MRI Questions & Answers

MR Safety

A Branch of The Adelaide MRI Web Site
Compilied by Greg Brown


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International MRI Safety Web Site

Q1.  All the texts I read have the American F.D.A. limits of SAR and gradient switching rates.
Do the same standards apply in Australia and other countries?

Q2.  What are the FDA limits on SAR ?


Q1. All the texts I read have the American F.D.A. limits of SAR and gradient switching rates.
Do the same standards apply in Australia and other countries?

A1.
No.  The FDA standards are cited different to those applied in Europe, Australia and New Zealand. The European community ascribes to the IEC International Electrotechnical Commission standard concerning particular requirements for safety- Magnetic resonance equipment for medical diagnosis. IEC 601-2-33:1995
Australia and New Zealand have adopted those standards fully and labelled them AS/NZS 3200.2.33:1996, so you can see they have been in effect for a few years.

European, Australian, and New Zealand SAR limits
 
Region Normal Mode Limit First Controlled Operation Mode Limit Second Controlled Operation Mode Limit
Whole body  1.5 W/kg averaged over 15 minutes  4 W/kg averaged over 15 minutes Above the First controlled mode limits
Head 3 W/kg averaged over 10 minutes 3 W/kg averaged over 10 minutes Above the First controlled mode limits
Local SAR 
(averaged over any 1 gram of tissue)
8 W/kg head and torso, 12 W/kg in extremities averaged over 5 minutes 8 W/kg head and torso, 12 W/kg in extremities averaged over 5 minutes Above the First controlled mode limits
 
The IEC and Standards Australia documents introduce the concept of different operatonal modes.

Where a system is capable of operating in the First or Second controlled operating mode, the system must provide for a deliberate action to change modes and provide warnings and advise on the need to establish that the mode is safe for the individual to be scanned.
They recommend a room temperature of less than or equal to 24 C for the SAR limits, any higher and the SAR limits must be reduced.   There is no mention of Humidity of air flow, both of which will affect heat dissipation.  Many MR rooms run Relative Humidity above 50% to minimise spike discharges from gradient eddy currents.

Q2.  What are the FDA limits on SAR ?

The FDA regulations are framed quite differentlyto the IEC AS and NZS regulations.

Thanks to Bart Schraa for these details


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