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Commentary |
1 Division of Imaging Sciences, King's College London, Guy's Campus, London SE1 9RT, UK, 2 Department of Medical Physics, Guy's and St Thomas' NHS Foundation Trust, London SE1 9RT, UK, 3 IMR Unit, St Mary's Hospital, Praed Street, London W2 1NY, UK, 4 Sir Peter Mansfield Magnetic Resonance Centre, University of Nottingham, University Park, Nottingham NG7 2RD, UK, 5 Centre for Medical Image Computing, New Engineering Building, University College London, Malet Place, London WC1E 6BT, 6 Cancer Research UK Clinical Magnetic Resonance Research Group, Institute of Cancer Research and Royal Marsden NHS Foundation Trust, Downs Road, Sutton SM2 5PT, UK, 7 Department of Radiology, University Hospital, Queen's Medical Centre, Nottingham NG7 2UH, UK, 8 Radiological Sciences Unit, the Hammersmith Hospitals NHS Trust and Imperial College London, Charing Cross Hospital, London W6 8RF and 9 Optics and Semiconductor Group, Department of Electrical and Electronic Engineering, Imperial College London, Exhibition Row, London SW7 2AT, UK
MRI has a commendable safety record, thanks to the combined efforts of manufacturers and expert users working in conjunction with government bodies over many years. But now we face the prospect of new safety legislation with significant implications for current and future practice. This legislation does not deal with concrete and familiar hazards, such as the missile effect and contact burns, but with occupational exposure to electromagnetic fields (EMF). It is being adopted not in dialogue with, but rather despite opposition from, the MR community.
In 2004 the European Union (EU) adopted a Directive limiting occupational exposure to EMF. The Physical Agents (EMF) Directive [1] must be incorporated into domestic law by each EU member state by 30 April 2008. It applies to all employment sectors, including medical MR, but only to workers, not to patients or volunteers.
MRI employs EMF in three frequency ranges, all within the scope of the Directive: the static magnetic field (0 Hz), the time-varying magnetic field generated by the imaging gradients (100–1000 Hz), and the radiofrequency (RF) field (10–100 MHz). Table 1
shows the exposure limit stipulated in the Directive in each case (for a representative frequency in the case of the gradients, as the limit is frequency-dependent over their range of operation). These are fundamental limits that it will be illegal to exceed. The limit over the gradient frequency range is set in terms of the weak electric current induced in the body by the time varying magnetic field, and that for RF fields is set in terms of specific absorption rate (SAR) to limit heating. As well as exposure limits the Directive contains "action values" expressed in terms of more easily measured quantities, such as magnetic flux density. These are set conservatively so that satisfying the action value guarantees compliance with the corresponding exposure limit. The draft Directive contained a static field exposure limit of 2 T, which was removed during negotiation partly as a result of lobbying by the MR community. It is possible that such a limit may be restored in a future revision of the Directive, with the level being determined by a review currently being undertaken by the International Commission on Non-ionising Radiation Protection (ICNIRP).
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contains worst-case estimates of exposure for an MR worker. For the static field, the estimate is based on someone entering the magnet bore, perhaps for cleaning or to position experimental apparatus. In the case of the gradients, it is based on a worker standing very close to the bore during imaging, perhaps to carry out an interventional procedure or provide patient care. In the case of RF, given that exposure is averaged over the whole body, it seems unlikely that the limit would be exceeded under foreseeable circumstances. In the absence of a static field limit, the gradient field limit poses the greatest problem. It will exclude staff from the vicinity of the bore during imaging, with the extent of the exclusion zone depending on magnet and gradient system design and choice of sequence. Since the limits are absolute, without scope for time averaging or relaxation for brief exposure, it will become illegal for an anaesthetist to lean into the bore even for a moment to check a patient, or for a radiographer or nurse to hold an anxious patient's hand.
Incorporation of these limits into law will make many interventional MR procedures illegal in Europe, closing off development of a field with tremendous clinical potential. It will make it more difficult to provide appropriate care for anaesthetised, monitored and anxious patients. It will affect manufacture of MR equipment, particularly if a static field limit is adopted, and hence threaten the UK's global position in this sector. It will give US researchers a significant advantage over European competitors, both in the development of MR methodology itself and in the growing exploitation of these techniques, for example in the pharmaceutical industry. Most importantly, it will mean that current and future MR techniques may be denied to patients, in many cases necessitating an examination with X-rays instead, with the resulting dose of ionizing radiation to both patient and staff. Modifying MR scanners to meet the regulations is not a practical solution. Even if technically feasible without significantly compromising performance, it would take many years to replace the installed base at considerable cost to manufacturers and hence to healthcare providers in producing special models for the European market.
