Abstract：Magneto-acoustic concentration tomography of magnetic nanoparticles (MNPs) using magnetic induction (MACT-MI) represents a novel approach for MNPs concentration tomography.However, existing MACT-MI research is confronted with two major issues: “excessively high excitation current amplitude and poor uniformity of the gradient magnetic field”, which restrict the system′s miniaturization and clinical application. To address the above issues, this study starts with reducing current excitation and improving magnetic field uniformity, and designs a gradient coil to enhance the quality of the gradient magnetic field in the imaging area, thereby promoting experimental research. Firstly, this study proposed an “improved helical gradient coil” design method. Based on the discrete flow technique, it combines the existing helical coil with the circular coil to form an improved helical coil system. The external coil of this system can improve the problem of magnetic field non-uniformity at the imaging boundary. The Artificial lemming algorithm (ALA) was applied to the model, with the root mean square error of the magnetic flux density at 4 147 discrete points in the imaging area as the fitness function, and optimization was carried out under multiple physical constraints such as current, radius, and spacing. Compared with the traditional Maxwell, matrix, and improved target field method (TFM) gradient coils under the same gradient magnetic field conditions, the current excitation amplitude of the improved helical gradient coil was reduced by 67%, 64%, and 54% respectively, and the non-uniformity of the magnetic field in the imaging area was reduced by 70%, 65%, and 31% respectively. In addition, to evaluate the performance of the improved gradient coil, a multi-physical field forward problem simulation model of MACT-MI was built in COMSOL, nine sets of magnetite nanoparticles, identical in size and uniformly distributed, were constructed within the imaging region,and the physical process was solved. Under the gradient magnetic fields generated by the traditional Maxwell, matrix, and improved TFM gradient coils, the average relative deviations of the magnetic force were 7.56%, 2.95%, and 1.68% respectively, and the average relative deviations of the acoustic pressure signals were 5.64%, 2.79%, and 2.37% respectively; under the gradient magnetic field generated by the improved helical gradient coil, the average relative deviations of the magnetic force and acoustic pressure signal were 1.36% and 2.34% respectively. The average deviations of the magnetic force and acoustic pressure compared to the other three gradient coils have decreased, and the standard deviations have also decreased. To evaluate coil stability, analysis was conducted using standard deviation and 95% confidence intervals. The coil designed in this study exhibited the smallest standard deviation and no overlap within the 95% confidence interval, thereby validating the high stability of the gradient coil. The results show that the improved helical coil proposed in this paper is superior to the Maxwell coil, matrix coil, and improved TFM coil in both the amplitude of the pulse excitation and the non-uniformity of the gradient magnetic field. Moreover, magnetic nanoparticles experience lower magnetic force and acoustic pressure deviations compared to other coils, resulting in greater stability in the magnetic field generated by the coil.
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2026, Vol. 41 
