Design and construction of a Maxwell-type induction coil for magnetic nanoparticle hyperthermia

Abstract

Purpose: We describe a modified Helmholtz induction coil, or Maxwell coil, that generates alternating magnetic fields (AMF) having field uniformity ( 10%) within a ¼ 3000 cm 3 volume of interest for magnetic hyperthermia research. Materials and methods: Two-dimensional finite element analysis (2D-FEA) was used for electromagnetic design of the induction coil set and to develop specifications for the required matching network. The matching network and induction coil set were fabricated using best available practices and connected to a 120 kW industrial induction heating power supply. System performance was evaluated by magnetic field mapping with a magnetic field probe, and tests were performed using gel phantoms. Results: Tests verified that the system generated a target peak AMF amplitude along the coil axis of $35 kA/m (peak) at a frequency of 150 ± 10 kHz while maintaining field uniformity to >90% of peak for a volume of $3000 cm 3 . Conclusions: The induction coil apparatus comprising three independent loops, i.e., Maxwell-type improves upon the performance of simple solenoid and Helmholtz coils by providing homogeneous flux density fields within a large volume while minimizing demands on power and stray fields. Experiments with gel phantoms and analytical calculations show that future translational research efforts should be devoted to developing strategies to reduce the impact of nonspecific tissue heating from eddy currents; and, that an inductor producing a homogeneous field has significant clinical potential for deep-tissue magnetic fluid hyperthermia. ARTICLE HISTORY

Magnetic nanoparticle hyperthermia for treating locally advanced unresectable and borderline resectable pancreatic cancers: the role of tumor size and eddy-current heating

“…MIONs (NanoMaterials Technology, Singapore) used in this study were citrate-stabilized dense polycrystalline core MIONs [58][59][60][61][62][63][64]. Briefly, MIONs were prepared by high-gravity controlled precipitation with thermal aging and stabilized with citric acid.…”

“…A photograph displaying the experimental set-up (AMF system, water jacket) is shown in Figure 1. Previously described, the AMF system comprised three main components: a power supply, an external impedance matching network, and a modified Maxwell coil [61]. The AMF coil comprises three independent loops capable of generating uniform AMF amplitudes in a cylindrical volume of !2.7 Â 10 3 cm 3 at 160 ± 10 kHz [61].…”

“…Previously described, the AMF system comprised three main components: a power supply, an external impedance matching network, and a modified Maxwell coil [61]. The AMF coil comprises three independent loops capable of generating uniform AMF amplitudes in a cylindrical volume of !2.7 Â 10 3 cm 3 at 160 ± 10 kHz [61]. Prior to experiments, the magnitude of the magnetic field was measured at the multiple locations with the AMF coil using a magnetic field probe (AMF Life Systems, Inc., Auburn Hills, MI) for several power settings to provide calibration for all studies [57,61,65,66].…”

“…The thermal and electrical properties of the tissues were derived from the IT'IS foundation database [68]. Homogeneous AMF distribution was assumed based on the previously reported Maxwell-type induction coil [61]. Constant blood perfusion and homogeneous tissue properties were assumed to reduce the non-linearity in the simulation.…”

“…Eddy current heating can be reduced by limiting the area of exposure to high-amplitude AMF, as well as reducing amplitude or duty. AMF coils generating homogeneous AMF in the target region reduce nonspecific tissue heating compared to currently available clinical MNPH systems by reducing the requirement for increased amplitude at the surface to compensate for reduced amplitude at the target [61]. Thus, coils generating homogeneous AMF will reduce superficial heating in deep tissue applications such as treating LAPC.…”