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.

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To present electromagnetic simulation setups for detailed analyses of respiration's impact on B +1 and E-fields, local specific absorption rate (SAR) and associated safety-limits for 7T cardiac imaging. Methods: Finite-difference time-domain electromagnetic field simulations were performed at five respiratory states using a breathing body model and a 16-element 7T body transceiver RF-coil array. B + 1 and SAR are analyzed for fixed and moving coil configurations. SAR variations are investigated using phase/amplitude shimming considering (i) a local SAR-controlled mode (here SAR calculations consider RF amplitudes and phases) and (ii) a channel-wise power-controlled mode (SAR boundary calculation is independent of the channels' phases, only dependent on the channels' maximum amplitude). Results: Respiration-induced variations of both B + 1 amplitude and phase are observed. The flip angle homogeneity depends on the respiratory state used for B + 1 shimming; best results were achieved for shimming on inhale and exhale simultaneously (|ΔCV| < 35%). The results reflect that respiration impacts position and amplitude of the local SAR maximum. With the local-SAR-control mode, a safety factor of up to 1.4 is needed to accommodate for respiratory variations while the power control mode appears respiration-robust when the coil moves with respiration (SAR peak decrease: 9% exhale→inhale).Instead, a spatially fixed coil setup yields higher SAR variations with respiration. Conclusion: Respiratory motion does not only affect the B +1 distribution and hence the image contrast, but also location and magnitude of the peak spatial SAR. Therefore, respiration effects may need to be included in safety analyses of RF coils applied to the human thorax.

To protect implant carriers in MRI from excessive radiofrequency (RF) heating it has previously been suggested to assess that hazard via sensors on the implant. Other work recommended parallel transmission (pTx) to actively mitigate implant‐related heating. Here, both ideas are integrated into one comprehensive safety concept where native pTx safety (without implant) is ensured by state‐of‐the‐art field simulations and the implant‐specific hazard is quantified in situ using physical sensors. The concept is demonstrated by electromagnetic simulations performed on a human voxel model with a simplified spinal‐cord implant in an eight‐channel pTx body coil at 3T. To integrate implant and native safety, the sensor signal must be calibrated in terms of an established safety metric (e.g., specific absorption rate [SAR]). Virtual experiments show that E‐field and implant‐current sensors are well suited for this purpose, while temperature sensors require some caution, and B1 probes are inadequate. Based on an implant sensor matrix bold-italicQnormals, constructed in situ from sensor readings, and precomputed native SAR limits, a vector space of safe RF excitations is determined where both global (native) and local (implant‐related) safety requirements are satisfied. Within this safe‐excitation subspace, the solution with the best image quality in terms of B1+ magnitude and homogeneity is then found by a straightforward optimization algorithm. In the investigated example, the optimized pTx shim provides a 3‐fold higher meanB1+ magnitude compared with circularly polarized excitation for a maximum implant‐related temperature increase ∆Timp≤1K. To date, sensor‐equipped implants interfaced to a pTx scanner exist as demonstrator items in research labs, but commercial devices are not yet within sight. This paper aims to demonstrate the significant benefits of such an approach and how this could impact implant‐related RF safety in MRI. Today, the responsibility for safe implant scanning lies with the implant manufacturer and the MRI operator; within the sensor concept, the MRI manufacturer would assume much of the operator's current responsibility.