MR safety standards

In most countries the MR safety standard IEC 60601-2-33 provides safety limits for MR exams, for ventilation, and for occupational exposure of MR workers. The International Electrotechnical Commission (IEC) developed a widely-used MR safety standard. The IEC MR safety standard is three-tiered. The NORMAL OPERATING MODE is for routine scanning of patients. The operator must take a deliberate action (usually an ACCEPT button) to enter the FIRST CONTROLLED OPERATING MODE. This mode provides higher scanner performance, but requires monitoring of the patient. Finally, there is a SECOND CONTROLLED OPERATING MODE used only for research purposes under limits controlled by an Investigational Review Board (IRB). The static magnetic field, gradient output and SAR levels for patients are based on current scientific literature related to safety, and that the level of exposure, the decision of leaving the normal operating mode and the possible need for physiological monitoring of the patients shall be a medical judgement as to the patients’ potential risk versus benefit.

The scanner employs a whole body gradient system whose IEC 60601-2-33 compliance volume is:

• a cylinder with axis coinciding with the magnet axis and with a radius of 0.20 meters, for cylindrical magnets, or
• a volume bound by planes parallel to the magnet poles and separated by a distance of 0.40 meters, for vertical-field magnets.

Table 1. IEC safety limits

Operating mode	Maximum allowed specific absorbed energy per MR Exam	Whole body SAR (W/Kg)	Head SAR (W/Kg)	Partial body SAR (W/Kg)	Local head/trunk SAR (W/Kg)	Local extremity SAR (W/Kg)	Short term SAR (W/Kg)	dB/dt (% mean PNS)
IEC Normal Mode	14.4 kj/kg (240 W⋅min/kg)	2	3.2		10	20	2 x long term	80% PNS
IEC 1st Controlled Operating Mode	14.4 kj/kg (240 W⋅min/kg)	4	3.2		20	40	2 x long term	100% PNS
IEC 2nd Controlled Operating Mode	14.4 kj/kg (240 W⋅min/kg)	IRB Limit	IRB Limit	IRB Limit	IRB Limit	IRB Limit	IRB Limit	IRB Limit

Local SAR is averaged over the worst-case 10 g. Short term SAR is averaged over 10 s. SAR limits are reduced if temperature can exceed 24 degrees C or if humidity exceeds 60%. Hearing protection (only earplugs have been validated) with NRR >/= 29 dB to reduce the A-weighted root-mean-squared sound pressure level below 99 dB(A) shall be used. IEC 60601-1 limits surface contact temperatures to 41 degrees C.