优化磁共振纳米医学中磁性纳米粒子的热疗效率

磁共振热疗(magnetic resonance hyperthermia)是近年来新兴的一种纳米医学治疗方法，由磁共振的硬件架构产生特定交变磁场，有效地加热磁性纳米粒子，以直接或间接地杀死癌细胞，体现诊疗一体化。提高磁性纳米粒子的加热效率是当前磁共振热疗领域亟待解决的难题之一。磁性纳米粒子的加热效率不仅与粒子本身的大小、性质以及尺寸分布有关，还和聚集状态有关。该研究利用3D Metropolis蒙特卡罗模拟方法，模拟了不同温度下磁性纳米粒子的磁共振热动力学行为及其团聚与分离现象；并通过修正过的郎之万方程，建立了相变临界温度与外加磁场频率的函数关系。模拟结果显示，磁性纳米粒子悬浮液中多聚体的相对含量随着温度的升高而降低，达到临界温度后，多聚体完全分离成单体；而提高交变磁场频率可以显著降低临界温度，且存在临界频率，高于此临界频率后临界温度不再受外加磁场频率影响，达到稳定。因而在临界频率下预热磁性纳米粒子悬浮液，使得多聚体分离成单体，可优化磁性纳米粒子的热疗效率。

Optimizing Magnetic Nanoparticle Hyperthermia Effect in Magnetic Resonance Nanomedicine

Magnetic resonance hyperthermia is a new nano-medical therapy that emerges in recent years. In the presence of external alternating magnetic fields produced by MR instruments, magnetic nanoparticles accumulated at the tumor site can generate heat through Neel relaxation and/or Brownian relaxation. Through magnetic resonance hyperthermia, magnetic nanoparticles can serve as “molecular bullets” to kill cancer cells, leaving surrounding healthy tissues unaffected. Such hyperthermic effects can also be used for thermal activation and control releasing of cancer drugs. One major challenge of magnetic resonance hyperthermia is to optimize the heating efficiency of magnetic nanoparticle suspension. Heating efficiency depends on the size, physical properties, and aggregation state of magnetic nanoparticles. In this study, the thermodynamic behavior of magnetic nanoparticles and the aggregation/disruption of monomers/clusters under different temperatures were studied by 3D Metropolis Monte Carlo method. The relationship between the critical temperature for aggregation/disruption and the frequency of external magnetic field has been established through revised Langevin function. Simulation results show that the relative content of aggregates in colloidal magnetic nanoparticle suspension decreased with the increase in temperature, and the aggregates disrupted completely into monomers at or above the critical temperature. In addition, increasing the frequency of external alternating magnetic field significantly lowered down the critical temperature, and there existed a critical frequency where the critical temperature stabilized and became unaffected by the frequency. Preheating the suspension under critical frequency will disrupt the aggregates into monomers and thus optimize the heating efficiency of magnetic nanoparticles.