Scanning patients that have metallic implants may cause tissue heating. This heating is quantified with electromagnetic simulations. We present an alternative method to quickly compute the electromagnetic response of implants by reducing the problem size using a low-rank inverse updating method. This method requires a pre-computed library of field responses at possible implant locations. Results show computed RF fields that are completely equivalent to full-wave simulations while being 1500 times faster. In particular, computations involving small geometric alterations become faster. This method facilitates rapid field computations and possibly enables online RF- or tier 4 ISO/TS 10974 safety assessment.