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Eddy currents are induced by rapidly switching currents of the gradient coils in an magnetic resonance imaging (MRI) scanner, which is typically cylindrical in shape; hence, the use of cylindrical coordinates is more numerically efficient. We develop the energy conserved splitting finite-difference time-domain (EC-S-FDTD) scheme in the cylindrical coordinates to estimate the eddy currents. The important feature is that this scheme conserves energy and is unconditionally stable. The transformation results in numerical singularities are formed at the internal boundary, and we formulate methods to handle these singularities and efficiently solve the equations. The accuracy of the scheme and energy conversation property are tested, and the performance of the proposed method will be evaluated using two numerical simulations, cylindrical transmission lines, and eddy currents induced by Z-gradient coils. Our numerical results indicate that current density distribution reflects the differences in the tissues by their dielectric properties and has second-order convergence in time and space while preserving energy conservation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]