The 4th edition of the International Electrotechnical Commission (IEC) standard 60601-2-33 Medical electrical equipment – Part 2-33: Particular requirements for the basic safety and essential performance of magnetic resonance equipment for medical diagnosis published in 2022 has an updated ‘controlled access area‘. The previous edition defined this area as being enclosed by the 0.5 mT (5 gauss) contour, colloquially known as ‘the 5 gauss line.’ The 0.5 mT limit for public access was based upon the possibility of effecting the function of cardiac pacemakers which utilised magnetically activated reed relay switches.
The new edition of IEC 60601-2-33 redefines the limit for controlled access to 0.9 mT. What does this mean in practice?
IEC 60601-2-33 4edn, 2022
The fourth edition of IEC 60601-2-33 redefines the limit for controlled access to 0.9 mT. What does this mean in practice? There are two principal considerations:
- technical
- regulatory
Technical consequences
The extent of the 0.9 mT contour will be marginally smaller than the 0.5 mT one. On the z-axis, this may reduce the extent by around 50 cm, allowing for a reduction in the installed footprint by about 20%. Approximate values are shown in the table and chart below.
| B0 (T) | z-distance to 0.5 mT | z-distance to 0.9 mT | Minimum area 0.5 mT (m2) | Minimum area 0.9 mT (m2) |
| 1.5 | 4.0 | 3.6 | 40 | 32 |
| 3.0 | 4.6 | 4.3 | 48 | 40 |
Regulatory consequences
IEC-60601-2-33 is an international technical standard for manufacturers of MRI equipment. However other local/national regulations, standards or guidance may apply (see here). At the time of writing these principally stipulate a 0.5 mT limit. In planning new MRI installations it is imperative to comply with the relevant national safety requirements and laws. Until these change, the ‘5 gauss line is still the 5 gauss line.’