Method to reduce non-specific tissue heating of small animals in solenoid coils

Abstract

Purpose Solenoid coils that generate time-varying or alternating magnetic fields (AMFs) are used in biomedical devices for research, imaging and therapy. Interactions of AMF and tissue produce eddy currents that deposit power within tissue, thus limiting effectiveness and safety. We aim to develop methods that minimise excess heating of mice exposed to AMFs for cancer therapy experiments. Materials and methods Numerical and experimental data were obtained to characterise thermal management properties of water using a continuous, custom water jacket in a four-turn simple solenoid. Theoretical data were obtained with method-of-moments (MoM) numerical field calculations and finite element method (FEM) thermal simulations. Experimental data were obtained from gel phantoms and mice exposed to AMFs having amplitude >50kA/m and frequency of 160 kHz. Results Water has a high specific heat and thermal conductivity, is diamagnetic, polar, and nearly transparent to magnetic fields. We report at least a two-fold reduction of temperature increase from gel phantom and animal models when a continuous layer of circulating water was placed between the sample and solenoid, compared with no water. Thermal simulations indicate the superior efficiency in thermal management by the developed continuous single chamber cooling system over a double chamber non-continuous system. Further reductions of heating were obtained by regulating water temperature and flow for active cooling. Conclusions These results demonstrate the potential value of a contiguous layer of circulating water to permit sustained exposure to high intensity alternating magnetic fields at this frequency for research using small animal models exposed to AMFs.

Image-guided thermal therapy with a dual-contrast magnetic nanoparticle formulation: A feasibility study

“…It is well established that exposing mice to AMF at ~150 kHz generates amplitude-dependent non-specific tissue heating that also increases with increased radius of exposed tissue [37,41,52,53]. Thus, temperature rise measured at the surface of a subcutaneous tumour is expected to always exceed rectal temperature rise until or unless cooling mechanisms become involved [39,41,52]. Active cooling with a water jacket thus provides a thermal-stabilising chamber that warms a hyp o thermic subject and provides cooling when hyp er thermic temperatures are encountered [39,41].…”

“…These are summarised briefly to provide context for discussion. The AMF system comprises three main components: 1) the power source, 2) an external impedance matching (capacitance) network, and 3) the load which is a modified solenoid inductor having either vertical configuration for nanoparticle heating characterisation [36,37]; or, horizontal configuration with water-jacketed thermal regulation for mouse experiments [39,41]. The power source, matching network, and closed-loop circulating water/water cooling system (200-L reservoir of distilled water) are the same components for both solenoids and for all AMF experiments.…”

“…The power supply was an 80-kW induction heating system manufactured by PPECO (Watsonville, CA) providing alternating current with variable frequency between 135 kHz and 440 kHz, which was adjusted for stable oscillation at 155 ± 5 kHz using capacitors, as previously described [37,39]. Field amplitude measurements were conducted with commercially available field probes (AMF Life Systems, Auburn Hills, MI) and solenoid calibrations were performed with copper wire [38].…”

“…Additionally, the water jacket may provide thermal regulation for anaesthetised mice that commonly lose thermo-regulatory control and become hypothermic. Further, operation of the coil with its cooling system can add either heat sink or additional heat source to hyperthermia experiments with mice that depends upon the power setting used [39,41]. Stabilising the temperature of the sample environment for mouse hyperthermia experiments facilitates data interpretation [41].…”

Modified Solenoid Coil That Efficiently Produces High Amplitude AC Magnetic Fields With Enhanced Uniformity for Biomedical Applications

“…Field amplitude can be dynamically controlled by adjusting the power supply output voltage. Heat station components and their operation have been previously described [23]. The mounted coil, along with the magnetic field probe described below, is depicted in Fig.…”