Magnetic nanoparticle hyperthermia enhanced by a rotating field

The heating efficiency of magnetite nanoparticles for therapeutic hyperthermia is shown to be substantially enhanced by applying a uniformly rotating magnetic field in place of a field directed along an axis, when all other factors are held constant. Optimization of the heating efficiency is actively pursued in order to keep the volume fraction of nanoparticles as low as possible, reducing the adverse effects emerging from nanoparticle accumulation in organs. The effect of a rotating magnetic field is calculated by solving rate equations for the magnetic moments of magnetite nanoparticles with predominant N\'eel relaxation and pictured as double-well systems. The model results in a simple expression for the power density generated by nanoparticles with random easy-axis directions. A thermal model of a tissue simulant is used to show that applying a rotating instead of a linear field permits us to more than halve the dose of nanoparticles needed to attain the target temperature in the tissue.