New ICNIRP guidelines for RF exposure: what you need to know
The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has just updated its guidelines for human exposures in the radio-frequency (RF) region: 100 kHz to 300 GHz [1]. These replace previous guidelines from 1998 [2] which were reaffirmed in 2009 [3]. Along with a timely review of the evidence for health effects, new restrictions and limits aim to address exposures from developing technologies, such as 5G. Differences in the guidelines are summarised in [4].
What changes are relevant for MRI?
The ICNIRP guidelines cover occupational exposure and that of the general public. They do not relate to patient exposures in MRI. However, one significant change is that the foetus is now considered to be a member the public.
According to ICNIRP pregnant workers should observe exposure limits for the general public.
Basic restrictions
The Basic Restrictions (the ‘limits’) that apply within the MR frequency range (10-300 MHz) are unchanged numerically, but the averaging times are different:
- Whole-body SAR now averaged over 30 minutes (formerly 6 min)
- Local SAR averaged over 6 minutes
- Limit values remain unchanged for exposure of 6 minutes or more
- Higher limits are defined for exposures of less than 6 minutes.
| Basic Restriction | SAR (W/kg) | ||
|---|---|---|---|
| Whole-body | Local (head & trunk) | Limbs | |
| Occupational | 0.4 | 10 | 20 |
| Public | 0.08 | 2 | 4 |
Reference Levels
Reference levels can be used to establish compliance with Basic Restrictions. They are defined in terms of easily measured incident quantities, e.g. the magnetic field. The whole-body levels are unchanged, but are now averaged over 30 minutes. Limits to magnetic flux density (B) are not explicitly defined but have been calculated in the tables below.
- Whole-body electric (E) and magnetic (B) fields and incident power density now averaged over 30 minutes (formerly 6 min)
- Local exposure metrics averaged over 6 minutes
- Limit values remain unchanged for exposure of 6 minutes or more
- Higher limits are defined for exposures of less than 6 minutes.
| Reference Levels (WB) | E field (V/m) | H field (A/m) | B field (µT) | Power density (W/m2) |
|---|---|---|---|---|
| Occupational | 61 | 0.16 | 0.2 | 10 |
| Public | 27.7 | 0.073 | 0.092 | 2 |
| Local exposure Reference Levels | E field (V/m) | H field (A/m) | B field (µT) | Power density (W/m2) |
|---|---|---|---|---|
| Occupational | 139 | 0.36 | 0.45 | 50 |
| Public | 62 | 0.163 | 0.2 | 10 |
What does this mean for MRI?
The MR community generally utilises the MR industry-specific standard IEC 60601-2-33 for both patient and occupational exposure limitation [5]. This remains unchanged. If anything, the new ICNIRP basic restrictions and reference levels allow more leeway for short-term (less than 6 minute) exposures to the RF.
The RF B1 field in air (but not in the patient’s body) attenuates rapidly outside the transmit coil. Measurements of the RF exposure to workers outside the bore for 1.5 and 3 T scanners have shown a maximum 6 minute time‐averaged RMS E field of less than 30 V/m and an H of less than 0.12 A/m [6]. These are below the relevant ICNIRP limits.
Pregnant staff are usually excluded from the MR examination room during acquisition- on account of the risk to foetal hearing from the acoustic noise. The new ICNIRP guidelines add further weight to this exclusion on account of the possible RF exposure.
More about occupational exposure can be found in chapter 13 of Essentials of MRI Safety.
References
- International Commission on Non-Ionizing Radiation Protection (2020). Guidelines on limiting exposure to electromagnetic fields (100 kHz to 300 GHz)”. Health Physics 118:483–524.
- International Commission on Non-Ionizing Radiation Protection (1998). Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz)”. Health Physics 74:494–522.
- International Commission on Non-Ionizing Radiation Protection (2009). ICNIRP statement on the “Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz)”. Health Physics 93:257–258.
- International Electrotechnical Commission (2015). Medical Electrical Equipment – Part 2–33: Particular Requirements for the Safety of Magnetic Resonance Equipment for Medical Diagnosis. IEC 60601-2-33 3.3 edn. Geneva: IEC.
- Capstick, M., McRobbie, D., Hand, J., et al. (2008). An investigation into occupational exposure to electromagnetic fields for personnel working with and around medical magnetic resonance imaging equipment. Report on Project VT/2007/017 of the European Commission Employment, Social Affairs and Equal Opportunities DG.