Towards an integrated radiofrequency safety concept for implant carriers in MRI based on sensor‐equipped implants and parallel transmission

Abstract

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.

Wirelessly interfacing sensor‐equipped implants and MR scanners for improved safety and imaging

“…In addition to ensuring RF safety, optimizations can be performed to improve image quality when an implant is present. Simulations have shown that using this strategy with an eight‐channel body coil at 3T, mean $$$ {B}_1^{+} $$$ can be increased three‐fold compared to CP excitation, while still maintaining a maximum temperature increase of the implant tip of ≤1 K 67 …”

“…Based on the demonstrated feasibility in this study, it is possible to establish a comprehensive safety strategy that includes native pTx safety (without implant). 42,67 Native pTx safety can be ensured through state-of-the-art electromagnetic field simulations conducted on human voxel models to calculate Q-matrices in advance, which enable the generation of safe excitation vectors. In the presence of an implant in a patient, the Q S of the implant can be measured in situ, transmitted to the MRI system, and combined with the native safety Q-matrices.…”

“…Therefore, further investigations are necessary to explore the robustness and uncertainties of the presented methodology for a larger set of realistic implant lead configurations. 67,79 While it is appealing to position the readout electronics in the implant casing to avoid modifications of the implant lead design, sensors embedded directly at the implant's lead tip could also be used. For example, it has been previously demonstrated that temperature sensors (e.g., thermistors) embedded at the implant tip can also be used to measure Q S and mitigate RF heating.…”

“…Temperature sensors might provide a more robust calibration setting, and the temperature readings can be also converted to lead tip SAR values. 67,79 These calibrations are in principle frequency-dependent and need to be performed at each field strengths, independently. Similar to the transfer function assessment of implants, the electromagnetic and thermal properties of the investigated tissue will alter lead tip heating and need to be accounted for as an uncertainty of this approach (Supporting Information Figures S3 and S4).…”

“…An alternative explanation is the temperature rise due to the "native," not implant related, SAR deposition. 67 Either way, this is not a real problem, since it occurs only at very low power depositions and a native RF safety assessment without implant must be performed regardless, for example, using Q-matrices and virtual observation points. 67,[87][88][89]…”