Parallel transmission medical implant safety testbed: Real‐time mitigation of RF induced tip heating using time‐domain E‐field sensors

Purpose To implement a modular, flexible, open‐source hardware configuration for parallel transmission (pTx) experiments on medical implant safety and to demonstrate real‐time mitigation strategies for radio frequency (RF) induced implant heating based on sensor measurements. Methods The hardware comprises a home‐built 8‐channel pTx system (scalable to 32‐channels), wideband power amplifiers and a positioning system with submillimeter precision. The orthogonal projection (OP) method is used to mitigate RF induced tip heating and to maintain sufficient B1+ for imaging. Experiments are performed at 297MHz and inside a clinical 3T MRI using 8‐channel pTx RF coils, a guidewire substitute inside a phantom with attached thermistor and time‐domain E‐field probes. Results Repeatability and precision are ~3% for E‐field measurements including guidewire repositioning, ~3% for temperature slopes and an ~6% root‐mean‐square deviation between B1+ measurements and simulations. Real‐time pTx mitigation with the OP mode reduces the E‐fields everywhere within the investigated area with a maximum reduction factor of 26 compared to the circularly polarized mode. Tip heating was measured with ~100 μK resolution and ~14 Hz sampling frequency and showed substantial reduction for the OP vs CP mode. Conclusion The pTx medical implant safety testbed presents a much‐needed flexible and modular hardware configuration for the in‐vitro assessment of implant safety, covering all field strengths from 0.5‐7 T. Sensor based real‐time mitigation strategies utilizing pTx and the OP method allow to substantially reduce RF induced implant heating while maintaining sufficient image quality without the need for a priori knowledge based on simulations or in‐vitro testing.