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 >50 kA/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.

Figure 1

Calculated allowable power deposition for a simulated cylinder of muscle tissue based on limits proposed by Atkinson et al. [25], with (A) varying frequency (f) and field amplitude (H) for cylinder having a radius, r= 0.15 m, and (B) varying field amplitude (H) and radius (r) of simulated tissue at a fixed frequency of 160 kHz. Blue zones display combinations that do not exceed maximum allowable limits, whereas red zones represent fields and exposures that exceed tolerance limits.

Figure 2

(A) Circuit diagram of the AMF system. (B) The four turn solenoid coil modelled using MoM. The coil inner diameter is 45 mm and coil length is 32 mm.

Figure 3

Picture of the 4-turn solenoid coil, water jacket and mouse chamber with anaesthetised mouse inside.

Figure 4

Magnetic field across the (x–z plane) in the coil in the sample volume with no water (A) and with water (B) in the jacket; Magnetic field profile along the middle of solenoid in z direction without water (C) and with water (D) for 8 kW (peak) input power to AMF coil.

Figure 5

Temperature profile at the end of 1200 s of AMF exposure. (A) No external cooling. (B) Dual chamber discontinuous cooling at 35°C. (C) Dual chamber discontinuous cooling at 27°C. (D) Single chamber continuous cooling at 35°C. (E) Comparison of maximum and minimum temperatures in the domain among cases B, C and D.

Figure 6

Comparison of net initial rate of temperature change in the gels for conditions I, II, and III.

Figure 7

Observed temporal change in temperatures of (A) individual mice at each of the three conditions (I, II, and III) with no AMF, and (B) with 20 min exposures to AMF having amplitude of 84 kA/m. Displayed are corrected (T(t) − T(0)) data obtained from each of four temperature probes (two placed s.c. in left and right thorax, one affixed to skin surface of abdomen, and one inserted into rectum.

Figure 8

Corrected temperature change as measured in the thorax (A) and rectum (B) for mouse exposed to AMF with no water, i.e. condition I (solid line), mouse exposed to AMF with water but no active cooling, i.e. condition II (dashed line), and mouse exposed to AMF with actively cooled water, i.e. condition III (dash-dot-dot). Subcutaneous thoracic temperatures were averaged for both left and right side. In all cases the temperature change for each temperature probe was calculated from measured temperatures and the total change of temperature was estimated after subtracting temperature changes obtained from sham controls.