In this study, we investigated whether different dual-channel transmission settings, such as the CPT versus the EPT, may determine different heat induction around an implant material dependent on its location within the magnetic field. While the heat induction steadily increased with the lateral offset from the isocenter for both settings, it was consistently higher for CPT than for EPT. To the best knowledge of the authors, this is the first study that reports a reduced heat induction through the use of an EPT in comparison to conventional CPT. It is also the first report to highlight that EPT reduces the heat induction near implants placed with a lateral offset from isocenter in comparison with CPT.
The main driving force for the development of dual-channel, parallel RF transmission technology was the reduction of standing-wave artifacts in MRI of the abdomen and thorax [12, 13]. However, the technique was also reported to reduce artifacts and improve image quality in other body regions, which was mainly explained with a better B1-field homogeneity [10] or a decrease in locally induced RF currents resulting in decreased shading artifacts around metallic implants [19].
For MRI-conditional devices, the safety is tested for traditional, single-channel, RF body coils. In our measurements, conducted in a phantom tank and with implant orientations as recommended for RF heating tests according to the ASTM standard F2182-11a [18], the EPT resulted in consistently less heating than the CPT. However, our findings cannot be generalized to the general assumption that EPT would always be preferable to, and safer than, CPT. Actual electric field patterns in a patient’s body depend on many factors that are hard to control in clinical examination. Certain configurations may lead to higher heating with EPT than with CPT. For example, Murbach et al. demonstrated that, in the absence of implants, EPT may generate a higher temperature increase near the fetus in pregnant women than CPT [16].
However, the presented results are important not only for clinical daily use, where the EPT setting should be preferably used if scanning implants, but also for safety test measurements to avoid false underestimations of heat induction around metallic objects when are conducted with EPT instead of CPT. If in vitro RF heating tests are performed following the ASTM standard F2182-11a [18]. However, using EPT instead of CPT (as required), the results could strongly underestimate the potential RF heating. Special care is needed, since some dual-transmit MR system from other vendors might force the use of EPT setting, which cannot be overruled in clinical software.
Some study limitations need recognition. First, all the temperature measurements of implant-related heating were performed over a time period of 2 min. This is a shorter time than the duration of many clinical acquisition sequences. However, our goal was not to provide evidence for the certification of an MRI-conditional label for the used implants, but to relatively compare the effects of two RF transmission settings. The scan duration (2 min) was chosen because an assessment of the relative heat induction was considered sufficient for a comparison of the two RF transmission settings. The temperature increases reached after 2 min were large enough for a robust quantification of the effects but small enough to keep the required cool-down periods between two successive experiments sufficiently short to allow the realization of this study. A scan duration of 15 min would result in a temperature increase of about twice as high, but deposit 7.5 times more energy with thus much longer waiting time. This would only lead to a higher likelihood of alteration of the implant or the temperature sensors and, therefore, potentially generate larger errors. In addition, we only used the body coil for RF transmission. The use of transmit head or knee coils may produce different results, while the choice of the receive coil seems of minimum relevance in this context. Finally, this was only a phantom study. Studying such effects in living humans is very challenging, and the positioning of temperature probes next to the implant is not realistic.
In conclusion, in this phantom study, comparatively testing different dual-channel transmission settings (CPT versus EPT), we demonstrated that the heat induction around metallic implants increases with increasing lateral implant distance from the isocenter and is more severe using RF with CPT than with EPT. However, further studies, including simulations and evaluations on humans, are needed to verify and generalize our findings.