Anatomical Model Uncertainty for RF Safety Evaluation of Metallic Implants Under MRI Exposure.

The Virtual Population (ViP) phantoms have been used in many dosimetry studies, yet, to date, anatomical phantom uncertainty in radiofrequency (RF) research has largely been neglected. The objective of this study is to gain insight, for the first time, regarding the uncertainty in RF-induced fields during magnetic resonance imaging associated with tissue assignment and segmentation quality and consistency in anatomical phantoms by evaluating the differences between two generations of ViP phantoms, ViP1.x and ViP3.0. The RF-induced 10g-average electric (E-) fields, tangential E-fields distribution along active implantable medical devices (AIMD) routings, and estimated AIMD heating were compared for five phantoms that are part of both ViP1.x and ViP3.0. The results demonstrated that differences exceeded 3 dB (-29%, +41%) for local quantities and 1 dB (±12% for field, ±25% for power) for integrated and volume-averaged quantities (e.g., estimated AIMD-heating and 10 g-average E-fields), while the variation across different ViP phantoms of the same generation can exceed 10 dB (-68% and +217% for field, -90% and +900% for power). In conclusion, the anatomical phantom uncertainty associated with tissue assignment and segmentation quality/consistency is larger than previously assumed, i.e., 0.6 dB or ±15% (k = 1) for AIMD heating. Further, multiple phantoms based on different volunteers covering the target population are required for quantitative analysis of dosimetric endpoints, e.g., AIMD heating, which depend on patient anatomy. Phantoms with the highest fidelity in tissue assignment and segmentation should be used, as these ensure the lowest uncertainty and possible underestimation of exposure. To verify that the uncertainty decreases monotonically with improved phantom quality, the evaluation of differences between phantom generations should be repeated for any improvement in segmentation. Bioelectromagnetics. 2019;40:458-471. © 2019 Bioelectromagnetics Society.

Virtual population-based assessment of the impact of 3 Tesla radiofrequency shimming and thermoregulation on safety and B1 + uniformity.

PURPOSE: To assess the effect of radiofrequency (RF) shimming of a 3 Tesla (T) two-port body coil on B1 + uniformity, the local specific absorption rate (SAR), and the local temperature increase as a function of the thermoregulatory response. METHODS: RF shimming alters induced current distribution, which may result in large changes in the level and location of absorbed RF energy. We investigated this effect with six anatomical human models from the Virtual Population in 10 imaging landmarks and four RF coils. Three thermoregulation models were applied to estimate potential local temperature increases, including a newly proposed model for impaired thermoregulation. RESULTS: Two-port RF shimming, compared to circular polarization mode, can increase the B1 + uniformity on average by +32%. Worst-case SAR excitations increase the local RF power deposition on average by +39%. In the first level controlled operating mode, induced peak temperatures reach 42.5°C and 45.6°C in patients with normal and impaired thermoregulation, respectively. CONCLUSION: Image quality with 3T body coils can be significantly increased by RF shimming. Exposure in realistic scan scenarios within guideline limits can be considered safe for a broad patient population with normal thermoregulation. Patients with impaired thermoregulation should not be scanned outside of the normal operating mode. Magn Reson Med 76:986-997, 2016. © 2015 Wiley Periodicals, Inc.

Quantification of RF-exposure of the fetus using anatomical CAD-models in three different gestational stages.

This study analyzes the exposure of pregnant women and their fetuses in three different gestational stages to electromagnetic radiation in the radio frequency range in the near- and the far-field using numerical modeling. For far-field exposure, the power density at which the basic restriction for the whole body SAR is reached is calculated for both the mother and the fetus at whole body resonance and at frequencies between 450 MHz and 2,450 MHz. The near-field exposure is assessed at 450 MHz, 900 MHz, and 2,450 MHz using half wavelength dipoles as generic sources located at different locations around the abdomen of the mother. For the investigated cases, the exposure of the mother is always below or on the order of magnitude of the basic restriction for exposure at the reference level. When applying the reference levels for the general public, the fetus is sufficiently shielded by the mother. However, the basic restrictions for general public exposure can be exceeded in the fetus when the mother is exposed at reference levels for occupational conditions. For plane wave exposure at occupational levels, the whole body SAR in the fetus can exceed the basic restrictions for the general population by at least 1.8 dB, and in the near-field of professional devices, the 10 g SAR can be non-compliant with the product standard for the general public by > 3.5 dB.

