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Heating induced by radio frequency (RF) energy may appear in tissues near implants located in a human subject undergoing magnetic resonance imaging examination. Lead shielding was proposed to reduce such heating below dangerous levels. In this article, we employed 3-D electromagnetic (EM) and thermal co-simulations to quantify the effectiveness of metallic shielding in the presence of two large sets of nonuniform incident fields at 128 MHz. Specifically, we used a lead EM model (LEM) and computed the RF responses, i.e., the net dissipated power and net temperature increase, above background, at the electrodes. We considered a set of single electrode leads with both straight and helical wires, comparing regular unshielded configurations to different implementations of a metallic shield. The lead length, the relative permittivity of the insulator material, and the lead electrode geometry were all independently varied. For leads with helical wires we observed that: 1) the metallic shield significantly modified the LEM; 2) in most cases, the RF responses substantially increased if metallic shielding was applied; and 3) the net temperature increase in close proximity to the shield can be substantially higher than the net temperature increase in close proximity to the lead electrode. Leads with helical and straight wires exhibited significantly different behavior. Using the results obtained from generic straight wire leads to predict the results for leads with helical wires can be significantly misleading. [ABSTRACT FROM AUTHOR]

The radio frequency (RF)-induced heating is a major concern when patients with medical devices are placed inside a magnetic resonance imaging (MRI) system. In this article, numerical studies are applied to investigate the potentials of using insulated materials to reduce the RF heating for external fixation devices. It is found that by changing the dielectric constant of the insulation material, the RF-induced heating at the tips of devices can be altered. This study indicates a potential technique of developing external fixation device with low MRI RF heating. [ABSTRACT FROM AUTHOR]

We reviewed articles using computational RF dosimetry to compare the Specific Anthropomorphic Mannequin (SAM) to anatomically correct models of the human head. Published conclusions based on such comparisons have varied widely. We looked for reasons that might cause apparently similar comparisons to produce dissimilar results. We also looked at the information needed to adequately compare the results of computational RF dosimetry studies. We concluded studies were not comparable because of differences in definitions, models, and methodology. Therefore we propose a protocol, developed by an IEEE standards group, as an initial step in alleviating this problem. The protocol calls for a benchmark validation study comparing the SAM phantom to two anatomically correct models of the human head. It also establishes common definitions and reporting requirements that will increase the comparability of all computational RF dosimetry studies of the human head. [ABSTRACT FROM AUTHOR]

Radio frequency (RF) shimming is a widely used method to improve the MRI quality by adjusting the magnitude and phase of the transmit elements in the RF coil and eliminating the field inhomogeneity in 3T MRI system. This process alters the field distribution inside the MRI borehole and changes both the level and location of the absorbed RF energy in patients. It is necessary to assess the impact of RF shimming on the risk of RF-induced heating for patients especially those who have implantable longitude medical electrodes in place during the MRI procedure. This paper investigates the impacts of RF shimming on the RF-induced heating of a generic pacemaker electrode using a transfer function method. Two different shimming schemes were studied. The first excitation scheme was designed to obtain a homogeneous ${B_{1 + }}$ field in the image region-chest region. The second condition maximized the temperature rise around the generic pacemaker electrode with different implant trajectories. All the results were normalized to 2 W/kg average whole-body specific absorption rate (SAR) and a Q-matrices method was used to improve the SAR calculation efficiency. The average temperature rise at the electrode tip under quadrature excitation was 3.7 °C while under optimized excitation was 2.3 °C. The worst-case temperature rise for a selected trajectory was found to be 33.17 °C, which was almost 6 times of the temperature rise under quadrature excitation. Therefore, the RF-shimming condition can significantly change the RF-induced heating of electrodes. To ensure patient safety under MRI, RF-shimming condition should be considered and thoroughly integrated into the RF heating assessment. [ABSTRACT FROM AUTHOR]

