On the assumption of negligible heat diffusion during the thermal measurement of a nonuniform specific absorption rate

The problem of measuring a spatially varying specific absorption rate (SAR) by thermal techniques is treated both analytically and numerically. It is shown that, unless the measurement is attempted at an inflection point of the SAR distribution, it will be confounded by thermal diffusion within the medium. Rules of thumb are provided to enable the experimenter to gauge the thermal conduction contributions (i.e. error) to the thermally determined SAR near a spatial extremum, The simplest of these is that the width t(m) [s] of the time window, over which temperature variation associated with SAR is measured, should satisfy the inequality t(m) less than or similar to alpha(2)/pi(2)D where alpha [m] is a characteristic length scale of the spatial variation of the SAR and D [m(2)/s] is the thermal diffusivity of the medium. The above considerations are valid, independent of the source of the deposited energy. (C) 1999 by Radiation Research Society.