On the measurement technique for specific absorption rate of nanoparticles in an alternating electromagnetic field

Accurate measurement of specific absorption rate (SAR) is essential for quantifying the power dissipation of magnetic nanoparticle suspensions in alternating magnetic fields, which have applications in cancer hyperthermia. Current SAR measurement setups usually comprise a coil surrounding a container holding the sample fluid. The temperature rise of the magnetic fluid is recorded once the field is turned on and SAR is determined from the initial slope of the temperature as a function of time. However several factors, including volume of the fluid sample, thermal properties of the container, positioning of the temperature sensor and non-uniformities in the magnetic field induced by the sample and coil geometry, may influence the reported SAR. To illustrate these effects theoretical and experimental investigations are carried out. The results show that the SAR measured on samples of relatively small volume may be subjected to errors associated with conductive heat losses to the container holding the sample. Numerical simulations also show that the fluid experiences internal convection during heating, which dictates the positioning of the temperature sensor. Moreover, the heat generation is shown to depend on sample geometry because this has an effect on the magnetic flux passing through the sample. Since SAR is proportional to squared field strength, small differences in the magnetic field may result in large differences in SAR. The findings reported here are also relevant to the measurement of power dissipation in capacitively coupled, radio-frequency heating of metallic nanoparticles.