18-04 Regulations and Legal Aspects
It is difficult to get a proper overview of regulations concerning MRI equipment on national or international levels.
Since the early 1980s, a number of national health and radiation protection boards established recommendations concerning magnetic resonance equipment and spectroscopic units [⇒ Hentschel, ⇒ ICNIRP, ⇒ Bureau of Radiological Health, ⇒ Radiological Protection Board 1981; 1992]. All limits set by them were recommended levels, not mandatory ones. In the meantime, however, some manufacturers started using field strengths beyond 2.0 T, different pulse sequences, and gradient switching procedures without any reported ill effects. Adjustments made do not cover all possible medical applications of MR imaging, although the US- American Food and Drug Administration extended the designation of ‘nonsignificant risk’ to MR systems with field strengths of up to 4.0 T in 1997 and 8.0 T in 2003 (4.0 T in neonates younger than one month) [⇒ U.S. Department of Health]; the International Electrotechnical Commission followed suit in 2016 [⇒ IEC 2016]. At present, there is an investigational device exemption of the US Food and Drug Administration for fields up to 9.4 Tesla.
For legal reasons, the owner of MR equipment has to ensure that the equipment does fulfil the local requirements. In some countries, the regulations are more stringent than in others; in other countries, they are nonexistent.
The RF energy during an MR examination must be adjusted to avoid producing a core temperature rise in excess of 1°C and localized heating greater than 38° C in the head (3.2 W/kg averaged over head mass), 39° C in the trunk (or 10 W/kg over any 10 grams), and 40° C in the extremities (or 10 W/kg over any 10 grams) [⇒ NEMA].
The legal requirements must be guaranteed by the manufacturer because the user in general is unable to check power output, gradient strength, or even field strength. This guarantee must cover authorized hardware and software updates after the initial installation. Specially designed computer programs usually supervise the power output of MR systems and will not allow or will interrupt any imaging or spectroscopy procedure exceeding those limits considered safe.
According to the specific FDA criteria for SAR (specific absorption rate) limits [⇒ U.S. Department of Health], the SAR must not be greater than:
4 W/kg averaged over the whole body for any 15-minute period;
3 W/kg averaged over the head for any 10-minute period; or
8 W/kg in any gram of tissue in the extremities for any period of 5 minutes.
Some European countries have also issued national SAR restriction. No common denominator has been found.
After many years of deliberation, the European Commission too has published some guidelines. They follow those of the International Commission on Non Ionizing Radiation Protection [⇒ European Commission, ⇒ ICNIRP].
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 [⇒ European Commission].