Exposure Guidelines for Magnetic Resonance Imaging

The Most Successful Application of Electromagnetics in Biology and Medicine

By James C. Lin

NOTE: This is an introduction to the article, which appeared in the June 2011 issue of the IEEE Microwave Magazine.
Click here to read the entire article.

The International Nonionizing Radiation Protection Commission (ICNIRP) is an independent group of experts with the responsibility to evaluate the state of knowledge about the effects of nonionizing radiation (NIR) on human health and well-being and to provide science-based advice and recommendations on protection against harmful effects of NIR that include static electric and magnetic fields, RF waves, visible light, and ultraviolet radiation.

In the paper, the author explains the ICNIRP’s scope of activity and how it operates. The products of the ICNIRP in fulfilling its aims to disseminate information and advice on the potential health hazards from exposure to NIR are published in the form of scientific reviews and reports and the proceedings of scientific meetings. The reviews, combined with risk assessments conducted in collaboration with the WHO, result in the recommendation by the ICNIRP of exposure guidelines. Examples of these are guidelines limiting exposure to electromagnetic fields, lasers and incoherent optical radiation, and ultraviolet radiation. The ICNIRP also publishes statements providing information and advice on specific topics of NIR protection and/or radiation protection issues related to specific devices or exposure situations.

The operation of MR equipment involves static, gradient, and RF magnetic fields. The paper addresses guidelines in each of these situations.

Exposure to static and gradient magnetic fields

The ICNIRP guidelines distinguish between exposure on the part of occupational personnel, that of the general public, and of patients. The article tabulates the ICNIRP recommendations in these areas. In addition, there are guidelines for motion within static magnetic fields, as vertigo and nausea may be experienced under these circumstances. The article tabulates the ICNIRP recommendations in these areas.

Exposure to RF magnetic fields

As is the case for static magnetic fields, the ICNIRP has developed recommendations for maximum exposure to RF magnetic fields for occupational personnel, general public, and patients. Exposure limits to RF fields are related to local or whole-body heating caused by the fields. For instance, data seem to indicate that no adverse effects occur if the increase in core body temperature does not exceed 1 °C. That value is halved in the case of infants, pregnant women, and persons with cardiovascular impairment.

The author concludes the paper with remarks on some challenging areas in setting guidelines:

  • Differences in heat specific absorption rates (SARs) and temperature distributions within the human anatomy
  • The mass of the volume over which SARs and temperatures are averaged
  • Dosimetry and thermal models of pregnant women
  • Features of SAR and heating distributions for high-field MR imaging.
  • Epidemiological studies of health effects in patients or MR workers.

AUTHOR

James C. Lin (lin@uic.edu) is with the Department of Electrical and Computer Engineering, and Department of Bioengineering, University of Illinois-Chicago 851 South Morgan Street, M/C 154 Chicago, Illinois 60607-7053 USA.