Defining the heating characteristics of ferromagnetic implants using calorimetry

Abstract

The induction heating of small, cylindrical ferromagnetic implants for localized tumors is currently under investigation. These thermal rods are implanted within a lesion in 1 cm2 arrays and subsequently exposed to an externally applied alternating magnetic field. Implants absorb energy from the field and transfer it as heat to the surrounding tissue. To achieve a uniform temperature rise throughout the tissue volume and to account for any field-rod misalignment, 400 mW of power per implant is used as the design specification. The temperature to necrose cells must be greater than 46°C. A calorimeter was constructed to confirm that the rod power output specification is satisfied at temperatures adequate for inducing cell death. The rods were designed to undergo a ferromagnetic to paramagnetic transition at temperatures of 55°C, 60°C, 65°C, and 70°C; this transition produces rods that are temperature self-regulating. Calorimetric results demonstrated that 55°C, 60°C, 65°C, and 70°C rods provided 400 mW at 47-51°C, 51-53.5°C, 57°C, and 62.5-63.5°C, respectively. Thermal rods provide sufficient power output at the temperatures necessary to cause thermal ablation of tumors. The higher-temperature rods give a greater margin to ensure that necrotizing temperatures can be achieved throughout the rod array even with minor misalignment. (C) 2000 John Wiley and Sons, Inc.