The rate at which radio-frequency (RF) electromagnetic (E-M) energy is absorbed by the human body can be quantified by the specific absorption rate (SAR). This SAR can be expressed as the power absorbed per unit of mass tissue, e.g. expressed in units of watts per kilogram. Even though in particular applications, SAR is used in relation to the absorption of different energy qualities, such as energy conveyed by ultrasound, the present disclosure relates to SAR as a measure of the absorption rate of RF electromagnetic energy specifically. SAR measures can, for example, be used to determine whether emissions from RF sources, such as mobile phones, magnetic resonance imaging scanners or microwave ovens, are within safety tolerance levels.
A distinction can be made between the whole-body averaged SAR (SARwb) and a local measure of SAR, such as SAR over a small sample volume of tissue, for example over 1 g or 10 g of tissue. A whole-body averaged SAR measure may, for example, be particularly suitable for expressing the influence of a relatively uniform RF E-M exposure.
It is known in the art that the number of man-made RF sources, and the transmitted RF energy density in occupied areas, has steadily increased over time in the past, and can be expected to increase further in at least the near future. Furthermore, possible adverse effects of absorbed RF energy in the human body may have given rise to a public concern. For example, dielectric heating of tissues due to absorption of radio-frequency electromagnetic fields in the human body is a known health effect. Furthermore, the equations known in the art to describe such heating may typically use the SAR as an input parameter. Therefore, the SAR has been implemented as a test measure for defining basic restrictions on RF E-M radiation exposure. Particularly, restrictions on the allowable SARwb have been imposed in many jurisdictions.
However, it may be difficult, or even impossible, to accurately measure the specific absorption ratio in tissue inside a living human. Therefore, reference levels have also been defined on the incident electromagnetic fields, as studied in the field of personal exposure assessment.
For example, it is known in the art to assess personal exposure by registering electric field strengths using personal exposimeters (PEM), e.g. devices that can be worn on the body to measure time-varying electric-field strengths in different frequency bands of interest. It is an advantage that such exposimeters can be worn and used by subjects without requiring an extensive training. The use of such devices is widespread. For example, exposimeters may be used by both scientists and RF workers, e.g. workers installing RF antennas or performing maintenance on RF antennas. However, exposimeters as known in the art may have various disadvantages, e.g. large measurement uncertainties. Particularly, an important disadvantage is that electric fields are measured, which only serve as a proxy for SARwb-values. Even though methods are known in the art to measure SARwb for a controlled source in an indoor environment, such methods may require a fixed set-up using off-body antennas and may only determine the absorption of a predetermined controlled emitted signal. Particularly, the SARwb of ambient radiation, e.g. of uncontrolled sources, may not be measurable by using such techniques.
Various methods are known in the art for performing a standardized measurement of averaged SAR values, e.g. 1 g or 10 g averaged SAR values, using a standardized phantom or human body model, e.g. an anthropomorphic phantom, to assess SAR values, such as the ESM-120 (Maschek, Germany), DASY (SPEAG, Switzerland), cSAR3D (SPEAG, Switzerland) and iSAR (SPEAG, Switzerland) systems. However, most of such methods known in the art are adapted for providing 1 g or 10 g averaged SAR values.
Specific numerical tools are known in the art, such as SEMCAD-X (SPEAG, Switzerland) and Sim4Life (SPEAG, Switzerland), that allow one to use numerical human body models in order to provide SARwb-values. For example, such numerical methods may use an MRI model of a specific subject. However, to achieve a highly specific estimate of SARwb for a particular human subject, such methods would also require a detailed measurement of the subject's dielectric properties, which may not be possible in living humans using methods known in the art.
Another approach for the determination of SARwb uses the concept of ‘room electromagnetics’, a theory which studies the propagation and absorption of electromagnetic fields using methods from room acoustics. This theory established a relationship between the reverberation time, a time constant of the decay of electromagnetic power in a room, and the electromagnetic radiation absorption in a room. For example, Bamba et al. disclosed an application of room electromagnetics for determining SARwb in “Experimental Assessment of Specific Absorption Rate Using Room Electromagnetics,” IEEE Transactions on Electromagnetic Compatibility, 54(4), pp. 747-757. However, such approach requires knowledge of the incident power density without the subject present, which cannot be directly determined when the monitored person is present.