# SAR (special absorption rate) & frequency

1. Sep 23, 2013

### Worldline

Hi
we know the relation between wavelength, frequency and energy :
The greater the energy, the larger the frequency and the shorter (smaller) the wavelength -> E=h$\upsilon$
On the other side, SAR is common property that measures absorbed energy.
Now if we calculating SAR for human brain tissue , we would have such graph :

But i don't understand why it behaves like that , why SAR doesn't increase with frequency ?

2. Sep 24, 2013

### marcusl

There's no graph, so I don't know what exactly you are referring to. In general, you shouldn't confuse quantum properties with the classical problem you are considering. Power absorption depends on geometry, EM wave coupling, and on the square of the amplitude of the wave. Coupling depends only weakly on frequency. As frequency rises, eddy currents are induced and dissipated in the highly conductive tissue. Higher frequencies don't change this until you get high enough to limit penetration due to skin depth shielding.

PS: It's Specific (not special) Absorption Rate. Specific means amount per kg of tissue.