No abstract

No abstract

The radio frequency (RF) electromagnetic field of magnetic resonance (MR) scanners can result in significant tissue heating due to the RF coupling with the conducting parts of medical implants. The objective of this article is to evaluate the advantages and shortcomings of a new four-tier approach based on a combined numerical and experimental procedure, designed to demonstrate safety of implants during MR scans. To the authors' best knowledge, this is the first study analyzing this technique. The evaluation is performed for 1.5 T MR scanners using a generic model of a deep brain stimulator (DBS) with a straight lead and a helical lead. The results show that the approach is technically feasible and provides sound and conservative information about the potential heating of implants. We demonstrate that (1) applying optimized tools results in reasonable uncertainties for the overall evaluation; (2) each tier reduces the overestimation by several dB at the cost of more demanding evaluation steps; (3) the implant with the straight lead would cause local temperature increases larger than 18 °C at the RF exposure limit for the normal operating mode; (4) Tier 3 is not sufficient for the helical implant; and (5) Tier 4 might be too demanding to be performed for complex implants. We conclude with a suggestion for a procedure that follows the same concept but is between Tier 3 and 4. In addition, the evaluation of Tier 3 has shown consistency with current scan practice, namely, the resulting heat at the lead tip is less than 3.5 °C for the straight lead and 0.7 °C for the helix lead for scans at the current applied MR scan restrictions for deep brain stimulation at a head average SAR of 0.1 W/kg.

In this paper, fetal exposure to uniform magnetic fields (MF) with different polarizations is quantified at 50 Hz. Numerical computations were performed on high-resolution pregnant models at 3, 7, and 9 months of gestational age (GA), that distinguish a high number of fetal tissues. Fetal whole-body and tissue-specific induced electric fields (E) and current densities (J) were analyzed as a function of both the extremely low frequency magnetic field (ELF-MF) polarization and GA. Additionally, the induced field variation due to changes in fetal position was analyzed by means of two new pregnant models. The uncertainty budget due to the grid resolution was also calculated. Finally, the compliance of the fetal exposure to the ICNIRP Guidelines was checked. A fetal exposure matrix was built at 50 Hz, which could be used to further investigate possible interaction mechanisms between ELF-MF and the associated health risk. Some specific findings were: (1) the induced fields increased with GA; (2) the maxima E were found in skin and fat tissues at each GA; (3) fetal tissue-specific exposure was modified as a function of GA and polarization; (4) the change of the fetal position in the womb significantly modified the induced E in some fetal tissues; (5) the induced fields were in compliance with ICNIRP Guidelines and the results were quite below the permitted threshold limit.

No abstract