This paper presents a study on strategies of searching for the worst case configuration of a multi-configuration implantable system under magnetic resonance imaging, which induces the highest radio frequency energy exposure among all possible configurations. The strategies are usually based on parameter search to avoid brutal study of a huge number of possible configurations. Comparison studies of four different searching strategies, i.e., single sequential search, sequential search with the second worst case validations, hotspots-based sequential search, and multi-iteration sequential search, are evaluated. It is found that the initial configuration and sequence of parameter search can significantly affect the searched worst case configuration. For the multi-configuration system considered in this investigation, the simple sequential search strategy leads to a high probability of search failure, which means that the worst case configuration may be easily missed. Single sequential search with the second worst case validations and multi-iteration sequential search can significantly reduce the probability of search failure. [ABSTRACT FROM AUTHOR]

This paper presents a novel technique for efficient evaluation of magnetic resonance imaging (MRI)-induced electric fields or induced voltages in the vicinity of implanted metallic leads. The technique is based on the reciprocity theorem in conjunction with the Huygens Principle. This approach allows one to decouple the micro-scale metallic lead simulation/measurement from the macro-level phantom human simulations within the MRI scanners. Consequently, the estimation of MRI-induced heating on an implanted lead, and the induced voltage on the pacemaker device can be greatly simplified. In addition, this method clearly explains the induced lead heating mechanism during MRI procedures. Several numerical examples, as well as measurement results are given to demonstrate the efficiency and accuracy of this method. [ABSTRACT FROM PUBLISHER]

This article presents a cascaded heterogeneous equivalent network for the evaluation of radiofrequency (RF) induced safety hazards on electrically long active implantable medical devices (AIMDs) exposed to magnetic resonance imaging systems. AIMDs in a highly heterogeneous human body are represented by cascaded transmission line models consisting of segments with different values of effective wavenumbers and characteristic impedances. Two terminations, implantable pulse generator (IPG), and lead-tip are modeled as lumped elements. Methods to extract values for lead-tip impedance, IPG impedance, wavenumber, and characteristic impedance of the AIMDs are proposed and validated through numerical simulations and experimental investigations. Based on these values, one can build up a unique AIMD model along each implanted trajectory inside human body models. The developed models can be used to accurately evaluate the RF-induced voltage and heating for each implantation scenario. [ABSTRACT FROM AUTHOR]

Background: The magnetic resonance imaging (MRI) radio frequency (RF) field induced heating on external fixation devices can be very high in the vicinity of device screws. Such induced RF heating is related to device constructs, device placements, as well as the device insertion depth into human subjects. In this study, computational modeling is performed to determine factors associated with such induced heating. Methods: Numerical modeling, based on the finite-difference time-domain (FDTD) method, is used to evaluate the temperature rises near external device screw tips inside the ASTM phantom for both 1.5-T and 3-T MRI systems. The modeling approach consists of 1) the development of RF coils for 1.5-T and 3-T, 2) the electromagnetic simulations of energy deposition near the screw tips of external fixation devices, and 3) the thermal simulations of temperature rises near the tips of these devices. Results: It is found that changing insertion depth and screw spacing could largely affect the heating of these devices. In 1.5-T MRI system, smaller insertion depth and larger pin spacing will lead to higher temperature rise. However, for 3-T MRI system, the relation is not very clear when insertion depth is larger than 5 cm or when pin spacing became larger than 20 cm. The effect of connection bar material on device heating is also studied and the heating mechanism of the device is analysed. Conclusions: Numerical simulation is used to study RF heating for external fixation devices in both 1.5-T and 3-T MRI coils. Typically, shallower insertion depth and larger pin spacing with conductive bar lead to higher RF heating. The heating mechanism is explained using induced current along the device and power decay inside ASTM phantom. [ABSTRACT FROM AUTHOR]

