Mitigation of eddy current heating during magnetic nanoparticle hyperthermia therapy

Abstract

Background: Magnetic nanoparticle hyperthermia therapy is a promising technology for cancer treatment, involving delivering magnetic nanoparticles (MNPs) into tumours then activating them using an alternating magnetic field (AMF). The system produces not only a magnetic field, but also an electric field which penetrates normal tissue and induces eddy currents, resulting in unwanted heating of normal tissues. Magnitude of the eddy current depends, in part, on the AMF source and the size of the tissue exposed to the field. The majority of in vivo MNP hyperthermia therapy studies have been performed in small animals, which, due to the spatial distribution of the AMF relative to the size of the animals, do not reveal the potential toxicity of eddy current heating in larger tissues. This has posed a non-trivial challenge for researchers attempting to scale up to clinically relevant volumes of tissue. There is a relative dearth of studies focused on decreasing the maximum temperature resulting from eddy current heating to increase therapeutic ratio.

Methods: This paper presents two simple, clinically applicable techniques for decreasing maximum temperature induced by eddy currents. Computational and experimental results are presented to understand the underlying physics of eddy currents induced in conducting, biological tissues and leverage these insights to mitigate eddy current heating during MNP hyperthermia therapy.

Results: Phantom studies show that the displacement and motion techniques reduce maximum temperature due to eddy currents by 74% and 19% in simulation, and by 77% and 33% experimentally.

Conclusion: Further study is required to optimise these methods for particular scenarios; however, these results suggest larger volumes of tissue could be treated, and/or higher field strengths and frequencies could be used to attain increased MNP heating when these eddy current mitigation techniques are employed.

Keywords: Eddy currents; Method of Auxiliary Sources; cancer therapy; hyperthermia; magnetic nanoparticle.

Figure 1

Modeled (a) magnetic and (b) electric field distributions of the single-turn coil with a magnetic core located in the xy-plane at z = −1.55cm. (c) Magnetic field strength along x, at y=0, for various z values. The transition between bimodal and unimodal behavior occurs at z ≈ 2. (d) Cross-sectional diagram of the experimental setup at y = 0 (i.e. – vertically bisecting the phantom), drawn to scale and further described in Section III-A.

Figure 2

Modeled cross sectional SAR distribution (xz-plane at y = 0) for a 0.6 S/m phantom with 1cm³ uniformly distributed MNP inclusion. Regions of high EC SAR to be blocked by a tissue displacer are marked with dashed lines.

Figure 3

(a) AMF system with a phantom experiment in progress. The generator, treatment table, thermal camera, and thermal camera software interface are shown. The induction coil (b) is obscured from view in (a) by the treatment table.

Figure 4

(a) A 2D cross-sectional diagram showing the geometry and relevant dimensions of the displacer. (b) 3D model of toroid section shaped tissue displacer. (c) Cross-sectional diagram of the experimental setup at y = 0 (drawn to scale).

Figure 4

(a) A 2D cross-sectional diagram showing the geometry and relevant dimensions of the displacer. (b) 3D model of toroid section shaped tissue displacer. (c) Cross-sectional diagram of the experimental setup at y = 0 (drawn to scale).

Figure 5

Top-down diagram of phantom positions during ECM-motion technique, note the phantom is positioned above the coil. (a) Coil position (copper colored ring) shown with 12 positions for placement of the center of the phantom. (b) Phantom (blue) in position 1, and (c) in position 2. Note that the phantom is simply translated to the next position and undergoes no rotation.

Figure 6

Modeled cross sectional SAR distributions of control and displaced phantoms at z = 0, 2, and 4cm (i.e. – the base of the phantom, the height of the displacer, and 2cm above the displacer).

Figure 7

SAR distribution for control and displaced phantoms. Cross-sections shown at y = 0cm.

Figure 8

Resulting temperature distribution at various time points for the control phantom at z = 0cm, and the displaced phantom at z = 2cm. The cross-sections shown each contain the maximum temperature.

Figure 9

Resulting temperature distribution at t = 1800s, y = 0cm, for the control and displaced phantoms.

Figure 10

Position of bottom half of (a) control phantom and (b) displaced phantom, after sectioning. Resulting cross-sectional temperature distributions at t = 1900s for (c) control and (d) displaced phantoms.

Figure 11

Diagram of phantom in (a) control position, i.e. - centered over coil, (b) resulting SAR distribution of phantom in control position, at z = 0cm, (c) diagram of phantom in position 1 (x = −2.5cm, y = 0cm), (d) resulting SAR distribution of phantom at position 1, (cross-section shown at z = 0cm).

Figure 12

SAR distribution of the centered control phantom, and of the motion phantom at position 1. Cross-sections shown at y = 0cm.

Figure 13

Resulting temperature distribution at the base of the phantom (z = 0cm), for various time points, with the phantom in the control position (centered) throughout the exposure, and having moved between the 12 offset positions at 30 second intervals throughout the exposure.

Figure 14

Resulting temperature distribution at t = 1800s, with the phantom in the control position throughout the exposure, and having moved between the 12 offset positions at 30 second intervals throughout the exposure. Cross-sections shown in the xz-plane at y = 0cm, i.e. – bisecting the phantoms.

Figure 15

Resulting cross sectional temperature distributions at t = 1890s (90s post-exposure) for (a) control and (b) motion phantoms.