MR safety: Why we may not need a 20T scanner

Fresh and perhaps exciting research might be presented on stage at the upcoming European Society for Magnetic Resonance in Medicine and Biology (ESMRMB) 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.

Dr. Peter Rinck, PhD, is a professor of diagnostic imaging and the president of the Council of the Round Table Foundation (TRTF) and European Magnetic Resonance Forum (EMRF).

When I started working with one of the whole-body MRI prototypes in Germany in the early 1980s, I sat down to find out more about possible side effects of MR examinations and wrote an overview of the risks and dangers.¹ 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.²

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 are put with wide safety margins to stay on the safe side. During the following decades these limits were slowly raised because no severe, 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, California, was deeply involved in basic research of MRI risks in the 1980s. In 1998, he wrote an article titled "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.³

Early days of 10T machines

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. He and a number of co-authors published a review of research opportunities and possible biophysical and physiological effects of MR equipment operating at 20 tesla.⁴ 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 tesla, and more scientific schemes exist for 20 tesla.

However, the mere idea of ultrahigh-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 ultrahigh fields.

In a 2007 article, Thomas A. Houpt and his collaborators wrote: "Rats, for instance, find entry into a 14.1-tesla magnet aversive ... After their first climb into 14.1-tesla, most rats refused to re-enter the magnet or climb past the 2-tesla 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."⁵

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 tesla than at 1.5 tesla. In addition, they found magnetophosphenes caused by motion of the eyes within the static field.

The results were published in a full paper in 1992 and considered proof 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 tesla and below.⁶

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 ultrahigh-field MR equipment operating at fields higher than 2 tesla. 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 tesla and, at a higher degree, at 7 tesla.^7,8,9 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 ultrahigh-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 co-authors stress in their paper about the 20-tesla project, these might become a main limiting factor in ultrahigh-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.⁴

It is also still unknown what happens to magneto-biomaterials in the human brain at high/ultrahigh 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.¹⁰

MRI equipment is excluded from the regulations of this directive. However, if MRI machines operating at 3 tesla or higher have a negative impact on the health of people working with these machines or in 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 tesla 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 tesla 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.¹¹ We should never forget this.

A detailed overview of the state of research in MRI safety can be found in the European MR Forum's e-textbook.¹²

Industry and taxpayer-sponsored researchers who try to push unproven ideas into the imaging healthcare 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 ultrahigh 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.

Dr. Peter Rinck, PhD, is a professor of diagnostic imaging and the president of the Council of the Round Table Foundation (TRTF) and European Magnetic Resonance Forum (EMRF).

References

1. Rinck PA. Risiken und Gefahren der NMR-Tomographie. Dtsch Med Wschr. 1983; 108: 992-994.
2. Bundesgesundheitsamt (der Bundesrepublik Deutschland) et al.: Empfehlungen zur Vermeidung gesundheitlicher Risiken verursacht durch magnetische und hochfrequente elektromagnetische Felder bei der NMR-Tomographie und In-vivo-NMR-Spektroskopie. Bundesgesundheitsblatt. 1984; 27: 92-96.
3. Budinger TF. MR safety: Past, present, and future from a historical perspective. Magn Reson Imaging Clin N Am. 1998;6:701-714.
4. Budinger TF, Bird MD, Frydman L, et al. Toward 20 T magnetic resonance for human brain studies: Opportunities for discovery and neuroscience rationale. MAGMA. 2016;29:617-639.
5. Houpt TA, Cassell JA, Riccardi C, DenBleyker MD, Hood A, Smith JC. Rats avoid high magnetic fields: Dependence on an intact vestibular system. Physiology & Behavior. 2007;92:741-747.
6. Schenck JF, Dumoulin CL, Redington RW, Kressel HY, Elliott RT, McDougall IL. Human exposure to 4.0-Tesla magnetic fields in a whole-body scanner. Med Phys. 1992;19:1089-1098.
7. Roberts DC, Marcelli V, Gillen JS, Carey JP, Della Santina CC, Zee DS. MRI magnetic field stimulates rotational sensors of the brain. Curr Biol. 2011;21:1635-1640.
8. van Nierop LEV, Slottje P, Zandvort MJV, De Vocht F, Kromhout H. Effects of magnetic stray fields from a 7 Tesla MRI scanner on neurocognition: A double-blind randomized crossover study. Occup Environ Med. 2012;69:761-768.
9. Schaap K, Portengen L, Kromhout H. Exposure to MRI-related magnetic fields and vertigo in MRI workers. Occup Environ Med. 2016;73:161-166.
10. European Commission, Directorate-General, for Employment, Social Affairs and Inclusion, Unit B3. Nonbinding guide to good practice for implementing Directive 2013/35/EU, Electromagnetic Fields, Volume 1: Practical Guide. Brussels: European Union, 2015.
11. Rinck PA. Gadolinium -- Will anybody learn from the debacle? Rinckside. 2015;26(9):23-26.
12. Rinck PA. Chapter Eighteen: Safety of Patients and Personnel. In: Magnetic Re­so­nan­ce in Me­di­ci­ne. The Basic Textbook of the European Magnetic Resonance Forum. 9th ed. 2016. E-ver­sion 9.8. http://magnetic-resonance.org/ch/18-01.html

The comments and observations expressed herein do not necessarily reflect the opinions of AuntMinnieEurope.com, nor should they be construed as an endorsement or admonishment of any particular vendor, analyst, industry consultant, or consulting group.