Radio-frequency resonant cavity measurements for rapid, accurate assessment of body composition and human exposure to electromagnetic fields

摘要

Body composition measurements play an important role in nutritional studies, renal medicine and sports science, while human exposure to electromagnetic fields (EMF) is an area of growing concern owing to the implementation of Directive 2013/35/EU on EMF. Resonant cavity techniques offer an attractive alternative to traditional methods in these fields, as they allow rapid, non-invasive measurements without using ionising radiation. At frequencies of a few tens of MHz, a large screened room can act as a cavity resonator. A human subject inside the room perturbs its low-order resonances, and the resulting shift in frequency depends on the tissue dielectric properties, which correlate strongly with water content. This approach has been well tested and shows good agreement with current methods of measuring total body water. The number of resonant modes increases rapidly with frequency, so if we instead use microwaves at 1GHz and above, many modes can be excited simultaneously. Adding a rotating paddle to the room creates a ‘stirred mode’ environment, where the body is effectively illuminated by microwave radiation from all directions. For EMF exposure studies this a more realistic scenario than considering only a single direction and polarisation. The average absorption cross section (ACS), which is closely related to specific absorption rate (SAR), can be rapidly obtained over a very broad band (1GHz to 15GHz and beyond), whereas the alternative is detailed computer simulations that take many hours for just a single frequency. At these microwave frequencies, the field penetration into tissues is a few cm, so the ACS gives useful information about the composition of tissues near the body surface. Normalising the ACS to body surface area gives us an ‘absorption efficiency’ that is independent of body size. Results will be presented of the relationship between this parameter and the thickness of subcutaneous body fat. Both techniques are comfortable for the subject, use safe levels (around 1mW) of non-ionising radiation, and allow measurements to be made in less than 10min.