基于改进目标场法梯度线圈的感应式磁声磁粒子浓度成像研究

Magneto-Acoustic Magnetic Particle Concentration Imaging Based on Improved Target Field Method Gradient Coil

感应式磁声磁粒子浓度成像(MACT-MI)是一种基于磁声耦合效应的磁性纳米粒子(MNPs)浓度成像新方法。由于其磁声信号的信噪比低,导致系统成像质量不佳,该文提出一种改善MACT-MI信噪比的新思路:首先,基于经典目标场法(TFM)设计梯度线圈的思路,将最小化磁场相对误差定义为适应度函数,并采用鲸鱼优化算法对电流密度系数进行优化,设计了一种结构灵活、激励简单的高质量梯度磁场线圈;其次,为研究梯度磁场空间分布特征对MACT-MI的影响,该文构建不同的MNPs分布模型,并利用多物理场仿真软件COMSOL对MACT-MI的物理过程进行数值求解,得到成像区域的磁力和声压分布。仿真结果表明,磁力和声压的有效波动范围与梯度磁场均匀空间的大小成正比,且随着均匀空间的缩小,磁力和声压的峰值也随之衰减,通过提高磁场的均匀度可以有效提高磁声信号的信噪比,改善系统成像质量。研究结果可为成像装备的设计以及MACT-MI的后续实验应用提供研究基础。

The magneto acoustic concentration tomography of magnetic nanoparticles(MNPs)with magnetic induction(MACT-MI)represents a novel approach to MNP concentration tomography.However,prior research revealed a limitation:the current MACT-MI yields a weak magneto acoustic signal due to the small magnetic force on MNPs,hindering experimental progress.Our study focuses on gradient magnetic field uniformity to enhance signal-to-noise ratio and imaging quality by designing a high-quality gradient magnetic field coil to amplify the magnetic force of MNPs.An improved gradient coil for MACT-MI is proposed.An intelligent optimization algorithm is incorporated to rectify the correlation error from representing continuous current density through the contour lines of the discrete flow function during the conventional target field method(TFM)coil design.This algorithm defines the fitness function for minimizing relative magnetic field error by optimizing current density coefficients.A gradient coil is produced for various coil spacing requirements in imaging.The proposed TFM coil shows a larger uniform space within the imaging area than the traditional gradient coil.Compared with the matrix coil,the TFM coil reduces the coil count by approximately 30%under equivalent conditions,decreases coil turns by about 65%,minimizes coil size by 84%,and lowers the maximum current by approximately 22%.Additionally,to investigate the influence of gradient magnetic field spatial distribution on MACT-MI,diverse distribution models for magnetic nano particles(MNPs)are constructed.With the multi physics simulation software COMSOL,the physical processes of MACT-MI are numerically solved.Variations in magnetic force and sound pressure acting on MNPs can be analyzed in both uniform and non-uniform magnetic fields.The magnetic force on MNPs exhibits enhanced smoothness and stability in a uniform magnetic field.Conversely,in a non-uniform field,as the gradient magnetic field's uniform space diminishes,the stability range of the magnetic force contracts significantly.Similarly,the sound pressure waveform is broad in a uniform field,accompanied by a wide effective range of sound pressure fluctuations.In contrast,sound pressure curve peaks gradually attenuate as the distance from the center increases in a non-uniform field.The results indicate a proportional relationship between the effective fluctuation range of magnetic force,sound pressure,and the uniform spatial extent of the gradient magnetic field.Moreover,as the uniform space decreases,the peak values of both magnetic force and sound pressure diminish.This phenomenon attests to the impact of the gradient magnetic field's spatial distribution characteristics on the magnitude of magnetic force and sound pressure amplitudes,which also contributes to the relatively weak magnetic-acoustic signals in MACT-MI.Enhancing the uniformity of the magnetic field emerges as apractical approach to bolstering the signal-to-noise ratio of the magnetic-acoustic signals.