(SAR) The Specific
Absorption Rate is defined as the RF power absorbed per unit of
mass of an object, and is measured in watts per
kilogram (W/kg).
The SAR describes the potential for heating of the patient's tissue due to the application of the RF
energy necessary to produce the MR signal.
Inhomogeneity of the RF field leads to a local exposure where most of the absorbed energy is applied to one body region rather than the entire person, leading to the concept of a local SAR. Hot spots may occur in the exposed tissue, to avoid or at least minimize effects of such theoretical complications, the
frequency and the power of the
radio frequency irradiation should be kept at the lowest possible level.
Averaging over the whole body leads to the global SAR.
It increases with
field strength,
radio frequency power and
duty cycle, transmitter-coil type and body size. The doubling of the
field strength from 1.5
Tesla (1.5T) to 3
Tesla (
3T) leads to a quadrupling of SAR. In high and ultrahigh fields, some of the multiple
echo, multiple-slice pulse
sequences may create a higher SAR than recommended by the agencies. SAR can be reduced by lower
flip angle and longer repetition times, which could potentially affect image contrast.
Normally no threatening increase in temperature could be shown. Even in high magnetic fields, the local temperature increases not more than 1°C. 2.1°C is the highest measured increase in skin temperature.
Eddy currents may heat up
implants and thus may cause local heating.
FDA SAR limits:
•
Whole body: 4W/kg/15-minute exposure averaged;
•
Head: 3W/kg/10-minute exposure averaged;
•
Head or torso: 8W/kg/5 minute exposure per gram of tissue;
•
Extremities: 12W/kg/5 minute exposure per gram of tissue.
IEC (International Electrotechnical Commission) SAR limits of some European countries:
All limits are averaged over 6 minutes.
•
Level 0 (normal operating mode): Whole body 2W/kg; Head 3.2W/kg; Head or Torso (local) 10W/kg;
Extremities (local) 20W/kg;
•
Level I (first level controlled operating mode): Whole body 4W/kg; Head 3.2W/kg; Head or Torso (local) 10W/kg; Extremities (local) 20W/kg;
•
Level II (second level controlled operating mode): All values are over Level I values.
(For more details: IEC 60601-2-33 (2002))
In most countries standard MRI systems are limited to a maximum SAR of 4 W/kg, so most scanning in level II is impossible.
For Level I, in addition to routine monitoring, particular caution must be exercised for patients who are sensitive to temperature increases or to RF
energy.
For Japan different SAR limits are valid.
•
] and 19 definitions [
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