high/ultra-high fields and what their function is – whether they are, e.g., bioreceptors or biosensors.
Impact of EU regulations
In July 2016, the European Commission's Directive on electromagnetic fields (EMF) came into force. At present, this directive addresses only short-term effects, not yet possible long-term effects .
MRI equipment is excluded from the regulations of this directive. However, if MRI machines operating at 3 T or higher have a negative impact on the health of people working with these machines or on patients, regulatory measures, including exposure thresholds, will have to be re-evaluated and it can be expected that the conditional derogation for MRI equipment from the requirement will be revoked.
Such a step might also negatively affect clinical MRI at 1.5 T and lower fields.
Science is always a progress report; however, perhaps one should rather focus on topics that promise no harm to animals and humans but rather some clearly positive outcome. As I see it, all examinations above 2 T should be considered experimental and not clinical, and patients should be informed, in writing, about possible side-effects. The gadolinium disaster has shown us that being reckless of danger can end in the mutilation and death of patients . We should never forget this.
A detailed overview of the state of research in MRI safety can be found in the European Magnetic Resonance Forum's (EMRF) e-textbook .
Industry and taxpayer-sponsored researchers who try to push unproven ideas into the imaging health care market may act unethically and against the benefit of patients and delivery of appropriate medical care to the general public. According to Budinger's review article , it might take quite some time until the “all clear” can (or cannot) be sounded for introducing research or even clinical machines in the ultra-high field range of MRI.
There is a lot of food for thought: Aren't there better projects, e.g., working at low field and developing a patient-friendly easy-to-handle MR machine for 95% or more of all clinical examinations, for the price of a medium-sized car? Taxpayers’ money should go into such projects.
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