PAD 2004/40/EC: Speed Limits

In Europe, the Physical Agents Directive (PAD) 2004/40/EC is supposed to become national law by April 2008 (but see Comment below on a five-year reprieve). It will restrict the EMF exposure of workers. This will cause problems in MRI. The gradient magnetic fields used in MRI (dB/dt) are such that MRI studies may no longer be possible where staff are required to be close to the magnet during scanning. This includes

  • Emergency department MRI scanning
  • Babies and other special populations
  • Interventional MR (i.e. MR-guided surgical procedures)
  • Research innovation

But the gradient fields are not the only source of a changing magnetic field. Simply moving through the main magnetic field creates a magnetic flux (dB/dt). This means that if you work with MR machines in Europe, you won’t be able to move very fast or you’ll reach the limits prescribed by the legislation!

Your speed limit will be:

  • 25 cm/s close to a 1.5 T magnet
  • 10 cm/s close to a 3 T magnet

A friend of mine remarked that this could result in some interesting control measures…

“Perhaps speed cameras and restrictive suits to slow down body motion.”

And if your employer allows you to exceed the limits, you’ll be able to sue them!

The PAD 2004/40/EC considers acute effects on workers, and draws from ICNIRP reports. Acute effects refers principally to peripheral nerve stimulation (PNS). However, the limits described for workers in the PAD are 100 times lower than levels which exclude PNS. MRI scanners are already designed to IEC safety standards to protect patients from PNS, which also has the effect of limiting staff exposure. So why make the limits of exposure more restrictive for workers? (Remember the PAD deals only with acute effects.) There is no evidence of adverse health effects in the gradient frequency range below the patient PNS threshold. To put this in context: the typical dB/dt operating range of an MRI scanner is 45 T/s, safely below the mean PNS threshold which is at 60 T/s. PNS becomes painful at 90 T/s, affects respiration at 900 T/s and cardiac function at 3600 T/s.

Some commentators have observed that no risk-benefit analysis was made before the Directive was submitted, breaking Common Market rules. Questions have been asked such as:

  • After 500 million MRI scans worldwide, why not use the same limits as imposed for patients?
  • Why is there no derogation for MRI workers? Derogations are used in (e.g.) the Noise (2003) and the Vibration (2002) Directives.
  • Why is IEC-60601-2-33 not being used?

Read more about the PAD: SenseAboutScience (see the 3-page briefing document).

Did you know: EMF exposure has been considered in healthcare contexts even as far back as 1869 when a healing device was patented by E. Smith (US patent 96,044).

See Comments on this post for developments and updates.