Purpose. To characterize and quantify the induced radiofrequency (RF) electric (E)-fields and B1+rms fields in patients undergoing magnetic resonance (MR) examinations; to provide guidance on aspects of RF heating risks for patients with and without implants; and to discuss some strengths and limitations of safety assessments in current ISO, IEC, and ASTM standards to determine the RF heating risks for patients with and without implants. Methods. Induced E-fields and B1+rms fields during 1.5 T and 3 T MR examinations were numerically estimated for high-resolution patient models of the Virtual Population exposed to ten two-port birdcage RF coils from head to feet imaging landmarks over the full polarization space, as well as in surrogate ASTM phantoms. Results. Worst-case B1+rms exposure greater than 3.5 μT (1.5 T) and 2 μT (3 T) must be considered for all MR examinations at the Normal Operating Mode limit. Representative induced E-field and specific absorption rate distributions under different clinical scenarios allow quick estimation of clinical factors of high and reduced exposure. B1 shimming can cause +6 dB enhancements to E-fields along implant trajectories. The distribution and magnitude of induced E-fields in the ASTM phantom differ from clinical exposures and are not always conservative for typical implant locations. Conclusions. Field distributions in patient models are condensed, visualized for quick estimation of risks, and compared to those induced in the ASTM phantom. Induced E-fields in patient models can significantly exceed those in the surrogate ASTM phantom in some cases. In the recent 19ε2 revision of the ASTM F2182 standard, the major shortcomings of previous versions have been addressed by requiring that the relationship between ASTM test conditions and in vivo tangential E-fields be established, e.g. numerically. With this requirement, the principal methods defined in the ASTM standard for passive implants are reconciled with those of the ISO 10974 standard for active implantable medical devices. [ABSTRACT FROM AUTHOR]

In this article, an electric (E) field generator is developed to generate uniform and high-strength E-fields at multiple frequencies. The system can be used to assess the radio frequency (RF) hazards of implantable medical devices (IMDs) in magnetic resonance imaging (MRI) environments. The novel system consists of three parts: 1) a signal generating and amplifying block; 2) a four-channel feeding network; and 3) a rectangular resonator box. Mixed modes are utilized to generate locally uniform E-fields inside the rectangular resonator at both 64 and 128 MHz. RF-induced heating for two passive IMD families are evaluated using both numerical and experimental methods inside the E-field generator. These results are compared with those obtained from standard American Society of Testing and Materials (ASTM) phantom testing. The results show that the novel E-field generator is capable of evaluating RF-induced heating for passive IMDs similar to that of the ASTM phantom. Furthermore, it integrates both 1.5- and 3-T RF-induced heating tests in a single system. It is much simpler compared to the ASTM test method which needs the RF coil as a source. Consequently, this small and efficient E-field generator can be used as an alternative system for RF-induced heating tests of passive IMDs in the MRI environment. [ABSTRACT FROM AUTHOR]

This article presents a novel test instrument and strategy for active implantable medical device (AIMD) model validation under a radiofrequency (RF) field generated by a magnetic resonance imaging (MRI) system. A high electric field generator is developed as an alternative instrument for standard MRI RF coils. This system is both compact and power efficient. In addition, a set of validation pathways are proposed based on the Hadamard matrix. These pathways provide low correlations among incident electric fields and yet generate relatively high temperature rises. Two generic AIMDs with different lengths are used to validate the strategy experimentally. It is demonstrated that this method provides an efficient and effective alternative for future AIMD model validation under MRI RF emissions. [ABSTRACT FROM AUTHOR]

In this article, we discuss the model validation for active implantable medical devices (AIMDs) used for safety evaluations under magnetic resonance imaging (MRI) radio frequency (RF) coil emission. A mathematical derivation is presented to provide the guidance on selecting meaningful pathways for the model validation. Suggested validation pathways from current ISO 10974 are used as examples. It is shown that these standard pathways are: 1) inefficient since validations from several pathways are theoretically redundant and 2) incomplete or false since significantly different AIMD models can have identical validation outputs. Based on the developed guidance, two sets of pathways are proposed. It is demonstrated that for efficient and correct model validation, the electric-field distributions along validation pathways should be orthogonal to each other or at least has low correlations between each other. These guidelines can be implemented for future AIMD model validations in ISO10974. [ABSTRACT FROM AUTHOR]

In this paper, an absorbing radio frequency shield (ARFS) is developed to reduce the radio frequency (RF)-induced heating for active implantable medical device (AIMD) during magnetic resonance imaging (MRI). Using the ARFS, the strength of the incident electric field within the shielded region is significantly decreased, and subsequently the induced temperature rises near the tip of AIMD. Both numerical and experimental studies show that the proposed design of ARFS can effectively reduce the temperature rise induced by an AIMD and can mitigate the MRI RF heating risks for patients with AIMDs. [ABSTRACT FROM AUTHOR]

