Cross-Scale Transcranial Magnetic Stimulation Coil for Joint Control of Focal Position and Orientation

Electronic focal control can rapidly reconfigure the induced electric field in transcranial magnetic stimulation without mechanical coil repositioning, but existing approaches face trade-offs among driving complexity, structural compactness, and joint control of focal position and orientation. This study proposes a compact dual-layer four-channel coil topology composed of semicircular elements. Coordinated modulation of channel-current amplitudes and polarities enables joint control of focal position and electric-field orientation. The control mechanism and geometric effects were first characterized using simplified analytical models, followed by evaluation in realistic rat and human head models and wet-phantom measurements with a rat-scale prototype. The same fundamental control mechanism was preserved across scales, whereas the quantitative behavior differed. In the rat model, the focal electric-field orientation changed by 137.22° with a maximum coupled focal displacement of 3.34 mm, while a 5.95 mm focal translation produced only a 0.69° orientation change. In the human model, a 120.42° orientation change was accompanied by a focal displacement of up to 21.17 mm, indicating stronger position-orientation coupling, while focal area could be reduced by 21.2%. Prototype measurements reproduced the simulated control trends, with mean angular deviations of 4.11°-5.64°. These results support the engineering feasibility and cross-scale adaptability of the proposed topology for flexible electric-field control; however, the achievable operating range remains anatomy dependent, and neurophysiological effects were not evaluated.