The Directive states, to paraphrase, that these limits are needed to protect workers from known short-term adverse effects on the central nervous system (CNS) occurring instantaneously on exposure to EMF above a well-defined threshold. This assertion is based on guidance issued by ICNIRP [2] in 1998, and exposure limits and action values in the Directive are adopted from this document without modification. The ICNIRP guidance was adopted by the UK National Radiological Protection Board (NRPB) in 2004 [3], also including a 2 T limit for static field exposure. We anticipate that UK regulators may attempt to include this static field in domestic law, even in the absence of a Europe-wide limit. Thus high-field MRI in the UK would be at a unique disadvantage globally.
What are the known short-term adverse effects that the Directive seeks to avoid? A recent paper [4] has considered ICNIRP and NRPB documents [2, 5] in more detail than is possible here. In the gradient frequency range, peripheral nerve stimulation (PNS), due to induction of electric currents by time varying magnetic fields, is an adverse effect that forms the basis for limitation of patient exposure [6, 7]. PNS occurs at a threshold current density of around 1 A m–2 – 100 times higher that the limit set in the Directive. The difference arises because ICNIRP occupational exposure guidelines rely on less well-established phenomena, such as alteration of visual evoked potentials and subtle cognitive effects. Evidence for most of these effects is sparse, often dating from the 1980s, in some cases presented in preliminary form at conferences rather than in full papers, and in other cases reported only in the 10–100 Hz frequency range but extrapolated to higher frequencies in the absence of more appropriate data.
ICNIRP concludes from these data that thresholds for acute CNS effects are exceeded above 100 mA m–2, but in view of the sparse evidence, applies a safety factor of 10, resulting in the 10 mA m–2 limit. In supporting the same limit, the NRPB acknowledges adoption of "a cautious approach... to indicate thresholds for adverse health effects that are scientifically plausible" [3]. Many things are scientifically plausible, but the exposure limits are supposed to be based not on hypothetical possibilities but on "known adverse health effects" causing "detectable impairment of... health", as opposed to biological effects that may or may not be harmful [2] if they exist at all. There is no substantial evidence for any such effects in the gradient frequency range below the PNS threshold.
It is clear that research is needed to establish a stronger scientific basis for EMF exposure limits. Efforts are being made to secure funding for such research in the UK and elsewhere. But this work will take time, and in the meantime there is also a need for a more balanced approach to risk management, or perhaps in this instance perceived risk management on the part of the regulators.
In relation to another EMF safety issue [8], risk management has been defined as "...the process by which the risks and benefits... are weighed against each other..." – not simply elimination of any plausible risk at whatever cost. This definition goes on to state that "the benefits may be real or potential, and direct (e.g. an improvement in health...) or indirect (e.g. making an industry more competitive)..." – considerations which all clearly apply to MRI in the UK. Furthermore, in view of the uncertainties involved and in the absence of any reported genuinely adverse effects, regulators might consider how "the balancing of risks and benefits should take account of the uncertainties in risk estimates and also the severity of the adverse effects that might result" [8].
Ensuring the safety of the MRI environment is a key objective for the MR community. Occupational exposure limits are likely to be an important component of safe operation, but the limits proposed in the EU Directive, especially for gradient frequencies, seem to be based on inadequate science. Their implementation will lead to curbs on the use of MRI in healthcare without any clear benefit to staff. Ironically, it may well lead to increased staff exposure to ionizing radiation: in the case of X-ray imaging, balancing of risk and benefit – not just for the individual, but for society as a whole – allows staff to incur some risk (not always negligible in the case of interventional procedures) in view of the benefit to patients, while this philosophy is not being extended to the case of MRI.
The view of the UK Health and Safety Executive (HSE) is that the Directive will have minimal impact on MRI, because it replicates existing NRPB guidance [3] with which they assume we already comply – overlooking the distinction between guidance, which competent professionals may consider alongside other factors as part of a wider risk assessment, and legally enforceable exposure limits. The HSE's own regulatory impact assessment of the draft Directive, without even considering the effect on MRI, states that "we are... unable to identify any health and safety benefits from the Directive", and concludes that "...the benefits... are very heavily outweighed by the costs" [9]. So there is one point at least on which the HSE and the imaging community are as one.
Received for publication July 15, 2005. Accepted for publication September 9, 2005.
References
This article has been cited by other articles:
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  F de Vocht, L Liket, A De Vocht, T Mistry, P Glover, P Gowland, and H Kromhout Exposure to alternating electromagnetic fields and effects on the visual and visuomotor systems Br. J. Radiol., October 1, 2007; 80(958): 822 - 828. [Abstract] [Full Text] [PDF]
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S F Riches, D J Collins, J W Scuffham, and M O Leach EU Directive 2004/40: field measurements of a 1.5 T clinical MR scanner Br. J. Radiol., June 1, 2007; 80(954): 483 - 487. [Abstract] [Full Text] [PDF]
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BJR Review of the Year - 2005. Br. J. Radiol., March 1, 2006; 79(939): 183 - 187. [Full Text] [PDF]
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