This paper presents a novel, efficient, and simple method to characterize the transfer function (TF) of leads for active implantable medical devices. The method characterizes the TF using a set of base functions and can be obtained using a few voltage measurements. This procedure requires a few direct measurements of the radiofrequency (RF)-induced voltages on the leads during magnetic resonance imaging (MRI) exposure, and the simulated, or measured, electric field generated by MRI RF coil inside the American Society for Testing and Materials (ASTM) phantom. Consequently, the measurement time of the TF can be reduced and the overall process significantly simplified compared to existing TF measurement methods. The theory of the new method is presented and two examples are given to demonstrate the accuracy and efficiency. [ABSTRACT FROM AUTHOR]

This study investigated the radiofrequency-induced heating due to interventional catheters at 1,5 T magnetic resonance image (MRI) based on a combined modeling and experimental method. Two types of interventional catheters, a “single wire” and a “dual wire” catheter, were studied. They were modeled inside a high-resolution anatomical human model along four trajectories. In total, 4 insertion depths, from 17.5 to 70 cm, and 13 scanning landmarks were studied to cover various clinically relevant scenarios. The computational model for the catheters was based on a transfer function approach, measured using the reciprocity theorem. Results of the study showed that the upper limit of the temperature rises near the catheter tip may reach up to 100 °C when scaled to the 2 W/kg average whole-body specific absorption rate (SAR). The computational model was validated experimentally by measuring the induced heating near the catheter in a gel-filled phantom. The data showed a good agreement between the result obtained by the combined method and the direct measurement method with a difference of less than 1 °C. The results suggest that the proposed combined approach, based on the transfer function method, may accurately and efficiently predict the temperature rises near ablation catheters at 1,5 T MRI exposure. [ABSTRACT FROM AUTHOR]

In this paper, a full-wave simulation-based method is proposed to determine the optimal tissue-simulating medium for magnetic resonance imaging radio-frequency safety assessment for patients with active implantable medical devices (AIMDs). Compared to the standard medium, the proposed tissue-simulating medium is optimized for each AIMD lead path in human bodies. The reciprocity-based transfer function model is used as a criterion for optimizing the medium parameters. Numerical investigations and validations are performed for a neurostimulator to demonstrate the method. [ABSTRACT FROM AUTHOR]

This paper investigates the radio frequency (RF)-induced heating for helical stents under 1.5T magnetic resonance imaging (MRI) birdcage RF coil electromagnetic emission.The effects of design parameters, such as pitch, gap, winding pattern, overall length, and diameter, on RF-induced heating are investigated. It is found that these parameters can have different levels of impacts on the overall heating of tissue near the stents. The peak 1 g averaged specific absorption rate (SAR), a parameter that is closely related to the RF-induced heating, is strongly influenced by the stent length and the worst-case peak 1 g averaged SAR often occurs when the stent length is around the half-wavelength of the incident electric field. The increasing diameter reduces the SAR values. At the same stent length and diameter, a higher RF-induced heating occurs for the stent with a larger pitch and a smaller gap. In addition, stents that have longer equivalent electrical lengths could reduce the heating. The SAR and current distributions are also extracted from simulation results to demonstrate the heating mechanism. Experimental measurements are performed to validate the results from simulation investigations. [ABSTRACT FROM AUTHOR]

In this paper, a transmission line model is applied to evaluate magnetic resonance imaging (MRI) radio frequency (RF)-induced fields on active implantable medical devices (AIMDs). The influences of the AIMD lead body, the lead tip, and the implantable pulse generator (IPG) are analyzed based on this model. The transmission line model, together with the transfer function method, forms a systematic approach for analyzing the MRI RF safety problems. It can be used to understand the mechanisms and predict the MRI RF safety risks for various IPGs and leads. Both numerical simulation and measurement results demonstrate the accuracy and the efficiency of this method. [ABSTRACT FROM AUTHOR]

