Mitigation of Intra-coil Eddy Currents in Split Gradient Coils in a Hybrid MRI-LINAC System

The aim of this study is to mitigate intra-gradient coil eddy currents in a hybrid MRI-LINAC system.

The tracks of the gradient coils are modified by cutting slits along the current flow direction. The electromagnetic model developed was first experimentally validated and then used to study the impacts of the slit conductors on the energized and surrounding coils. In this study, gradient coils were slit with different numbers of sub-tracks and driven by a current with frequencies ranging from 100 Hz to 10 kHz. The proposed configuration was assessed by evaluating a number of system parameters, such as the gradient magnetic field, the power loss generated in the surrounding unenergized coil (hereafter referred to as passive coils), and the performance of the energized coil.

It was found that at a typical operating frequency of 1 kHz and compared with a conventional (no cut) split coil structure, the new coil pattern (with four slits) offered improved coil parameters. 1) The average power loss dissipated in the surrounding passive coil was significantly reduced by 85%, 2) the cuts largely reduced the secondary field generated by the eddy currents in the passive coil, which was reduced to about 4% of that produced by the uncut coil and, 3) the performance of the energized coil with slit tracks was significantly improved. Some typical gradient coil parameters, such as the figure of merit, efficiency (η), and η²/R (where η is the efficiency and R is the resistance), were improved by 8.0%, 11.9%, and 45.7%, respectively.

The new slit coil structure is effective in mitigating intra-coil eddy current effects, which is an important issue in the MRI-LINAC system.