Table 6.1 Reference Levels for Static Magnetic Fields B 0 and Basic SAR Restrictions for the
Public and Magnetic Resonance Imaging (MRI) Patients
Local RF EMF Transmit Coils
B 0,WB
SAR WB SAR PB
SAR HEAD SAR HEAD SAR TRUNK SAR LIMBS
T
W/kg
W/kg
W/kg
W/kg
W/kg
W/kg
General public
0.4 a
0.08 a
–
–
–
0.4 a
0.4 a
Patients with medical
0.0005 b
–
–
–
–
–
–
implants
Workers
2–8 c,d
0.4 a
–
2–8 c,d
2–8 c,d
2–8 c,d
8 a
Normal operating mode
4 c
2 b
2–10 b,e
3.2 b
10 b
10 b
20 b
First-level controlled
8 c
4 b
4–10 b,e
–
20 b
20 b
40 b
operating mode
Second-level controlled
>8 c
>4 b
>4–10 b,e
>3.2 b
>20 b
>20 b
>40 b
operating mode
a ICNIRP (2009a).
b IEC 60601-2-33.
c ICNIRP (2009b).
d Head and trunk, for specific tasks in controlled environment.
e 10 × M exposed /M WB.
WB = whole body; PB = partial body.
353
Medical Devices and Systems Exposure and Dosimetry
6.3.1.4.1 Normal Operating Mode
This mode includes routine examinations with MRI outputs that are low enough to avoid
any physiologic stress to the patient. Switched magnetic gradients should not induce
electric field strengths exceeding 80% of the threshold of peripheral nerve stimulation.
If not differently determined individually or justified by shorter durations, the induced
electric field strength is limited to E i ≤ 2.2 V/m. In the case of whole-body exposure, the
gradient is limited to dB/dt ≤ 20 T/s. To avoid undue heat stress, SAR values should meet
the following limits: SAR WB ≤ 2 W/kg, SAR PP ≤ 2M EX /M WB W/kg, SAR HEAD ≤ 3.2 W/kg.
6.3.1.4.2 First-Level Controlled Operating Mode
This mode is dedicated for special MRI examinations. It requires controlling patients
by medical supervision because one or more electromagnetic parameters may be large
enough to cause physiological stress to patients. Switched magnetic gradients should
not induce electric field strengths exceeding 100% of the threshold of peripheral nerve
stimulation. Although the exposure limit of the head equals those of normal operation
mode, the SAR WB values for the whole-body and partial-body exposure are increased by
a factor of 2. Therefore, the following limits should be met: SAR WB ≤ 4 W/kg, SAR PP ≤
4M EX /M WB W/kg, SAR HEAD ≤ 3.2 W/kg.
6.3.1.4.3 Second-Level Controlled Operating Mode
In this mode, apart from controlling patients by medical supervision, an ethical
approval is required because one or more electromagnetic parameters exceed first-level
controlled operating mode limits and may be high enough to produce significant risks
for patients (Table 6.1).
Public and Magnetic Resonance Imaging (MRI) Patients
Local RF EMF Transmit Coils
B 0,WB
SAR WB SAR PB
SAR HEAD SAR HEAD SAR TRUNK SAR LIMBS
T
W/kg
W/kg
W/kg
W/kg
W/kg
W/kg
General public
0.4 a
0.08 a
–
–
–
0.4 a
0.4 a
Patients with medical
0.0005 b
–
–
–
–
–
–
implants
Workers
2–8 c,d
0.4 a
–
2–8 c,d
2–8 c,d
2–8 c,d
8 a
Normal operating mode
4 c
2 b
2–10 b,e
3.2 b
10 b
10 b
20 b
First-level controlled
8 c
4 b
4–10 b,e
–
20 b
20 b
40 b
operating mode
Second-level controlled
>8 c
>4 b
>4–10 b,e
>3.2 b
>20 b
>20 b
>40 b
operating mode
a ICNIRP (2009a).
b IEC 60601-2-33.
c ICNIRP (2009b).
d Head and trunk, for specific tasks in controlled environment.
e 10 × M exposed /M WB.
WB = whole body; PB = partial body.
353
Medical Devices and Systems Exposure and Dosimetry
6.3.1.4.1 Normal Operating Mode
This mode includes routine examinations with MRI outputs that are low enough to avoid
any physiologic stress to the patient. Switched magnetic gradients should not induce
electric field strengths exceeding 80% of the threshold of peripheral nerve stimulation.
If not differently determined individually or justified by shorter durations, the induced
electric field strength is limited to E i ≤ 2.2 V/m. In the case of whole-body exposure, the
gradient is limited to dB/dt ≤ 20 T/s. To avoid undue heat stress, SAR values should meet
the following limits: SAR WB ≤ 2 W/kg, SAR PP ≤ 2M EX /M WB W/kg, SAR HEAD ≤ 3.2 W/kg.
6.3.1.4.2 First-Level Controlled Operating Mode
This mode is dedicated for special MRI examinations. It requires controlling patients
by medical supervision because one or more electromagnetic parameters may be large
enough to cause physiological stress to patients. Switched magnetic gradients should
not induce electric field strengths exceeding 100% of the threshold of peripheral nerve
stimulation. Although the exposure limit of the head equals those of normal operation
mode, the SAR WB values for the whole-body and partial-body exposure are increased by
a factor of 2. Therefore, the following limits should be met: SAR WB ≤ 4 W/kg, SAR PP ≤
4M EX /M WB W/kg, SAR HEAD ≤ 3.2 W/kg.
6.3.1.4.3 Second-Level Controlled Operating Mode
In this mode, apart from controlling patients by medical supervision, an ethical
approval is required because one or more electromagnetic parameters exceed first-level
controlled operating mode limits and may be high enough to produce significant risks
for patients (Table 6.1).