**4. Specific absorption rate**

The extension result of the EM field is the elevation temperature of the tissue in a limited region caused by the generated heat which could affect on the biological system. The heat diffusion and its influence on the tissue are specified by the Specific Absorption Rate (SAR). SAR magnitude can not increase above the level of irradiation which becomes harmful. SAR magnitude is depending mainly on various factors such as the position of the antenna related the human tissue, the intensity field of the antenna, and the power.

The specific absorption rate (SAR) is an indication tool of absorption where the EM field exposure in the human body could be estimated. SAR is mainly used to measure the absorbed power in the tissues after irradiation by external field so that it gets the heat of tissue to be increased. The biological tissue heat could be checked via a safe method which is mainly depending on measuring the temperature elevation in tissues. It is mentioned that the temperature and SAR are evaluated in the human tissues so the precaution measures are verified. SAR is a function of many factors. Those factors affect on the absorption of EM field. SAR variations are depending on the properties of the wave, properties of the body and environment properties. Regarding the properties of the wave, the extent of SAR changes is depending directly on the features of the signal, such as frequency and polarization. In another side the dependence with the tissue human body, the SAR value depends on the type of tissue (e.g., geometry, size, age, and dielectric properties) and tissue orientation/exact location (e.g., the situation of the body; front or back incidence). SAR, in addition, depends on the exposure states, e.g., environmental exposure (indoor and outdoor) and influences of other objects in the field near the exposed body [24].

It is important to understand SAR calculation with respect to averaging design. There are two procedures to accomplish SAR calculations: first is point SAR, and second is averaging SAR (i.e., mass or volume). Point SAR is the value regardless of the averaging of the mass and the maximum SAR of all the grid cells is provided. When the absorbed power in each grid divided by grid mass, the point SAR is evaluated. While in the averaged SAR, a cube of known mass, e.g., 1 or 10 g, is utilized for every point, then the loss of the power density is integrated on this region. Then, the power loss in the integral form is divided by the mass cube [1, 24].

Because the electric field is usually not spatially uniform, SAR is averaged over a volume of tissue regarding the type of the source. It is useful to mention that the electric field is not fixed in time, so that, the probe is used for short-term timeaveraging [1].