Evaluation of the RF heating of a generic deep brain stimulator exposed in 1.5 T magnetic resonance scanners.

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.

Local SAR enhancements in anatomically correct children and adult models as a function of position within 1.5 T MR body coil.

Usage of magnetic resonance imaging (MRI) is continuously increasing due to its excellent soft-tissue contrast and improving diagnostic values. MRI also has the advantage that it operates without ionizing radiation. The main safety concerns are torque, acceleration by the static field, nerve stimulation by the gradient fields, and tissue heating by the radio-frequency (RF) fields. This paper investigates if children and fetuses are at higher risks than adults when the current RF regulations are applied. We analyzed and compared local absorption hotspots, i.e., the peak spatial specific absorption rate averaged over any 10 g (psSAR10g) for five adults, three children of ages 5, 11 and 14 years, and 1 pregnant female (36 weeks' gestation) in 10 different Z-positions (head to calves). In the First Level Operating Mode (4 W/kg whole-body averaged exposure), the psSAR10g values found for adults were as large as 60 W/kg in the trunk and 104 W/kg in the extremities. The corresponding values for children were 43 and 58 W/kg, respectively, and 14 W/kg for the unborn child. Modeling of worst case anatomical RF loops can substantially increase the psSAR10g values, i.e., by factor >>2. The results suggest that local exposure for children and fetuses is smaller than for adults (15-75%), i.e., no special considerations for children and the unborn child are needed regarding psSAR10g due to RF. However, the local thermal load of the unborn may be significantly increased due to the high exposure average (up to 4 W/kg) of the non-perfused amniotic fluid.

Analysis of the local worst-case SAR exposure caused by an MRI multi-transmit body coil in anatomical models of the human body.

Multi-transmit coils are increasingly being employed in high-field magnetic resonance imaging, along with a growing interest in multi-transmit body coils. However, they can lead to an increase in whole-body and local specific absorption rate (SAR) compared to conventional body coils excited in circular polarization for the same total incident input power. In this study, the maximum increase of SAR for three significantly different human anatomies is investigated for a large 3 T (128 MHz) multi-transmit body coil using numerical simulations and a (generalized) eigenvalue-based approach. The results demonstrate that the increase of SAR strongly depends on the anatomy. For the three models and normalization to the sum of the rung currents squared, the whole-body averaged SAR increases by up to a factor of 1.6 compared to conventional excitation and the peak spatial SAR (averaged over any 10 cm(3) of tissue) by up to 13.4. For some locations the local averaged SAR goes up as much as 800 times (130 when looking only at regions where it is above 1% of the peak spatial SAR). The ratio of the peak spatial SAR to the whole-body SAR increases by a factor of up to 47 and can reach values above 800. Due to the potentially much larger power deposition, additional, preferably patient-specific, considerations are necessary to avoid injuries by such systems.

Assessment of the radio-frequency electromagnetic fields induced in the human body from mobile phones used with hands-free kits.

In this study, the radiation emission from mobile phones when used with wireless and wired hands-free kits (HFK) was evaluated to determine the necessity for a dedicated compliance procedure and the extent to which the use of wired and wireless HFK can reduce human exposure. The specific absorption rates (SAR) from wireless HFK were determined experimentally. Wired HFK were evaluated dosimetrically while connected to mobile phones (GSM900/1800, UMTS1950) under maximized current coupling onto the HFK cable and various wire routing configurations. In addition, experimentally validated simulations of a wired HFK and a mobile phone operating on anatomical whole-body models were performed. The maximum spatial peak SAR in the head when using wired HFK was more than five times lower than ICNIRP limits. The SAR in the head depends on the output power of the mobile phone, the coupling between the antenna and cable, external attenuation and potential cable specific attenuation. In general, a wired HFK considerably reduces the exposure of the entire head region compared to mobile phones operated at the head, even under unlikely worst-case coupling scenarios. However, wired HFK may cause a localized increase of the exposure in the region of the ear inside the head under worst-case conditions. Wireless HFK exhibit a low but constant exposure.