This paper investigates the radio frequency (RF)-induced heating in American Society for Testing and Materials phantom with a generic external fixation device for both birdcage (BC) coil and transverse electromagnetic (TEM) coil. Two typical orientations, i.e., fixture bar perpendicular to and parallel to the coil axis, have been considered. To compare the specific absorption rate and temperature rise in the two different RF coils at 3 T, both electromagnetic and thermal simulations were performed. It is found that tibia external fixation device with orientation parallel to the coil axis will induce severe heating effect in both coils. However, pelvic external fixation device induces larger heating level in TEM coil than BC coil. Furthermore, increasing the pin length or pin spacing of the pelvic external fixation device will cause a larger heating level in TEM coil, while such changes have no substantial effects on BC coil. We show that the transversely distributed electric field in TEM coil forms the underlying reason why pelvic external fixation device induces a higher heating level in TEM coil than BC coil. This finding also indicates that metallic external fixation devices with large dimensions along transverse direction, while might be safe in BC coil, can experience significant heating issue in TEM coil. [ABSTRACT FROM PUBLISHER]

In this work, the magnetic resonance imaging (MRI) radio frequency (RF)-induced heating mechanism for an esophageal stent is discussed from the prospective of forward scattering and the reciprocity theorem. We conclude that the RF-induced heating largely depends on the induced currents on the surface of the stent. Based on this understanding, a novel implantable segmented stent structure is developed to reduce the RF-induced heating of an esophageal stent under MRI procedures. By using a segmented structure, the specific absorption rate (SAR) for a 10-mm-radius and 98-mm-length stent decreased from 53.0 to 14.2 W/kg at a whole-body average SAR of 2 W/kg, and the maximum temperature rise over 15-min MRI exam is reduced from 4.52 to 1.72 $^\circ$ C. The simulated results and experimental measurements agree with each other within the combined uncertainty and demonstrate the effectiveness of the structured stent design. Related to RF-induced heating, this novel design can be used to develop inherently safe esophageal stents. [ABSTRACT FROM AUTHOR]

Goal: This study aims at a systematic assessment of five computational models of a birdcage coil for magnetic resonance imaging (MRI) with respect to accuracy and computational cost. Methods : The models were implemented using the same geometrical model and numerical algorithm, but different driving methods (i.e., coil “defeaturing”). The defeatured models were labeled as: specific (S2), generic (G32, G16), and hybrid (H16, \H16fr\text{-forced}). The accuracy of the models was evaluated using the “symmetric mean absolute percentage error” (“SMAPE”), by comparison with measurements in terms of frequency response, as well as electric ( \\vec E\) and magnetic ( \ \vec B \) field magnitude. Results: All the models computed the \ \vec B \ within 35% of the measurements, only the S2, G32, and H16 were able to accurately model the \\vec E\ inside the phantom with a maximum SMAPE of 16%. Outside the phantom, only the S2 showed a SMAPE lower than 11%. Conclusions: Results showed that assessing the accuracy of \ \vec B \ based only on comparison along the central longitudinal line of the coil can be misleading. Generic or hybrid coils — when properly modeling the currents along the rings/rungs — were sufficient to accurately reproduce the fields inside a phantom while a specific model was needed to accurately model \\vec E\ in the space between coil and phantom. Significance: Computational modeling of birdcage body coils is extensively used in the evaluation of radiofrequency-induced heating during MRI. Experimental validation of numerical models is needed to determine if a model is an accurate representation of a physical coil. [ABSTRACT FROM AUTHOR]

In this paper, a quick and efficient approach is proposed to estimate in vivo RF-induced heating from small medical implants under MRI procedure. An efficient methodology is developed to correlate the incident electric field to the induced temperature rise for stents of arbitrary shapes. A 10-cm-long titanium rod and a 6-cm-long intestine stent are used as examples to validate the approach under in vitro testing conditions. In vivo temperature rise estimations are then performed on an anatomically corrected adult male model. Numerical and experimental studies demonstrate the efficiency and accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach to minimize the radio frequency (RF)-induced heating from external fixation devices under magnetic resonance imaging (MRI) procedure. With proper placement and selection of absorption material, one can alter the electromagnetic field and current distributions around and/or on medical devices so that the MRI RF field-induced heating can be reduced. The absorption-material-based RF heating reduction schemes are analyzed and validated. Numerical and experimental studies are conducted to illustrate the potentials of reducing the RF heating for external fixation devices. A design strategy based on response surface methodology is presented for material selection, and it is shown that absorption materials can significantly reduce the RF heating effects from 645 to 172 W/kg in terms of 1-g spatial-averaged SAR for the device under this investigation. [ABSTRACT FROM AUTHOR]