UK scientists have welcomed a proposed delay to European laws tightening the limits on emissions from scanners that medical staff can be exposed to.
The EU physical agents directive is expected to be postponed for four years for further scientific reviews.
Experts argued the limits, set at around 100 times lower than those for patients undergoing magnetic resonance imaging scans, were overly cautious.
MPs had warned the restrictions could stymie research and hinder diagnoses.
MRI scanners use magnetic fields to build up a picture of the inside of the body.
The stronger the magnetic resonance, the more accurate the image.
"Something like half a billion people have been exposed worldwide at 100 times the levels proposed and if there were adverse effects, they would have been detected"
The directive, which was due to be implemented from 2008, put limits on exposure of operating staff to electromagnetic fields from zero to 300GHz.
But the proposals proved controversial among doctors and scientists who feared such limits would block pioneering advances and mean patients, particularly in vulnerable groups such as children and those with cancer, would miss out on the best treatment.
Under the legislation there would have been difficulties using the most powerful, modern scanners, they warned.
And research into interventional MRI - treating conditions with less invasive surgery guided by MRI, for example - would become illegal.
Evidence
At very high doses, exposure to a magnetic field can cause the muscles to twitch and some people have reported feeling dizzy or sick when moving their heads quickly around a high-field magnet.
But no obvious health risks among operators or patients have been noted in the past couple of decades of widespread use.
Dr Stephen Keevil, consultant physicist at Guy's and St Thomas's NHS Foundation Trust in London, said the directive as it stands would be a threat to clinical practice and research.
"The bottom line is the directive is about acute effects and something like half a billion people have been exposed worldwide at 100 times the levels proposed and if there were adverse effects, they would have been detected."
In 2006 the Commons science and technology committee reviewed the directive and was "alarmed" to discover it was based on a 10-year-old risk assessment.
Phil Willis, chairman of the committee, said scientists and doctors had given compelling arguments over a number of years about the threat that the directive posed to use of MRI.
"I hope that the outcome of the process that the European Commission has now initiated will be a directive that accurately reflects the proven value of MRI."
The British Institute of Radiology has also raised concerns about the legislation.
Andrew Jones, consultant clinical scientist at the institute, said: "This postponement will allow essential time for further research and data to be collected on the implications of the impact of the proposed directive."
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