EU LEGISLATION The International Commission on Non-ionising Radia-
tion Protection (ICNIRP) recommended limits for expo- sure to static magnetic fields in 1994 [3] and to time-vary- ing fields in 1998 [4]. These were the basis of the exposure limits in the 2004 directive, although a static field limit was excluded as this was under review at the time by ICNIRP. The limits recommended by ICNIRP were extremely con- servative, with a large safety factor between the PNS thresh- old in particular and the exposure limit.
“.... even walking at normal speed close to a scanner on-field, would have become illegal.....”
IMPLICATIONS OF THE DIRECTIVE MRI uses EMF in three different frequency ranges: a static magnetic field, switched magnetic field gradients (hundreds to thousands of hertz) and a radiofrequency (RF) field (tens to hundreds of megahertz). The RF limits in the directive did not pose a particular problem for MRI, but the limits in the gradient frequency range, because they are set so far below the PNS threshold, certainly did. It was also found that work- ers could exceed the low-frequency exposure limits simply by walking through the static field, and hence experiencing a time-varying field by virtue of their movement. Thus any activity requiring staff members to stand close to the scan- ner during imaging (e.g. caring for a sick or anaesthetised patient, interventional MRI, some types of research), or even walking at normal speed close to a scanner on-field, would have become illegal [5]. The scale of the problem is clear from Table 1, which compares the exposure limits in the directive with peak occupational exposures in MRI. However, an exist- ing international standard concerning MRI (IEC 60601-2-33) provides adequate protection from adverse effects without the need for large safety margins, so there would have been no benefit to MRI worker health and safety from the directive. Indeed there would very likely have been a negative impact on both worker and patient safety, as procedures rendered illegal by the directive would need to be transferred to other imaging modalities, in most cases involving ionising radiation.
“.... Tere was opposition to the concept of excluding a particular sector and
hence appearing to give less protection to a specific group of workers .....”
CURRENT STATUS The MRI community welcomed postponement of the trans- position deadline and the proposal to exclude MRI from the exposure limits. Lobbying, with the support of numerous patient groups, had generated a great deal of understanding and support in the European Parliament, and trouble-free passage of the revised directive seemed likely. However, dur- ing discussion within Council working groups it soon became clear that several member states had strong objections to the proposal, including large countries with considerable voting
APRIL/MAY 2012
Switched gradients
power, such as Germany and Poland. There was opposition to the concept of excluding a particular sector and hence appear- ing to give less protection to a specific group of workers, despite the fact that the IEC standard and proposed harmon- ised safe working practices would provide protection from adverse effects in a different way. The revised directive also included higher exposure limits
based on new ICNIRP advice [6], and some argued, incor- rectly, that this would be sufficient to solve the MRI problem. As the postponed deadline drew closer, it became clear that more work would be needed to find an acceptable solution;
FigUre 2. A pediatric cardiologist carrying out MRI-guided cardiac catheterisation. This would be illegal under the original EMF Directive.
Frequency Exposure limit Estimated maximum occupational exposure
in MRI 0 Hz Static magnetic field
<1 Hz (typical) (generated by movement of subject)
None
Current density 40 mAm-2 head and trunk
3 T (clinical) 9.4 T (research)
200 – 400 mAm-2 (CNS)
ELV exceeded 0.5 - 1 m from magnet if moving at 1 ms-1
1 kHz (typical)
Current density 10 mAm-2 head and trunk
SAR
0.4 W kg-1 whole body
RF
10s – 100s MHz
10 W kg-1 head and trunk
20 W kg-1 limbs
All SAR values averaged over 6 mins, localised SAR averaged over 10g of tissue
Not expected to exceed ELV because of spatial and tempo- ral averaging.
> 200 mAm-2 (CNS) ELV exceeded ≈ 1 m from end of gradient coils
TaBle 1. Occupational exposures in MRI compared to exposure limits in EMF directive. DI EUROPE 11
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