A triple-axis hybrid-linearity selective gradient field system design in magnetic hyperthermia: Providing a spheroidal shaped field-free region

Magnetic nanoparticles (MNPs) have emerged as transformative agents in precision oncology due to their tumor-targeting specificity, non-invasive nature, and biocompatibility, particularly in magnetic hyperthermia (MH). Conventional targeted MH systems rely on single-axis gradient selective fields to generate irregular field-free regions (FFRs). This leads to anisotropic heating and compromised targeting accuracy. To address these limitations, this study introduces a triple-axis hybrid-linearity gradient field system that applies a single-axis nonlinear adjustment field to a double-axis linear basement field. This configuration achieves approximate-isotropic gradient distribution, producing spheroidal FFRs with enhanced spatial symmetry. Finite-element simulations demonstrate an obvious reduction in heating zone volume compared to conventional single- or dual-axis systems, significantly reducing off-target heating effects. Experiment using Fe3O4 nanoparticles confirms selective MH efficacy, with FFR temperatures rising by 15.4 °C while unselected regions remain unheated (temperature rise less than 2 °C). An adaptive pre-mapping protocol further optimizes targeting precision, reducing FFR positioning errors to below 20% within a 50 mm workspace. This resolves anisotropic expansion issues inherent to purely linear systems. Despite requiring higher energy input (25 A DC current) than traditional setups, the system enables spatially confined energy deposition with reduced collateral tissue damage. These advancements highlight its clinical potential for tumor-specific therapy and controlled drug delivery. Future efforts should focus on enhancing power efficiency through coil geometry optimization. Integrating real-time field monitoring could also facilitate the real-time selective heating accuracy and operational stability. By proposing a possible system design, this work establishes a novel hybrid-linear modeling and design method for a precise magnetic hyperthermia system.