Specific absorption rate

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Specific absorption rate (SAR) is a measure of the rate at which energy is absorbed by the body when exposed to a radio frequency (RF) electromagnetic field. It is defined as the power absorbed per mass of tissue and has units of watts per kilogram [1]. SAR is usually averaged either over the whole body, or over a small sample volume (typically 1g or 10g of tissue). The value cited is then the maximum level measured in the body part studied over the stated volume or mass. It can be calculated from the electric field within the tissue as:

SAR = \frac{\sigma E^2}{\rho}

where

σ is the sample electrical conductivity
E is the RMS electric field
ρ is the sample density

SAR is used to measure exposure to fields between 100kHz and 10 GHz[2]. It is commonly used to measure power absorbed from mobile phones and during MRI scans. The value will depend heavily on the geometry of the part of the body that is exposed to the RF energy, and on the exact location and geometry of the RF source. Thus tests must be made with each specific source, such as a mobile phone model, and at the intended position of use. For example, when measuring the SAR due to a mobile phone the phone is placed at the head in a talk position. The SAR value is then measured at the location that has the highest absorption rate in the entire head, which in the case of a mobile phone is often as close to the phone's antenna as possible. Various governments have defined safety limits for exposure to RF energy produced by mobile devices that mainly exposes the head or a limb for the RF energy:

  • United States: the FCC requires that phones sold have a SAR level at or below 1.6 watts per kilogram (W/kg) taken over a volume of 1 gram of tissue.
  • European Union: CENELEC specify SAR limits within the EU, following IEC standards. For mobile phones, and other such hand-held devices, the SAR limit is 2 W/kg averaged over 10g of tissue (IEC 62209-1). For Magnetic Resonance Imaging the limits (described in IEC 60601-2-33) are slightly more complicated:


Averaging time 6 minutes
Whole body SAR Partial body SAR Head SAR Local SAR
Body Region whole body exposed body part head head trunk extremities
Operating Mode ↓ (W/kg) (W/kg) (W/kg) (W/kg) (W/kg) (W/kg)
Normal 2 2 - 10 (b) 3.2 10 (c) 10 20
1st Level Controlled 4 4 - 10 (b) 3.2 10 (c) 10 20
2nd Level Controlled >4 >(4 - 10) (b) >3.2 >10 (c) >10 >20
Short term SAR The SAR limit over any 10s period shall not exceed three times the stated values

Notes:

(a) Local SAR is determined over a mass of 10g.

(b) The limit scales dynamically with the ratio "exposed patient mass / patient mass":

NORMAL OPERATING MODE: Partial body SAR = 10 W/kg – (8 W/kg * exposed patient mass / patient mass)
FIRST LEVEL CONTROLLED OPERATING MODE: Partial body SAR = 10 W/kg – (6 W/kg * exposed patient mass / patient mass)

(c) In cases where the orbit is in the field of a small local RF transmit coil, care should be taken to ensure that the temperature rise is limited to 1 °C.

For whole body exposure there is a limit of 0.08 watt/kg averaged over the whole body.[3]


[edit] See also

[edit] References

  1. ^ Jianming Jin (1998). Electromagnetic Analysis and Design in Magnetic Resonance Imaging. CRC Press. pp. §5.3.3 pp. 226 ff. ISBN 978-0849396939. 
  2. ^ http://www.icnirp.org/documents/emfgdl.pdf
  3. ^ http://www.icnirp.org/documents/emfgdl.pdf A whole-body average SAR of 0.4 W/kg has therefore been chosen as the restriction that provides adequate protection for occupational exposure. An additional safety factor of 5 is introduced for exposure of the public, giving an average whole-body SAR limit of 0.08 W/kg.

[edit] External links

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