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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.
-
Normal operating mode (suitable for all individuals)
-
First Level Controlled Mode (suitable for subjects on whom a medical decision
ensuring they can handle the increased effect (SAR or Gradients)
-
Second Level Controlled Operating Mode, which could be described as a full
research (no limits ) mode.
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.
-
In MR exposures up to 1 hour the total body exposure should be limited
to a total energy deposition of 120 Wmin/kg in order not to overload the
thermoregulatory system.
-
To avoid overheating any local area, the product of time and local SAR
should not exceed:
- 60 Wmin/kg averaged over the head
- 120 Wmin/kg averaged over the trunk
- 180 Wmin/kg averaged over the extremities,
provided that the instantaneous SAR does not exceed:
- 4 W/kg averaged over the head
- 8 W/kg averaged over the trunk
- 12 W/kg averaged over the extremities.
-
To protect poorly perfused tissues, they should not be exposed to a local
SAR of more than 10 W/kg, averaged over 0.01 kg for more than 10 min.
-
For exposures to infants, pregnant women, or patients with an cardiocirculatory,
or cerebral vascular impairment, a reduction of those values by a factor
two is recommended.
Thanks to Bart Schraa for these details
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