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

1. Introduction

In recent years magnetic resonance (MR) has emerged as a diagnostic tool for clinical in vivo imaging (MRI) and spectroscopy (MRS). Magnetic resonance clinical imaging offers important advantages which have stimulated
rapid development of various systems and applications. Information is obtained on chemical and structural body properties and their pathology which in some cases cannot be obtained by other imaging modalities such as computerized tomography (CT) or ultrasound. Other advantages include the availability of images of the body in any cross-sectional orientation without loss of image quality, and the possibility of identification of vascular structures without introduction of an intravenous contrast agent⁽³²⁾.

Magnetic resonance devices use a strong magnetic field, a time-varying magnetic field and a radiofrequency (RF) field to obtain images of the body in selected planes. The physical properties utilized are the magnetic moment and spin properties of specific nuclei contained in biological molecules. The most common nucleus for imaging is the single proton hydrogen atom but imaging using other nuclei is being developed. Properties of such nuclei is phosphorus (³¹P), carbon (¹³C), sodium (²³Na) and others are used for in vivo MRS.

As with any technology, even when beneficial in medical applications, it is necessary to consider carefully the potential health hazards and to develop and implement proper safety precautions. There are several safety factors that have to be considered with respect to clinical use of MR for human beings. In this safety code biological effects of fields used in MR are briefly reviewed. Information is also given on guidelines regarding MR in other countries. General guidance is provided regarding what patient and operator exposure levels are considered safe at the present time. Advice is also provided with respect to cardiac pacemakers and metallic implants. However, other safety issues not directly related to human exposure to electromagnetic fields are not addressed here. These include possible injury by flying projectiles (because of the forces acting on ferromagnetic objects in the static magnetic field), injury due to cryogenic magnet quench, and electromagnetic interference by MR fields with other medical devices (e.g. ECG monitors). Advice regarding these problems is given in another Health and Welfare publication⁽²³⁾.