Numerical safety study of currents induced in the patient during rotations in the static field produced by a hybrid MRI-LINAC system

MRI-LINAC is a new image-guided radiotherapy treatment system that combines magnetic resonance imaging (MRI) and a linear particle accelerator (LINAC) into a single unit. Moving (i.e., rotating or translating) the patient inside the strong magnetic field of the split MRI-LINAC magnet can potentially induce high levels of electric fields and corresponding current densities in the conducting tissues. The prediction and assessment of patient safety in terms of electromagnetic field exposure have received very little attention for a split cylindrical MRI magnet configuration, especially in the vicinity of the gap region. In this novel numerical study, based on the quasi-static finite-difference method, rotation-induced electric fields and current densities are calculated considering a split 1-T magnet and a tissue-accurate 2-mm-resolution human body model. The patient was modeled in both axial and radial orientations relative to the magnet gap in a number of treatment/imaging scenarios. It was found that rotating the patient in the radial orientation produced an order of magnitude larger field exposure in the central nervous system than when the patient was rotated in the axial orientation. Also, rotating the patient with periods lower than about Trot = 43.3 s may result in field exposures above the limits set out in the international safety guidelines. The novel results of this investigation can provide useful insights into the safe use of the MRI-LINAC technology and optimal orientations of the patient during the treatment.